MAXIM MAX1400

19-1430; Rev 0; 2/99
+5V, 18-Bit, Low-Power, Multichannel,
Oversampling (Sigma-Delta) ADC
The MAX1400 18-bit, low-power, multichannel, serialoutput ADC uses a sigma-delta modulator with a digital
decimation filter to achieve true 16-bit accuracy. The
user-selectable decimation factor of the digital filter
allows the conversion resolution to be reduced in
exchange for a higher output data rate. The device
achieves true 16-bit performance at an output data rate of
up to 480sps. In addition, the modulator sampling
frequency may be optimized for either lowest power
dissipation or highest throughput rate. The MAX1400
operates from +5V.
This device offers three fully differential input channels
that can be independently programmed with a gain
between +1V/V and +128V/V. Furthermore, it can compensate an input-referred DC offset (such as system offset) up to 117% of the selected full-scale range. These
three differential channels may also be configured to
operate as five pseudo-differential input channels. Two
additional, fully differential system-calibration channels
are provided for gain and offset error correction. External
access is provided to the multiplexer (mux) output to
facilitate additional signal processing.
The MAX1400 can be configured to scan all signal inputs
sequentially and provide the results through the serial
interface with minimum communications overhead. When
used with a 2.4576MHz or 1.024MHz master clock, the
digital decimation filter can be programmed to produce
zeros in its frequency response at the line frequency and associated harmonics, ensuring excellent
line rejection without the need for further post-filtering.
The MAX1400 comes in a 28-pin SSOP package.
Applications
♦ 16-Bit Performance with No Missing Codes
to 480sps
♦ Low Quiescent Current
250µA (operating mode)
2µA (power-down mode)
♦ 3 Fully Differential or 5 Pseudo-Differential Signal
Input Channels
♦ 2 Additional Fully Differential Calibration
Channels/Auxiliary Input Channels
♦ Access to the Mux Output/ADC Input
♦ Programmable Gain and Offset
♦ Fully Differential Reference Inputs
♦ Converts Continuously or On Command
♦ Automatic Channel Scanning and Continuous
Data Output Mode
♦ Operates with +5V Analog Supply and
+3V or +5V Digital Supply
♦ SPI™/QSPI™-Compatible 3-Wire Serial Interface
♦ 28-Pin SSOP Package
Pin Configuration
TOP VIEW
CLKIN 1
28 SCLK
CLKOUT 2
27 DIN
26 DOUT
RESET 4
Portable Weigh Scales
Loop-Powered Systems
Pressure Transducers
25 INT
MUXOUT+ 5
MUXOUT- 6
Ordering Information
TEMP. RANGE
♦ 18-Bit Resolution, Sigma-Delta ADC
CS 3
Portable Industrial Instruments
PART
Features
PIN-PACKAGE
24 VDD
MAX1400
23 DGND
ADCIN+ 7
22 CALOFF+
ADCIN- 8
21 CALOFF-
AGND 9
20 REFIN+
V+ 10
19 REFIN-
MAX1400CAI
0°C to +70°C
28 SSOP
AIN1 11
MAX1400EAI
-40°C to +85°C
28 SSOP
AIN2 12
17 CALGAIN-
AIN3 13
16 AIN6
AIN4 14
15 AIN5
SPI and QSPI are trademarks of Motorola, Inc.
18 CALGAIN+
SSOP
________________________________________________________________ Maxim Integrated Products
1
For free samples & the latest literature: http://www.maxim-ic.com, or phone 1-800-998-8800.
For small orders, phone 1-800-835-8769.
MAX1400
General Description
MAX1400
+5V, 18-Bit, Low-Power, Multichannel,
Oversampling (Sigma-Delta) ADC
ABSOLUTE MAXIMUM RATINGS
Maximum Current Input into Any Pin ..................................50mA
Continuous Power Dissipation (TA = +70°C)
28-Pin SSOP (derate 9.52mW/°C above +70°C) ........524mW
Operating Temperature Ranges
MAX1400CAI .....................................................0°C to +70°C
MAX1400EAI...................................................-40°C to +85°C
Storage Temperature Range .............................-60°C to +150°C
Lead Temperature (soldering, 10sec) .............................+300°C
V+ to AGND, DGND .................................................-0.3V to +6V
VDD to AGND, DGND ...............................................-0.3V to +6V
AGND to DGND.....................................................-0.3V to +0.3V
Analog Inputs to AGND................................-0.3V to (V+ + 0.3V)
Analog Outputs to AGND .............................-0.3V to (V+ + 0.3V)
Reference Inputs to AGND...........................-0.3V to (V+ + 0.3V)
CLKIN and CLKOUT to DGND...................-0.3V to (VDD + 0.3V)
All Other Digital Inputs to DGND..............................-0.3V to +6V
All Digital Outputs to DGND .......................-0.3V to (VDD + 0.3V)
Stresses beyond those listed under “Absolute Maximum Ratings” may cause permanent damage to the device. These are stress ratings only, and functional
operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to
absolute maximum rating conditions for extended periods may affect device reliability.
ELECTRICAL CHARACTERISTICS
(V+ = +5V ±5%, VDD = +2.7V to +5.25V, VREFIN+ = +2.50V, REFIN- = AGND, fCLKIN = 2.4576MHz, TA = TMIN to TMAX, unless otherwise noted. Typical values are at TA = +25°C.)
PARAMETER
SYMBOL
CONDITIONS
MIN
TYP
MAX
UNITS
STATIC PERFORMANCE
Noise-Free Resolution
No missing codes guaranteed by design;
for filter settings with FS1 = 0
Output Noise
Depends on filter setting and selected gain
Integral Nonlinearity
INL
Bipolar mode, filter settings with FS1 = 0
16
Table 16
-0.0015
Nominal Gain (Note 1)
Unipolar Offset Error
Unipolar Offset Drift
Positive Full-Scale Error
(Note 2)
Full-Scale Drift (Note 3)
Gain Error (Note 4)
Gain-Error Drift (Note 5)
Bipolar Negative Full-Scale Error
Bipolar Negative Full-Scale Drift
2
0.0015
%FSR
2
%FSR
0.98
Relative to nominal offset of 1% FSR
-1
For gains of 1, 2, 4
0.5
For gains of 8, 16, 32, 64, 128
0.3
Bipolar Zero Error
Bipolar Zero Drift
Bits
-2.0
µV/°C
2.0
For gains of 1, 2, 4
0.8
For gains of 8, 16, 32, 64, 128
0.3
µV/°C
For gains of 1, 2, 4, 8, 16, 32, 64
-2.5
2.5
For gain of 128
-3.5
3.5
For gains of 1, 2, 4
0.8
For gains of 8, 16, 32, 64, 128
0.3
-2
2
For gain of 128
-3
3
1
For gain of 128
5
-2.5
2.5
For gain of 128
-3.5
3.5
0.8
For gains of 8, 16, 32, 64, 128
0.3
_______________________________________________________________________________________
%FSR
ppm/°C
For gains of 1, 2, 4, 8, 16, 32, 64
For gains of 1, 2, 4
%FSR
µV/°C
For gains of 1, 2, 4, 8, 16, 32, 64
For gains of 1, 2, 4, 8, 16, 32, 64
%FSR
%FSR
µV/°C
+5V, 18-Bit, Low-Power, Multichannel,
Oversampling (Sigma-Delta) ADC
(V+ = +5V ±5%, VDD = +2.7V to +5.25V, VREFIN+ = +2.50V, REFIN- = AGND, fCLKIN = 2.4576MHz, TA = TMIN to TMAX, unless otherwise noted. Typical values are at TA = +25°C.)
PARAMETER
SYMBOL
CONDITIONS
MIN
TYP
MAX
UNITS
OFFSET DAC
Offset DAC Range (Note 6)
Offset DAC Resolution
Offset DAC Full-Scale Error
Unipolar mode
-116.7
116.7
Bipolar mode
-58.35
58.35
Unipolar mode
16.7
Bipolar mode
8.35
Input referred
-2.5
Additional Noise from Offset
DAC (Note 7)
%FSR
2.5
Offset DAC Zero-Scale Error
DAC code = 0000
%FSR
%FSR
0
%FSR
0
µVRMS
ANALOG INPUTS/REFERENCE INPUTS (Specifications for AIN and REFIN, unless otherwise noted.)
Common-Mode Rejection
CMR
At DC
90
For filter notch 50Hz, ±0.02 · fNOTCH,
MF1 = 0, MF0 = 0, fCLKIN = 2.4576MHz (Note 8)
150
For filter notch 60Hz, ±0.02 · fNOTCH,
MF1 = 0, MF0 = 0, fCLKIN = 2.4576MHz (Note 8)
150
dB
Normal Mode 50Hz Rejection
(Note 8)
NMR
For filter notch 50Hz, ±0.02 · fNOTCH,
MF1 = 0, MF0 = 0, fCLKIN = 2.4576MHz
100
dB
Normal Mode 60Hz Rejection
(Note 8)
NMR
For filter notch 60Hz, ±0.02 · fNOTCH,
MF1 = 0, MF0 = 0, fCLKIN = 2.4576MHz
100
dB
Common-Mode Voltage Range
(Note 9)
REFIN and AIN for BUFF = 0
VAGND
V+
V
Absolute Voltage Range Input
REFIN and AIN for BUFF = 0
VAGND
- 30mV
V+
+ 30mV
V
Absolute and Common-Mode
AIN Voltage Range
BUFF = 1
VAGND
+ 200mV
V+
- 1.5
V
DC Input Leakage Current
(Note 10)
REFIN and AIN for
BUFF = 0
AIN Input Current (Note 10)
BUFF = 1
AIN Input Capacitance
(Note 11)
BUFF = 0
TA = +25°C
Gain = 1
34
Gain = 2
38
Gain = 4
45
Gain = 8, 16, 32, 64, 128
60
BUFF = 1, all gains
AIN Differential Voltage Range
(Note 12)
40
TA = TMIN to TMAX
Unipolar input range (U/B bit = 1)
Bipolar input range (U/B bit = 0)
pA
10
nA
10
nA
pF
30
0 to VREF /
gain
±VREF /
gain
V
________________________________________________________________________________________
3
MAX1400
ELECTRICAL CHARACTERISTICS (continued)
MAX1400
+5V, 18-Bit, Low-Power, Multichannel,
Oversampling (Sigma-Delta) ADC
ELECTRICAL CHARACTERISTICS (continued)
(V+ = +5V ±5%, VDD = +2.7V to +5.25V, VREFIN+ = +2.50V, REFIN- = AGND, fCLKIN = 2.4576MHz, TA = TMIN to TMAX, unless otherwise noted. Typical values are at TA = +25°C.)
PARAMETER
SYMBOL
AIN and REFIN Input Sampling
Frequency
fS
REFIN+ - REFIN- Voltage
(Note 13)
CONDITIONS
MIN
TYP
MAX
(Table 15)
±5% for specified performance; functional with
lower VREF
UNITS
Hz
2.50
V
+10
µA
LOGIC INPUTS
Input Current
IIN
-10
All inputs except
CLKIN
Input Low Voltage
VIL
CLKIN only
VIH
CLKIN only
Input Hysteresis
VHYS
0.8
0.4
VDD = 5V
0.8
VDD = 3.3V
All inputs except
CLKIN
Input High Voltage
VDD = 5V
VDD = 3.3V
All inputs except
CLKIN
VDD = 5V
VDD = 3.3V
V
0.4
2
2
VDD = 5V
3.5
VDD = 3.3V
2.4
V
VDD = 5V
200
VDD = 3.3V
200
mV
LOGIC OUTPUTS
DOUT
and INT
Output Low Voltage (Note 14)
VOL
CLKOUT
DOUT
and INT
Output High Voltage (Note 14)
VOH
CLKOUT
Floating State Leakage Current
Floating State Output
Capacitance
VDD = 5V, ISINK = 800µA
0.4
VDD = 3.3V, ISINK = 100µA
V
VDD = 5V, ISINK = 10µA
0.4
VDD = 3.3V, ISINK = 10µA
VDD = 5V, ISOURCE = 200µA
VDD = 3.3V, ISOURCE = 100µA
VDD = 5V, ISOURCE = 10µA
VDD = 3.3V, ISOURCE = 10µA
IL
4
VDD - 0.3
V
4
VDD - 0.3
-10
CO
10
µA
9
pF
0.1
µA
TRANSDUCER BURN-OUT (Note 15)
Current
IBO
Initial Tolerance
Drift
±10
%
±0.05
%/°C
POWER REQUIREMENTS
V+ Voltage
V+
VDD Voltage
VDD
Power-Supply Rejection V+
(Note 16)
PSR
4
For specified performance
PSR
4.75
5.25
V
2.7
5.25
V
(Note 16)
(Note 17)
_______________________________________________________________________________________
dB
dB
+5V, 18-Bit, Low-Power, Multichannel,
Oversampling (Sigma-Delta) ADC
(V+ = +5V ±5%, VDD = +2.7V to +5.25V, VREFIN+ = +2.50V, REFIN- = AGND, fCLKIN = 2.4576MHz, TA = TMIN to TMAX, unless otherwise noted. Typical values are at TA = +25°C.)
PARAMETER
SYMBOL
CONDITIONS
MIN
TYP
MAX
UNITS
ANALOG POWER-SUPPLY CURRENT (Measured with digital inputs at either DGND or VDD, external CLKIN, burn-out currents
disabled, X2CLK = 0, CLK = 0 for 1.024MHz, CLK = 1 for 2.4576MHz.)
V+ Standby Current (Note 18)
PD bit = 1, external clock stopped
Normal mode,
MF1 = 0,
MF0 = 0
2X mode,
MF1 = 0,
MF0 = 1
V+ Current
1.024MHz
2.4576MHz
1.024MHz
2.4576MHz
IV+
4X mode,
MF1 = 1,
MF0 = 0
8X mode,
MF1 = 1,
MF0 = 1
1.024MHz
2.4576MHz
1.024MHz
2.4576MHz
1
10
Buffers off
175
210
Buffers on
370
420
Buffers off
250
300
Buffers on
610
700
Buffers off
245
Buffers on
610
Buffers off
0.42
0.55
Buffers on
1.2
1.5
Buffers off
0.42
Buffers on
1.2
Buffers off
1.8
2.2
Buffers on
4.8
6
Buffers off
1.8
Buffers on
4.8
Buffers off
1.8
2.2
Buffers on
4.8
6
µA
µA
mA
DIGITAL POWER-SUPPLY CURRENT (Measured with digital inputs at either DGND or VDD, external MCLKIN, burn-out currents
disabled, X2CLK = 0, CLK = 0 for 1.024MHz, CLK = 1 for 2.4576MHz.)
VDD Standby Current (Note 18)
3.3V Digital Supply Current
5V Digital Supply Current
IDD
IDD
PD bit = 1, external clock stopped
1
10
Normal mode,
MF1 = 0, MF0 = 0
1.024MHz
70
200
2.4576MHz
150
300
2X mode,
MF1 = 0, MF0 = 1
1.024MHz
0.08
2.4576MHz
0.17
4X mode,
MF1 = 1, MF0 = 0
1.024MHz
0.11
2.4576MHz
0.22
8X mode,
MF1 = 1, MF0 = 1
1.024MHz
0.15
2.4576MHz
0.32
0.45
Normal mode,
MF1 = 0, MF0 = 0
1.024MHz
150
300
2.4576MHz
235
450
2X mode,
MF1 = 0, MF0 = 1
1.024MHz
0.13
2.4576MHz
0.28
4X mode,
MF1 = 1, MF0 = 0
1.024MHz
0.17
2.4576MHz
0.36
8X mode,
MF1 = 1, MF0 = 1
1.024MHz
0.24
2.4576MHz
0.53
µA
µA
0.35
0.40
mA
µA
0.50
0.60
mA
0.80
_______________________________________________________________________________________
5
MAX1400
ELECTRICAL CHARACTERISTICS (continued)
MAX1400
+5V, 18-Bit, Low-Power, Multichannel,
Oversampling (Sigma-Delta) ADC
ELECTRICAL CHARACTERISTICS (continued)
(V+ = +5V ±5%, VDD = +2.7V to +5.25V, VREFIN+ = +2.50V, REFIN- = AGND, fCLKIN = 2.4576MHz, TA = TMIN to TMAX, unless otherwise noted. Typical values are at TA = +25°C.)
PARAMETER
SYMBOL
CONDITIONS
MIN
TYP
MAX
UNITS
5V POWER DISSIPATION (V+ = VDD = +5V, digital inputs = 0 or VDD, external CLKIN, burn-out currents disabled, X2CLK = 0,
CLK = 0 for 1.024MHz, CLK = 1 for 2.4576MHz.)
Normal mode,
MF1 = 0,
MF0 = 0
2X mode,
MF1 = 0,
MF0 = 1
Power Dissipation
2.4576MHz
1.024MHz
2.4576MHz
PD
4X mode,
MF1 = 1,
MF0 = 0
8X mode,
MF1 = 1,
MF0 = 1
Standby Power Dissipation
1.024MHz
(Note 18)
1.024MHz
2.4576MHz
1.024MHz
2.4576MHz
Buffers off
1.45
2.55
Buffers on
2.43
3.60
Buffers off
2.43
3.75
Buffers on
4.23
5.75
Buffers off
1.88
Buffers on
3.70
Buffers off
3.50
5.25
Buffers on
7.4
10.0
Buffers off
2.95
Buffers on
6.85
Buffers off
10.8
14.0
Buffers on
25.8
33.0
Buffers off
10.2
Buffers on
25.2
Buffers off
11.7
15.0
Buffers on
26.7
34.0
10
100
mW
µW
Note 1: Nominal gain is 0.98. This ensures a full-scale input voltage may be applied to the part under all conditions without causing saturation of the digital output data.
Note 2: Positive Full-Scale Error includes zero-scale errors (unipolar offset error or bipolar zero error) and applies to both unipolar
and bipolar input ranges. This error does not include the nominal gain of 0.98.
Note 3: Full-Scale Drift includes zero-scale drift (unipolar offset drift or bipolar zero drift) and applies to both unipolar and bipolar
input ranges.
Note 4: Gain Error does not include zero-scale errors. It is calculated as (full-scale error - unipolar offset error) for unipolar ranges
and as (full-scale error - bipolar zero error) for bipolar ranges. This error does not include the nominal gain of 0.98.
Note 5: Gain-Error Drift does not include unipolar offset drift or bipolar zero drift. It is effectively the drift of the part if zero-scale
error is removed.
Note 6: Use of the offset DAC does not imply that any input may be taken below AGND.
Note 7: Additional noise added by the offset DAC is dependent on the filter cutoff, gain, and DAC setting. No noise is added for a
DAC code of 0000.
Note 8: Guaranteed by design or characterization; not production tested.
Note 9: The input voltage must be within the Absolute Input Voltage Range specification.
Note 10: All AIN and REFIN pins have identical input structures. Leakage is production tested only for the AIN3, AIN4, AIN5,
CALGAIN, and CALOFF inputs.
Note 11: The dynamic load presented by the MAX1400 analog inputs for each gain setting is discussed in detail in the Switching
Network section. Values are provided for the maximum allowable external series resistance. Note that this value does not
include any additional capacitance added by the user to the MUXOUT_ or ADCIN_ pins.
Note 12: The input voltage range for the analog inputs is with respect to the voltage on the negative input of its respective differential or pseudo-differential pair. Table 5 shows which inputs form differential pairs.
Note 13: VREF = VREFIN+ - VREFIN-.
Note 14: These specifications apply to CLKOUT only when driving a single CMOS load.
Note 15: The burn-out currents require a 500mV overhead between the analog input voltage and both V+ and AGND to operate
correctly.
6
_______________________________________________________________________________________
+5V, 18-Bit, Low-Power, Multichannel,
Oversampling (Sigma-Delta) ADC
MAX1400
Note 16: Measured at DC in the selected passband. PSR at 50Hz will exceed 120dB with filter notches of 25Hz or 50Hz and FAST
bit = 0. PSR at 60Hz will exceed 120dB with filter notches of 20Hz or 60Hz and FAST bit = 0.
Note 17: PSR depends on gain. For a gain of +1V/V, PSR is 70dB typical. For a gain of +2V/V, PSR is 75dB typical. For a gain of
+4V/V, PSR is 80dB typical. For gains of +8V/V to +128V/V, PSR is 85dB typical.
Note 18: Standby power-dissipation and current specifications are valid only with CLKIN driven by an external clock and with the
external clock stopped. If the clock continues to run in standby mode, the power dissipation will be considerably higher.
When used with a resonator or crystal between CLKIN and CLKOUT, the actual power dissipation and IDD in standby
mode will depend on the resonator or crystal type.
TIMING CHARACTERISTICS
(V+ = +5V ±5%, VDD = +2.7V to +5.25V, AGND = DGND, fCLKIN = 2.4576MHz; input logic 0 = 0V; logic 1 = VDD, TA = TMIN to TMAX,
unless otherwise noted.) (Notes 19, 20, 21)
PARAMETER
Master Clock Frequency
SYMBOL
fCLKIN
CONDITIONS
Crystal oscillator or clock
externally supplied for
specified performance
(Notes 22, 23)
MIN
TYP
MAX
X2CLK = 0
0.4
2.5
X2CLK = 1
0.8
5.0
UNITS
MHz
Master Clock Input Low Time
fCLKIN LO
tCLKIN = 1 / fCLKIN, X2CLK = 0
0.4 ·
tCLKIN
ns
Master Clock Input High Time
fCLKIN HI
tCLKIN = 1 / fCLKIN, X2CLK = 0
0.4 ·
tCLKIN
ns
X2CLK = 0, N = 2(2 · MF1 + MF0)
INT High Time
tINT
X2CLK = 1, N = 2(2 · MF1 + MF0)
RESET Pulse Width Low
t2
280 / N
· tCLKIN
ns
560 / N
· tCLKIN
100
ns
SERIAL-INTERFACE READ OPERATION
INT to CS Setup Time (Note 8)
t3
0
ns
SCLK Setup to Falling Edge CS
t4
30
ns
CS Falling Edge to SCLK Falling
Edge Setup Time
t5
30
ns
SCLK Falling Edge to Data Valid
Delay (Notes 24, 25)
t6
SCLK High Pulse Width
t7
100
ns
SCLK Low Pulse Width
t8
100
ns
CS Rising Edge to SCLK Rising
Edge Hold Time (Note 21)
t9
0
ns
Bus Relinquish Time After SCLK
Rising Edge (Note 26)
t10
SCLK Rising Edge to INT High
(Note 27)
t11
VDD = 5V
0
80
VDD = 3.3V
0
100
VDD = 5V
10
70
VDD = 3.3V
10
100
ns
ns
VDD = 5V
100
ns
VDD = 3.3V
200
ns
_______________________________________________________________________________________
7
MAX1400
+5V, 18-Bit, Low-Power, Multichannel,
Oversampling (Sigma-Delta) ADC
TIMING CHARACTERISTICS (continued)
(V+ = +5V ±5%, VDD = +2.7V to +5.25V, AGND = DGND, fCLKIN = 2.4576MHz; input logic 0 = 0V; logic 1 = VDD, TA = TMIN to TMAX,
unless otherwise noted.) (Notes 19, 20, 21)
PARAMETER
SYMBOL
CONDITIONS
MIN
TYP
MAX
UNITS
SERIAL-INTERFACE WRITE OPERATION
SCLK Setup to Falling Edge CS
t12
30
ns
CS Falling Edge to SCLK Falling
Edge Setup Time
t13
30
ns
Data Valid to SCLK Rising Edge
Setup Time
t14
30
ns
Data Valid to SCLK Rising Edge
Hold Time
t15
0
ns
SCLK High Pulse Width
t16
100
ns
SCLK Low Pulse Width
t17
100
ns
CS Rising Edge to SCLK Rising
Edge Hold Time
t18
0
ns
Note 19: All input signals are specified with tr = tf = 5ns (10% to 90% of VDD).
Note 20: See Figure 4.
Note 21: Timings shown in tables are for the case where SCLK idles high between accesses. The part may also be used with the
SCLK idling low between accesses, provided CS is toggled. In this case SCLK in the timing diagrams should be inverted
and the terms “SCLK Falling Edge” and “SCLK Rising Edge” exchanged in the specification tables. If CS is permanently
tied low, the part should only be operated with SCLK idling high between accesses.
Note 22: CLKIN duty cycle range is 45% to 55%. CLKIN must be supplied whenever the MAX1400 is not in standby mode. If no
clock is present, the device can draw higher current than specified.
Note 23: The MAX1400 is production tested with fCLKIN at 2.5MHz (1MHz for some IDD tests).
Note 24: Measured with the load circuit of Figure 1 and defined as the time required for the output to cross the VOL or VOH limits.
Note 25: For read operations, SCLK active edge is falling edge of SCLK.
Note 26: Derived from the time taken by the data output to change 0.5V when loaded with the circuit of Figure 1. The number is then
extrapolated back to remove effects of charging or discharging the 50pF capacitor. This ensures that the times quoted in
the timing characteristics are true bus-relinquish times and are independent of external bus loading capacitances.
Note 27: INT returns high after the first read after an output update. The same data can be read again while INT is high, but be
careful not to allow subsequent reads to occur close to the next output update.
100µA
at VDD = +3.3V
800µA
at VDD = +5V
100µA
at VDD = +3.3V
200µA
at VDD = +5V
TO
OUTPUT
PIN
50pF
Figure 1. Load Circuit for Bus Relinquish Time and VOL and
VOH Levels
8
_______________________________________________________________________________________
+5V, 18-Bit, Low-Power, Multichannel,
Oversampling (Sigma-Delta) ADC
PIN
NAME
FUNCTION
1
CLKIN
Clock Input. A crystal can be connected across CLKIN and CLKOUT. Alternatively, drive CLKIN with a
CMOS-compatible clock at a nominal frequency of 2.4576MHz or 1.024MHz, and leave CLKOUT unconnected. Frequencies of 4.9152MHz and 2.048MHz may be used if the X2CLK control bit is set to 1.
2
CLKOUT
Clock Output. When deriving the master clock from a crystal, connect the crystal between CLKIN and
CLKOUT. In this mode, the on-chip clock signal is not available at CLKOUT. Leave CLKOUT unconnected
when CLKIN is driven with an external clock.
3
CS
Chip-Select Input. Active-low logic input used to enable the digital interface. With CS hard-wired low, the
MAX1400 operates in its 3-wire interface mode with SCLK, DIN and DOUT used to interface to the device.
CS is used either to select the device in systems with more than one device on the serial bus, or as a
frame-synchronization signal for the MAX1400 when a continuous SCLK is used.
4
RESET
Active Low Reset Input. Drive low to reset the control logic, interface logic, digital filter and analog modulator to power-on status. RESET must be high and CLKIN must be toggling in order to exit reset.
MUXOUT+
Positive Analog Mux Output. The positive differential output signal from the part’s internal input multiplexer. Use this signal in conjunction with MUXOUT- and a high-quality external amplifier for additional signal
processing before conversion. Return the processed output through ADCIN+ and ADCIN-. Connect
MUXOUT+ directly to ADCIN+ if external processing is not required.
6
MUXOUT-
Negative Analog Mux Output. The negative differential output signal from the part’s internal input multiplexer. Use this signal in conjunction with MUXOUT+ and a high-quality external amplifier for additional
signal processing before conversion. Return the processed output through ADCIN+ and ADCIN-.
Connect MUXOUT- directly to ADCIN- if external processing is not required.
7
ADCIN+
Positive Analog Input. A direct input to the positive buffer and the positive differential input terminal of the
ADC, bypassing the input mux. This signal forms a differential input pair with ADCIN-. Connect ADCIN+ to
MUXOUT+ when direct access is not required.
8
ADCIN-
Negative Analog Input. A direct input to the negative buffer and the negative differential input terminal of
the ADC - bypassing the input mux. This signal forms a differential input pair with ADCIN+. Connect
ADCIN- to MUXOUT- when direct access is not required.
9
AGND
Analog Ground. Reference point for the analog circuitry. AGND connects to the IC substrate.
10
V+
11
AIN1
Analog Input Channel 1. May be used as a pseudo-differential input with AIN6 as common, or as the positive input of the AIN1/AIN2 differential analog input pair (see Communications Register section).
12
AIN2
Analog Input Channel 2. May be used as a pseudo-differential input with AIN6 as common, or as the negative input of the AIN1/AIN2 differential analog input pair (see Communications Register section).
13
AIN3
Analog Input Channel 3. May be used as a pseudo-differential input with AIN6 as common, or as the positive input of the AIN3/AIN4 differential analog input pair (see Communications Register section).
14
AIN4
Analog Input Channel 4. May be used as a pseudo-differential input with AIN6 as common, or as the negative input of the AIN3/AIN4 differential analog input pair (see Communications Register section).
15
AIN5
Analog Input Channel 5. Used as a differential or pseudo-differential input with AIN6 (see
Communications Register section).
16
AIN6
Analog Input 6. May be used as a common point for AIN1 through AIN5 in pseudo-differential mode, or as
the negative input of the AIN5/AIN6 differential analog input pair (see Communications Register section).
5
Analog Positive Supply Voltage (+4.75V to +5.25V)
_______________________________________________________________________________________
9
MAX1400
Pin Description
+5V, 18-Bit, Low-Power, Multichannel,
Oversampling (Sigma-Delta) ADC
MAX1400
Pin Description (continued)
PIN
FUNCTION
CALGAIN-
Negative Gain Calibration Input. Used for system-gain calibration. It forms the negative input of a fully
differential input pair with CALGAIN+. Normally these inputs are connected to reference voltages in the
system. When system gain calibration is not required and the auto-sequence mode is used, the
CALGAIN+/CALGAIN- input pair provides an additional fully differential input channel.
18
CALGAIN+
Positive Gain Calibration Input. Used for system gain calibration. It forms the positive input of a fully differential input pair with CALGAIN-. Normally these inputs are connected to reference voltages in the system.
When system gain calibration is not required and the auto-sequence mode is used, the CALGAIN+/
CALGAIN- input pair provides an additional fully differential input channel.
19
REFIN-
Negative Differential Reference Input. Bias REFIN- between V+ and AGND, provided that REFIN+ is more
positive than REFIN-.
20
REFIN+
Positive Differential Reference Input. Bias REFIN+ between V+ and AGND, provided that REFIN+ is more
positive than REFIN-.
CALOFF-
Negative Offset Calibration Input. Used for system offset calibration. It forms the negative input of a fully
differential input pair with CALOFF+. Normally these inputs are connected to zero-reference voltages in
the system. When system offset calibration is not required and the auto-sequence mode is used, the
CALOFF+/CALOFF- input pair provides an additional fully differential input channel.
22
CALOFF+
Positive Offset Calibration Input. Used for system offset calibration. It forms the positive input of a fully
differential input pair with CALOFF-. Normally these inputs are connected to zero-reference voltages in the
system. When system offset calibration is not required and the auto-sequence mode is used, the
CALOFF+/CALOFF- input pair provides an additional fully differential input channel.
23
DGND
24
VDD
Digital Supply Voltage (+2.7V to +5.25V)
25
INT
Interrupt Output. A logic low indicates that a new output word is available from the data register. INT
returns high upon completion of a full output word read operation. INT also returns high for short periods
(determined by the filter and clock control bits) if no data read has taken place. A logic high indicates
internal activity, and a read operation should not be attempted under this condition. INT can also provide
a strobe to indicate valid data at DOUT (MDOUT = 1).
26
DOUT
Serial Data Output. DOUT outputs data from the internal shift register containing information from the
Communications Register, Global Setup Registers, Transfer Function Registers, or Data Register. DOUT
can also provide the digital bit stream directly from the Σ-∆ modulator (MDOUT = 1).
27
DIN
17
21
28
10
NAME
SCLK
Digital Ground. Reference point for digital circuitry.
Serial Data Input. Data on DIN is written to the input shift register and later transferred to the
Communications Register, Global Setup Registers, Special Function Register or Transfer Function
Registers, depending on the register selection bits in the Communications Register.
Serial Clock Input. Apply an external serial clock to transfer data to and from the MAX1400. This serial
clock can be continuous, with data transmitted in a train of pulses, or intermittent. If CS is used to frame
the data transfer, then SCLK may idle high or low between conversions and CS determines the desired
active clock edge (see Selecting Clock Polarity). If CS is tied permanently low, SCLK must idle high
between data transfers.
______________________________________________________________________________________
+5V, 18-Bit, Low-Power, Multichannel,
Oversampling (Sigma-Delta) ADC
Circuit Description
The MAX1400 is a low-power, multichannel, serialoutput, sigma-delta ADC designed for applications with
a wide dynamic range, such as weigh scales and pressure transducers. The functional block diagram in
Figure 2 contains a switching network, a modulator, a
PGA, two buffers, an oscillator, an on-chip digital filter,
and a bidirectional serial communications port.
Three fully-differential input channels feed into the
switching network. Each channel may be independently programmed with a gain between +1V/V and
+128V/V. These three differential channels may also be
configured to operate as five pseudo-differential input
channels. Two additional, fully differential system-calibration channels allow system gain and offset error to
be measured. These system-calibration channels can
be used as additional differential signal channels when
dedicated gain and offset error correction channels are
not required.
Two chopper-stabilized buffers are available to isolate
the selected inputs from the capacitive loading of the
PGA and modulator. Three independent DACs provide
compensation for the DC component of the input signal
on each of the differential input channels.
The sigma-delta modulator converts the input signal into
a digital pulse train whose average duty cycle represents
the digitized signal information. The pulse train is then
processed by a digital decimation filter, resulting in a
conversion accuracy exceeding 16 bits. The digital filter’s
decimation factor is user-selectable, which allows the
conversion result’s resolution to be reduced to achieve a
higher output data rate. When used with 2.4576MHz or
1.024MHz master clocks, the decimation filter can be
programmed to produce zeros in its frequency response
at the line frequency and associated harmonics. This
ensures excellent line rejection without the need for further post-filtering. In addition, the modulator sampling
frequency can be optimized for either lowest power dissipation or highest output data rate.
The MAX1400 can be configured to sequentially scan
all signal inputs and to transmit the results through the
serial interface with minimum communications overhead. The output word contains a result identification
tag to indicate the source of each conversion result.
ADCIN+
MUXOUT+
CALOFF+
CALGAIN+
AIN1
AIN2
AIN3
AIN4
AIN5
AIN6
CALOFFCALGAIN-
CLOCK
GEN
DIVIDER
MAX1400
CLKIN
CLKOUT
BUFFER
SWITCHING
NETWORK
V+
MODULATOR
PGA
DIGITAL
FILTER
VDD
DGND
V+
AGND
BUFFER
DAC
MUXOUTINTERFACE
AND CONTROL
ADCINAGND
REFIN+
REFIN-
SCLK
DIN
DOUT
INT
CS
RESET
Figure 2. Functional Diagram
______________________________________________________________________________________
11
MAX1400
_______________Detailed Description
MAX1400
+5V, 18-Bit, Low-Power, Multichannel,
Oversampling (Sigma-Delta) ADC
Serial Digital Interface
The serial digital interface provides access to eight onchip registers (Figure 3). All serial-interface commands
begin with a write to the communications register
(COMM). On power-up, system reset, or interface reset,
the part expects a write to its communications register.
The COMM register access begins with a 0 start bit.
The COMM register R/W bit selects a read or write
operation, and the register select bits (RS2, RS1, RS0)
select the register to be addressed. Hold DIN high
when not writing to COMM or another register (Table 1).
The serial interface consists of five signals: CS, SCLK,
DIN, DOUT, and INT. Clock pulses on SCLK shift bits
into DIN and out of DOUT. INT provides an indication
that data is available. CS is a device chip-select input
as well as a clock polarity select input (Figure 4).
Using CS allows the SCLK, DIN, and DOUT signals to
be shared among several SPI-compatible devices.
When short on I/O pins, connect CS low and operate
the serial digital interface in CPOL = 1, CPHA = 1 mode
using SCLK, DIN, and DOUT. This 3-wire interface
mode is ideal for opto-isolated applications.
Furthermore, a microcontroller (such as a PIC16C54 or
80C51) can use a single bidirectional I/O pin for both
sending to DIN and receiving from DOUT (see
Applications Information), because the MAX1400 drives
DOUT only during a read cycle.
Additionally, connecting the INT signal to a hardware
interrupt allows faster throughput and reliable, collisionfree data flow.
The MAX1400 features a mode where the raw modulator data output is accessible. In this mode the DOUT
and INT functions are reassigned (see the Modulator
Data Output section).
Table 1. Control Register Addressing
RS2
RS1
RS0
TARGET REGISTER
0
0
0
Communications Register
0
0
1
Global Setup Register 1
0
1
0
Global Setup Register 2
0
1
1
Special Function Register
1
0
0
Transfer Function Register 1
1
0
1
Transfer Function Register 2
1
1
0
Transfer Function Register 3
1
1
1
Data Register
COMMUNICATIONS REGISTER
DIN
RS2 RS1 RS0
GLOBAL SETUP REGISTER 1
GLOBAL SETUP REGISTER 2
REGISTER
SELECT
DECODER
SPECIAL FUNCTION REGISTER
XFER FUNCTION REGISTER 1
XFER FUNCTION REGISTER 2
XFER FUNCTION REGISTER 3
DOUT
DATA REGISTER D17–D10
DATA REGISTER D9–D2
DATA REGISTER D1–D0/CID
Figure 3. Register Summary
t11
INT
t1
t3
CS
t12
t4
SCLK
(CPOL = 1)
t16
t7
t13
t5
t9
t18
t8
t17
SCLK
(CPOL = 0)
t14
t15
DIN
(DURING
WRITE)*
DOUT
(DURING
READ)*
MSB D6
D5
D4
D3
D2
D1
D0
MSB D6
D5
D4
D3
D2
D1
D0
t6
t10
*DOUT IS HIGH IMPEDANCE DURING THE WRITE CYCLE; DIN IS IGNORED
DURING THE READ CYCLE.
Figure 4. Serial-Interface Timing
12
______________________________________________________________________________________
+5V, 18-Bit, Low-Power, Multichannel,
Oversampling (Sigma-Delta) ADC
Data-Ready Signal (DRDY bit true or INT = low)
The data-ready signal indicates that new data may be
read from the 24-bit data register. After the end of a successful data register read, the data-ready signal
becomes false. If a new measurement completes before
the data is read, the data-ready signal becomes false.
The data-ready signal becomes true again when new
data is available in the data register.
The MAX1400 provides two methods of monitoring the
data-ready signal. INT provides a hardware solution
(active low when data is ready to be accessed), while
the DRDY bit in the COMM register provides a software
solution (active high).
Read data as soon as possible once data-ready
becomes true. This becomes increasingly important for
faster measurement rates. If the data-read is delayed
significantly, a collision may result. A collision occurs
when a new measurement completes during a dataregister read operation. After a collision, information in
the data register is invalid. The failed read operation
must be completed even though the data is invalid.
Resetting the Interface
Reset the serial interface by clocking in 32 1s.
Resetting the interface does not affect the internal registers.
If continuous data output mode is in use, clock in eight
0s followed by 32 1s. More than 32 1s may be clocked
in, since a leading 0 is used as the start bit for all operations.
Continuous Data Output Mode
When scanning the input channels (SCAN = 1), the serial interface allows the data register to be read repeatedly without requiring a write to the COMM register.
The initial COMM write (01111000) is followed by 24
clocks (DIN = high) to read the 24-bit data register.
Once the data register has been read, it can be read
again after the next conversion by writing another 24
clocks (DIN = high). Terminate the continuous data output mode by writing to the COMM register with any
valid access.
Modulator Data Output (MDOUT = 1)
Single-bit, raw modulator data is available at DOUT for
custom filtering when MDOUT = 1. INT provides a modulator clock for data synchronization. Data is valid on
the falling edge of INT. Write operations can still be
performed, however, read operations are disabled.
After MDOUT is returned to 0, valid data is accessed
by the normal serial-interface read operation.
On-Chip Registers
Communications Register
0/DRDY: (Default = 0) Data Ready Bit. On a write, this
bit must be reset to 0 to signal the start of the Communications Register data word. On a read, a 1 in this
location (0/DRDY) signifies that valid data is available in
the data register. This bit is reset after the data register
is read or, if data is not read, 0/DRDY will go low at the
end of the next measurement.
RS2, RS1, RS0: (Default = 0, 0, 0) Register Select
Bits. These bits select the register to be accessed
(Table 1).
R/W: (Default = 0) Read/Write Bit. When set high, the
selected register is read; when R/W = 0, the selected
register is written.
RESET: (Default = 0) Software Reset Bit. Setting this
bit high causes the part to be reset to its default powerup condition (RESET = 0).
STDBY: (Default = 0) Standby Power-Down Bit. Setting
the STDBY bit places the part in “standby” condition,
shutting down everything except the serial interface
and the CLK oscillator.
Communications Register
First Bit (MSB)
FUNCTION
Name
Defaults
DATA RDY
(LSB)
REGISTER SELECT BITS
0/DRDY
RS2
RS1
RS0
R/W
RESET
STDBY
FSYNC
0
0
0
0
0
0
0
0
______________________________________________________________________________________
13
MAX1400
Selecting Clock Polarity
The serial interface can be operated with the clock
idling either high or low. This is compatible with
Motorola’s SPI interface operated in (CPOL = 1, CPHA
= 1) or (CPOL = 0, CPHA = 1) mode. Select the clock
polarity by sampling the state of SCLK at the falling
edge of CS. Ensure that the setup times t4/t12 and t5/t13
are not violated. If CS is connected to ground, resulting
in no falling edge on CS, SCLK must idle high (CPOL =
1, CPHA = 1).
MAX1400
+5V, 18-Bit, Low-Power, Multichannel,
Oversampling (Sigma-Delta) ADC
FSYNC: (Default = 0) Filter Sync Bit. When FSYNC = 0,
conversions are automatically performed at a data rate
determined by CLK, FS1, FS0, MF1, and MF0 bits.
When FSYNC = 1, the digital filter and analog modulator
are held in reset, inhibiting normal self-timed operation.
This bit may be used to convert on command to minimize the settling time to valid output data, or to synchronize operation of a number of MAX1400s. FSYNC does
not reset the serial interface or the 0/DRDY flag. To clear
the 0/DRDY flag while FSYNC is active, simply read the
data register.
Global Setup Register 1
A1, A0: (Default = 0, 0) Channel-Selection Control Bits.
These bits (combined with the state of the DIFF, M1,
and M0 bits) determine the channel selected for conversion according to Tables 8, 9, and 10. These bits
are ignored if the SCAN bit is set.
MF1, MF0: (Default = 0, 0) Modulator Frequency Bits.
MF1 and MF0 determine the ratio of CLKIN oscillator frequency to modulator operating frequency. They affect
the output data rate, the position of the digital filter notch
frequencies, and the power dissipation of the device.
Achieve lowest power dissipation with MF1 = 0 and MF0
= 0. Highest power dissipation and fastest output data
rate occur with these bits set to 1, 1 (Table 2).
CLK: (Default = 1) CLK Bit. The CLK bit is used in conjunction with X2CLK to tell the MAX1400 the frequency
of the CLKIN input signal. If CLK = 0, a CLKIN input frequency of 1.024MHz (2.048MHz for X2CLK = 1) is
expected. If CLK = 1, a CLKIN input frequency of
2.4576MHz (4.1952MHz for X2CLK = 1) is expected.
This bit affects the decimation factor in the digital filter
and thus the output data rate (Table 2).
FS1, FS0: (Default = 0, 1) Filter Selection Bits. These
bits (in conjunction with the CLK bit) control the decimation ratio of the digital filter. They determine the out-
put data rate, the position of the digital filter-frequency
response notches, and the noise present in the output
result. (Table 2).
FAST: (Default 0) FAST Bit. FAST = 0 causes the digital filter to perform a SINC3 filter function on the modulator data stream. The output data rate will be
determined by the values in the CLK, FS1, FS0, MF1,
and MF0 bits (Table 2). The settling time for SINC3
function is 3 [1 / (output data rate)]. In SINC3 mode, the
MAX1400 automatically holds the DRDY signal false
(after any significant configuration change) until settled
data is available. FAST = 1 causes the digital filter to
perform a SINC1 filter function on the modulator data
stream. The signal-to-noise ratio achieved with this filter
function is less than that of the SINC3 filter; however
SINC1 settles in a single output sample period, rather
than a minimum of three output sample periods for
SINC3. When switching from SINC1 to SINC3 mode, the
DRDY flag will be deasserted and reasserted after the
filter has fully settled. This mode change requires a
minimum of three samples.
Global Setup Register 2
SCAN: (Default = 0) Scan Bit. Setting this bit to a 1
causes sequential scanning of the input channels as
determined by DIFF, M1, and M0 (see Scanning
(SCAN-mode) section). When SCAN = 0, the MAX1400
repeatedly measures the unique channel selected by
A1, A0, DIFF, M1, and M0.
M1, M0: (Default 0, 0) Mode Control Bits. These bits
control access to the calibration channels CALOFF and
CALGAIN. When SCAN = 0, setting M1 = 0 and M0 = 1
selects the CALOFF input, and M1 = 1 and M0 = 0
selects the CALGAIN input (Table 3). When SCAN = 1
and M1 ≠ M0, the scanning sequence includes both
CALOFF and CALGAIN inputs (Table 4). When SCAN is
set to 1 and the device is scanning the available input
Global Setup Register 1
First Bit (MSB)
FUNCTION
(LSB)
MODULATOR
FREQUENCY
CHANNEL SELECTION
Name
FILTER SELECTION
A1
A0
MF1
MF0
CLK
FS1
FS0
FAST
0
0
0
0
1
0
1
0
Defaults
Global Setup Register 2
First Bit (MSB)
FUNCTION
Name
Defaults
14
(LSB)
MODE CONTROL
SCAN
M1
M0
BUFF
DIFF
BOUT
RESERVED
X2CLK
0
0
0
0
0
0
0
0
______________________________________________________________________________________
+5V, 18-Bit, Low-Power, Multichannel,
Oversampling (Sigma-Delta) ADC
AIN3/AIN4, and AIN5/AIN6. The available input channels for each mode are tabulated in Table 5. Note that
DIFF also affects the scanning sequence when the part
is placed in SCAN mode (Table 4).
BOUT: (Default = 0) Burnout Current Bit. Setting BOUT
= 1 connects 100nA current sources to the selected
analog input channel. This mode is used to check that
a transducer has not burned out or opened circuit. The
burnout current source must be turned off (BOUT = 0)
before measurement to ensure best linearity.
RESERVED: (Default = 0) Reserved Bit. A 0 must be
written to this location.
X2CLK: (Default = 0) Times-Two Clock Bit. Setting this
bit to 1 selects a divide-by-2 prescaler in the clock signal path. This allows use of a higher frequency crystal
or clock source and improves immunity to asymmetric
clock sources.
Table 2. Data Output Rate vs. CLK, Filter Select, and Modulator Frequency Bits
CLKIN
FREQ.
CLKIN
FREQ.
X2CLK = 0
fCLKIN
(MHz)
X2CLK = 1
fCLKIN
(MHz)
CLK
MF1
MF0
FS1, FS0
(0, 0)
(sps)*
FS1, FS0
(0, 1)
(sps)*
FS1, FS0
(1, 0)
(sps)
FS1, FS0
(1, 1)
(sps)
1.024
2.048
0
0
0
20
25
100
200
1.024
2.048
0
0
1
40
50
200
400
1.024
2.048
0
1
0
80
100
400
800
1.024
2.048
0
1
1
160
200
800
1600
2.4576
4.9152
1
0
0
50
60
300
600
2.4576
4.9152
1
0
1
100
120
600
1200
2.4576
4.9152
1
1
0
200
240
1200
2400
2.4576
4.9152
1
1
1
400
480
2400
4800
AVAILABLE OUTPUT DATA RATES
* Data rates offering noise-free 16-bit resolution.
Note: When FAST = 0, f-3dB = 0.262 · Data Rate. When FAST = 1, f-3dB = 0.443 · Data Rate.
Default condition is in bold print.
Table 3. Special Modes Controlled by M1, M0 (SCAN = 0)
M1
M0
DESCRIPTION
0
0
Normal Mode: The device operates normally.
0
1
Calibrate Offset: In this mode the MAX1400 converts the voltage applied across CALOFF+
and CALOFF-. The PGA gain, DAC, and format settings of the selected channel (defined by
DIFF, A1, A0) are used.
1
0
Calibrate Gain: In this mode the MAX1400 converts the voltage applied across CALGAIN+
and CALGAIN-. The PGA gain, DAC, and format settings of the selected channel (defined by
DIFF, A1, A0) are used.
1
1
Reserved: Do not use.
______________________________________________________________________________________
15
MAX1400
channels, selection of either calibration mode (01 or 10)
will cause the scanning sequence to be extended to
include a conversion on both the CALGAIN+/
CALGAIN- input pair and the CALOFF+/CALOFF- input
pair. The exact sequence depends on the state of the
DIFF bit (Table 4). When scanning, the calibration
channels use the PGA gain, format, and DAC settings
defined by the contents of Transfer Function Register 3.
BUFF: (Default = 0) The BUFF bit controls operation of
the input buffer amplifiers. When this bit is 0, the internal buffers are bypassed and powered down. When
this bit is set high, the buffers drive the input sampling
capacitors and minimize the dynamic input load.
DIFF: (Default = 0) Differential/Pseudo-Differential Bit.
When DIFF = 0, the part is in pseudo-differential mode,
and AIN1–AIN5 are measured respective to AIN6, the
analog common. When DIFF = 1, the part is in differential mode with the analog inputs defined as AIN1/AIN2,
MAX1400
+5V, 18-Bit, Low-Power, Multichannel,
Oversampling (Sigma-Delta) ADC
Table 4. SCAN Mode Scanning
Sequences (SCAN = 1)
DIFF
M1
M0
SEQUENCE
0
0
0
AIN1–AIN6, AIN2–AIN6, AIN3–AIN6,
AIN4–AIN6, AIN5–AIN6
0
0
1
AIN1–AIN6, AIN2–AIN6, AIN3–AIN6,
AIN4–AIN6, AIN5–AIN6, CALOFF,
CALGAIN
0
1
0
AIN1–AIN6, AIN2–AIN6,
AIN3–AIN6, AIN4–AIN6,
AIN5–AIN6, CALOFF, CALGAIN
1
0
0
AIN1–AIN2, AIN3–AIN4, AIN5–AIN6
1
0
1
AIN1–AIN2, AIN3–AIN4,
AIN5–AIN6, CALOFF, CALGAIN
1
1
0
AIN1–AIN2, AIN3–AIN4,
AIN5–AIN6, CALOFF, CALGAIN
Note: All other combinations reserved.
Table 5. Available Input Channels
(SCAN = 0)
DIFF
M1
M0
AVAILABLE CHANNELS
0
0
0
AIN1–AIN6, AIN2–AIN6,
AIN3–AIN6, AIN4–AIN6
0
0
1
CALOFF
0
1
0
CALGAIN
1
0
0
AIN1–AIN2, AIN3–AIN4, AIN5–AIN6
1
0
1
CALOFF
1
1
0
CALGAIN
Special Function Register (Write-Only)
MDOUT: (Default = 0) Modulator Out Bit. MDOUT = 0
enables data readout on the DOUT pin, the normal condition for the serial interface. MDOUT = 1 changes the
function of the DOUT and INT pins, providing raw, single-bit modulator output instead of the normal serialdata interface output. This allows custom filtering
directly on the modulator output, without going through
the on-chip digital filter. The INT pin provides a clock to
indicate when the modulator data at DOUT should be
sampled (falling edge of INT). Note that in this mode,
the on-chip digital filter continues to operate normally.
When MDOUT is returned to 0, valid data may be
accessed through the normal serial-interface read
operation.
FULLPD: (Default = 0) Complete Power-Down Bit.
FULLPD = 1 forces the part into a complete powerdown condition, which includes the clock oscillator. The
serial interface continues to operate. The part requires a
hardware reset to recover correctly from this condition.
Note: Changing the reserved bits in the special-function register from the default status of all 0s will select
one of the reserved modes and the part will not operate
as expected. This register is a write-only register.
However, in the event that this register is mistakenly
read, clock 24 bits of data out of the part to restore it to
the normal interface-idle state.
Transfer-Function Registers
The three transfer-function registers control the method
used to map the input voltage to the output codes. All
of the registers have the same format. The mapping of
control registers to associated channels depends on
the mode of operation and is affected by the state of
M1, M0, DIFF, and SCAN (Tables 8, 9, and 10).
Special Function Register (Write-Only)
First Bit (MSB)
FUNCTION
(LSB)
RESERVED BITS
RESERVED BITS
Name
0
0
MDOUT
0
0
0
0
FULLPD
Defaults
0
0
0
0
0
0
0
0
Transfer-Function Register
First Bit (MSB)
FUNCTION
Name
Defaults
16
(LSB)
PGA GAIN CONTROL
OFFSET CORRECTION
G2
G1
G0
U/B
D3
D2
D1
D0
0
0
0
0
0
0
0
0
______________________________________________________________________________________
+5V, 18-Bit, Low-Power, Multichannel,
Oversampling (Sigma-Delta) ADC
mapped to the correct output range. Note U/B must be
set before a conversion is performed; it will not affect
any data already held in the output register.
Selecting bipolar mode does not imply that any input
may be taken below AGND. It simply changes the gain
and offset of the part. All inputs must remain within their
specified operating voltage range.
CALGAIN and CALOFF
When not in scan mode (SCAN = 0), A1 and A0 select
which transfer function applies to CALGAIN and
CALOFF. In scan mode (SCAN = 1), CALGAIN and
CALOFF are always mapped to transfer-function register 3. Note that when scanning while M1 ≠ M0, the scan
sequence includes both CALGAIN and CALOFF channels (Table 4). CALOFF always precedes CALGAIN,
even though both channels share the same channel ID
tag (Table 11).
Note that changing the status of any active channel
control bits will cause INT to immediately transition high
and the modulator/filter to be reset. INT will reassert
after the appropriate digital-filter settling time. The control settings of the inactive channels may be changed
freely without affecting the status of INT or causing the
filter/modulator to be reset.
Offset-Correction DACs
Bits D3–D0 control the offset-correction DAC. The DAC
range depends on the PGA gain setting and is
expressed as a percentage of the available full-scale
input range (Table 7).
D3 is a sign bit, and D2–D0 represent the DAC magnitude. Note that when a DAC value of 0000 is programmed (the default), the DAC is disconnected from
the modulator inputs. This prevents the DAC from
degrading noise performance when offset correction is
not required.
PGA Gain
Bits G2–G0 control the PGA gain according to Table 6.
G0
BIPOLAR
DAC VALUE
(% of FSR)
UNIPOLAR
DAC VALUE
(% of FSR)
D2
D1
D0
0
0
0
0
0
0
0
1
+8.3
+16.7
0
0
1
0
+16.7
+33.3
0
0
1
1
+25
+50
0
1
0
0
+33.3
+66.7
0
1
0
1
+41.6
+83.3
0
1
1
0
+50
+100
PGA GAIN
0
1
1
1
+58.3
+116.7
0
0
0
Table 6. PGA Gain Codes
G1
Table 7. DAC Code vs. DAC Value
D3
Unipolar/Bipolar Mode
The U/B bit places the channel in either bipolar or
unipolar mode. A 0 selects bipolar mode, and a 1
selects unipolar mode. This bit does not affect the analog-signal conditioning. The modulator always accepts
bipolar inputs and produces a bitstream with 50%
ones-density when the selected inputs are at the same
potential. This bit controls the processing of the digitalfilter output, such that the available output bits are
G2
Transfer-Function Register Mapping
Tables 8, 9, and 10 show the channel-control register
mapping in the various operating modes.
DAC not connected
0
0
0
x1
1
0
0
1
x2
1
0
0
1
-8.3
-16.7
0
1
0
x4
1
0
1
0
-16.7
-33.3
x8
1
0
1
1
-25
-50
1
0
0
-33.3
-66.7
0
1
1
DAC not connected
1
0
0
x16
1
1
0
1
x32
1
1
0
1
-41.6
-83.3
1
1
0
-50
-100
1
1
1
-58.3
-116.7
1
1
0
x64
1
1
1
1
x128
1
______________________________________________________________________________________
17
MAX1400
Analog Inputs AIN1 to AIN6
Inputs AIN1 and AIN2 map to transfer-function register
1, regardless of scanning mode (SCAN = 1) or singleended vs. differential (DIFF) modes. Likewise, AIN3 and
AIN4 inputs always map to transfer-function register 2.
Finally, AIN5 always maps to transfer-function register 3
(input AIN6 is analog common).
MAX1400
+5V, 18-Bit, Low-Power, Multichannel,
Oversampling (Sigma-Delta) ADC
Table 8. Transfer-Function Register Mapping—Normal Mode (M1 = 0, M0 = 0)
SCAN
DIFF
A1
A0
CHANNEL
TRANSFER
FUNCTION REG.
0
0
0
0
0
0
0
0
1
1
1
1
1
1
1
1
1
0
0
0
0
1
1
1
1
0
0
0
0
0
1
1
1
1
0
0
1
1
0
0
1
1
X
X
X
X
X
X
X
X
1
0
1
0
1
0
1
0
1
X
X
X
X
X
X
X
X
1
AIN1–AIN6
AIN2–AIN6
AIN3–AIN6
AIN4–AIN6
AIN1–AIN2
AIN3–AIN4
AIN5–AIN6
1
1
2
2
1
2
3
Do Not Use
AIN1–AIN6
AIN2–AIN6
AIN3–AIN6
AIN4–AIN6
AIN5–AIN6
AIN1–AIN2
AIN3–AIN4
AIN5–AIN6
1
1
2
2
3
1
2
3
Do Not Use
X = Don’t care
Table 9. Transfer-Function Register Mapping—Offset-Cal Mode (M1 = 0, M0 = 1)
SCAN
DIFF
A1
A0
0
0
0
0
0
0
0
0
1
1
0
0
0
0
1
1
1
1
0
0
0
0
1
1
0
0
1
1
X
X
0
1
0
1
0
1
0
1
X
X
1
0
X
X
1
1
1
1
1
1
1
1
1
1
0
0
0
0
1
1
1
1
1
1
X
X
X
X
X
X
X
X
X
1
X
X
X
X
X
X
X
X
X
1
CHANNEL
TRANSFER
FUNCTION REG.
CALOFF+–CALOFFCALOFF+–CALOFFCALOFF+–CALOFFCALOFF+–CALOFFCALOFF+–CALOFFCALOFF+–CALOFFCALOFF+–CALOFFDo not use
AIN1–AIN6
AIN2–AIN6
AIN3–AIN6
AIN4–AIN6
AIN5–AIN6
CALOFF+–CALOFFCALGAIN+–CALGAINAIN1–AIN2
AIN3–AIN4
AIN5–AIN6
CALOFF+–CALOFFCALGAIN+–CALGAINDo not use
X = Don’t care
18
______________________________________________________________________________________
1
1
2
2
1
2
3
1
1
2
2
3
3
3
1
2
3
3
3
+5V, 18-Bit, Low-Power, Multichannel,
Oversampling (Sigma-Delta) ADC
MAX1400
Table 10. Transfer-Function Register Mapping—Gain-Cal Mode (M1 = 1, M0 = 0)
TRANSFER
FUNCTION REG.
SCAN
DIFF
A1
A0
CHANNEL
0
0
0
0
0
0
0
0
0
0
0
0
1
1
1
1
0
0
1
1
0
0
1
1
0
1
0
1
0
1
0
1
1
0
X
X
AIN1–AIN6
1
1
0
X
X
AIN2–AIN6
1
1
0
X
X
AIN3–AIN6
2
1
0
X
X
AIN4–AIN6
2
1
0
X
X
AIN5–AIN6
3
1
1
1
1
1
1
1
1
0
0
1
1
1
1
1
1
X
X
X
X
X
X
X
1
X
X
X
X
X
X
X
1
CALGAIN+–CALGAINCALGAIN+–CALGAINCALGAIN+–CALGAINCALGAIN+–CALGAINCALGAIN+–CALGAINCALGAIN+–CALGAINCALGAIN+–CALGAINDo not use
CALOFF+–CALOFFCALGAIN+–CALGAINAIN1–AIN2
AIN3–AIN4
AIN5–AIN6
CALOFF+–CALOFFCALGAIN+–CALGAINDo not use
1
1
2
2
1
2
3
3
3
1
2
3
3
3
X = Don’t care
Data Register (Read-Only)
The data register is a 24-bit, read-only register. Any
attempt to write data to this location will have no effect.
If a write operation is attempted, 8 bits of data must be
clocked into the part before it will return to its normal
idle mode, expecting a write to the communications
register.
Data is output MSB first, followed by three reserved 0
bits and a 3-bit channel ID tag indicating the channel
from which the data originated.
D17–D0: The conversion result. D17 is the MSB. The
result is in offset binary format. 00 0000 0000 0000
0000 represents the minimum value and 11 1111 1111
1111 1111 represents the maximum value. Inputs
exceeding the available input range are limited to the
corresponding minimum or maximum output values.
0: These reserved bits will always be 0.
CID2–0: Channel ID tag (Table 11).
Data Register (Read-Only) Bits
First Bit (Data MSB)
DATA BITS
D17
D16
D15
D14
D9
D8
D7
D6
D13
D12
D11
D10
D5
D4
D3
D2
DATA BITS
(Data LSB)
(LSB)
RESERVED BITS
DATA BITS
D1
D0
‘0’
‘0’
CHANNEL ID TAG
‘0’
CID2
CID1
CID0
______________________________________________________________________________________
19
MAX1400
+5V, 18-Bit, Low-Power, Multichannel,
Oversampling (Sigma-Delta) ADC
Table 11. Channel ID Tag Codes
CID2
CID1
CID0
CHANNEL
0
0
0
AIN1–AIN6
0
0
1
AIN2–AIN6
0
1
0
AIN3–AIN6
0
1
1
AIN4–AIN6
1
0
0
AIN1–AIN2
1
0
1
AIN3–AIN4
1
1
0
AIN5–AIN6
1
1
1
Calibration
Switching Network
A switching network provides selection between three
fully differential input channels or five pseudo-differential channels, using AIN6 as a shared common. The
switching network provides two additional fully differential input channels intended for system calibration,
which may be used as extra fully differential signal
channels. Table 12 shows the channel configurations
available for both operating modes.
Scanning (SCAN-mode)
To sample and convert the available input channels
sequentially, set the SCAN control bit in the global
setup register. The sequence is determined by DIFF
(fully differential or pseudo-differential) and by the
mode control bits M1 and M0 (Tables 8, 9, 10). With
SCAN set, the part automatically sequences through
each available channel, transmitting a single conversion result before proceeding to the next channel. The
MAX1400 automatically allows sufficient time for each
conversion to fully settle, to ensure optimum resolution
before asserting the data-ready signal and moving to
the next available channel. The scan rate is, therefore,
dependent on the clock bit (CLK), the filter control bits
(FS1, FS0), and the modulator frequency selection bits
(MF1, MF0).
Burnout Currents
The input circuitry also provides two “burnout” currents.
These small currents may be used to test the integrity
of the selected transducer. They can be selectively
enabled or disabled by the BOUT bit in the global
setup register.
Table 12. Input Channel Configuration in Fully and Pseudo-Differential Modes
(SCAN = 0)
M1
M0
DIFF
A1
A0
HIGH INPUT
LOW INPUT
0
0
0
0
0
MODE
AIN1
AIN6
0
0
0
0
1
AIN2
AIN6
0
0
0
1
0
AIN3
AIN6
0
0
0
1
1
0
0
X
X
X
0
1
X
X
X
CALOFF+**
CALOFF-**
1
0
X
X
X
CALGAIN+**
CALGAIN-**
0
0
1
0
0
AIN1
AIN2
0
0
1
0
1
AIN3
AIN4
0
0
1
1
0
0
1
X
X
1
0
X
X
PseudoDifferential
Fully
Differential
AIN4
AIN6
AIN5*
AIN6*
AIN5
AIN6
X
CALOFF+**
CALOFF-**
X
CALGAIN+**
CALGAIN-**
X = Don’t care
* This combination is available only in pseudo-differential mode when using the internal scanning logic
** These combinations are only available in the calibration modes.
20
______________________________________________________________________________________
+5V, 18-Bit, Low-Power, Multichannel,
Oversampling (Sigma-Delta) ADC
REXT
RMUX
CEXT
Dynamic Input Impedance at the
Channel Selection Network
When used in unbuffered mode (BUFF = 0), the analog
inputs present a dynamic load to the driving circuitry.
The size of the sampling capacitor and the input sampling frequency (Figure 5) determine the dynamic load
seen by the driving circuitry. The MAX1400 samples at a
constant rate for all gain settings. This provides a maximum time for the input to settle at a given data rate. The
dynamic load presented by the inputs varies with the
gain setting. For gains of +2V/V, +4V/V, and +8V/V, the
input sampling capacitor increases with the chosen
gain. Gains of +16V/V, +32V/V, +64V/V, and +128V/V
present the same input load as the x8 gain setting.
When designing with the MAX1400, as with any other
switched-capacitor ADC input, consider the advantages and disadvantages of series input resistance. A
series resistor reduces the transient-current impulse to
the external driving amplifier. This improves the amplifier phase margin and reduces the possibility of ringing.
The resistor spreads the transient-load current from the
MUXOUT
CPIN
ADCIN RSW
CST
CPIN
CSAMPLE
CC
Figure 5. Analog Input, Unbuffered Mode (BUFF = 0)
sampler over time due to the RC time constant of the
circuit. However, an improperly chosen series resistance can hinder performance in fast 16-bit converters.
The settling time of the RC network can limit the speed
at which the converter can operate properly, or reduce
the settling accuracy of the sampler. In practice, this
means ensuring that the RC time constant—resulting
from the product of the driving source impedance and
the capacitance presented by both the MAX1400’s
input and any external capacitances—is sufficiently
small to allow settling to the desired accuracy. Tables
13a–13d summarize the maximum allowable series
resistance vs. external capacitance for each MAX1400
gain setting in order to ensure 16-bit performance in
unbuffered mode.
Table 13a. REXT, CEXT Values for Less than 16-Bit Gain Error in Unbuffered (BUFF = 0)
Mode—1x Modulator Sampling Frequency (MF1, MF0 = 00); X2CLK = 0; fCLKIN = 2.4576MHz
PGA GAIN
EXTERNAL RESISTANCE REXT (kΩ)
CEXT = 0pF
CEXT = 50pF
CEXT = 100pF
CEXT = 500pF
CEXT = 1000pF
CEXT = 5000pF
1
38
18
12.5
3.8
2.1
0.57
2
38
18
12.5
3.8
2.1
0.57
4
29
16
11.1
3.5
2.0
0.53
8, 16, 32,
64, 128
20
12.7
9.3
3.2
1.8
0.48
Table 13b. REXT, CEXT Values for Less than 16-Bit Gain Error in Unbuffered (BUFF = 0)
Mode—2x Modulator Sampling Frequency (MF1, MF0 = 00); X2CLK = 0; fCLKIN = 2.4576MHz
PGA GAIN
EXTERNAL RESISTANCE REXT (kΩ)
CEXT = 0pF
CEXT = 50pF
CEXT = 100pF
CEXT = 500pF
CEXT = 1000pF
CEXT = 5000pF
1
19
9.2
6.2
1.9
1.0
0.28
2
19
9.2
6.2
1.9
1.0
0.28
4
14
8.0
5.5
1.7
1.0
0.26
8, 16, 32,
64, 128
10
6.3
4.6
1.6
0.92
0.24
______________________________________________________________________________________
21
MAX1400
External Access to Mux Outputs
The MAX1400 provides access to the switching-network output and the modulator input with the MUXOUT
and ADCIN pins. This allows the user to share a single
high-performance amplifier for additional signal conditioning of all input channels.
MAX1400
+5V, 18-Bit, Low-Power, Multichannel,
Oversampling (Sigma-Delta) ADC
Table 13c. REXT, CEXT Values for Less than 16-Bit Gain Error in Unbuffered (BUFF = 0)
Mode—4x Modulator Sampling Frequency (MF1, MF0 = 10 ); X2CLK = 0; fCLKIN =
2.4576MHz
EXTERNAL RESISTANCE REXT (kΩ)
PGA GAIN
CEXT = 0pF
CEXT = 50pF
CEXT = 100pF
CEXT = 500pF
CEXT = 1000pF
CEXT = 5000pF
1
9.3
4.5
3.0
0.94
0.53
0.14
2
9.3
4.5
3.0
0.94
0.53
0.14
4
7.2
3.9
2.7
0.87
0.50
0.13
8, 16, 32,
64, 128
5.0
3.1
2.3
0.79
0.45
0.12
Table 13d. REXT, CEXT Values for Less than 16-Bit Gain Error in Unbuffered (BUFF = 0)
Mode—8x Modulator Sampling Frequency (MF1, MF0 = 11); X2CLK = 0; fCLKIN =
2.4576MHz
EXTERNAL RESISTANCE REXT (kΩ)
PGA GAIN
CEXT = 0pF
CEXT = 50pF
CEXT = 100pF
CEXT = 500pF
CEXT = 1000pF
CEXT = 5000pF
1
4.5
2.2
1.5
0.46
0.26
0.069
2
4.5
2.2
1.5
0.46
0.26
0.069
4
3.5
1.9
1.3
0.42
0.24
0.069
8, 16, 32,
64, 128
2.4
1.5
1.1
0.38
0.22
0.059
Input Buffers
The MAX1400 provides a pair of input buffers to isolate
the inputs from the capacitive load presented by the
PGA/modulator (Figure 6). The buffers are chopper stabilized to reduce the effect of their DC offsets and lowfrequency noise. Since the buffers can represent more
than 50% of the total analog power dissipation, they
may be shut down in applications where minimum
power dissipation is required and the capacitive input
REXT
CEXT
RMUX MUXOUT
CPIN
CST
ADCIN
CPIN
load is not a concern. Disable the buffers in applications
where the inputs must operate close to AGND or V+.
When used in buffered mode, the buffers isolate the
inputs from the sampling capacitors. The samplingrelated gain error is dramatically reduced in this mode.
A small dynamic load remains from the chopper stabilization. The multiplexer exhibits a small input leakage
current of up to 10nA. With high source resistances,
this leakage current may result in a DC offset.
RIN
CAMP
CSAMPLE
Figure 6. Analog Input, Buffered Mode (BUFF = 1)
22
______________________________________________________________________________________
CC
+5V, 18-Bit, Low-Power, Multichannel,
Oversampling (Sigma-Delta) ADC
PGA GAIN
EXTERNAL RESISTANCE REXT (kΩ)
CEXT = 0pF
CEXT = 50pF
CEXT = 100pF
CEXT = 500pF
CEXT = 1000pF
CEXT = 5000pF
1
10
10
10
10
10
10
2
10
10
10
10
10
10
4
10
10
10
10
10
10
8
10
10
10
10
10
10
16
10
10
10
10
10
10
32
10
10
10
10
10
10
64
10
10
10
10
10
10
128
10
10
10
10
10
10
Reference Input
PGA
The MAX1400 is optimized for ratiometric measurements and includes a fully differential reference input.
Apply the reference voltage across REFIN+ and REFIN-,
ensuring that REFIN+ is more positive than REFIN-.
REFIN+ and REFIN- must be between AGND and V+.
The MAX1400 is specified with a +2.5V reference when
operating with a +5V analog supply (V+).
A programmable gain amplifier (PGA) with a userselectable gain of x1, x2, x4, x8, x16, x32, x64, or x128
(Table 6) precedes the modulator. Figure 8 shows the
default bipolar transfer function with the following illustrated codes: 1) PGA = 0, DAC = 0; 2) PGA = 3, DAC =
0; or 3) PGA = 3, DAC = 3.
Modulator
Tables 16a and 16b show the rms noise for typical output frequencies (notches) and -3dB frequencies for the
MAX1400 with f CLKIN = 2.4576MHz. The numbers
given are for the bipolar input ranges with V REF =
+2.50V, with no buffer (BUFF = 0) and with the buffer
inserted (BUFF = 1). These numbers are typical and
are generated at a differential analog input voltage of 0.
Figure 7 shows graphs of Effective Resolution vs. Gain
and Notch Frequency. The effective resolution values
were derived from the following equation:
The MAX1400 performs analog-to-digital conversion
using a single-bit, second-order, switched-capacitor
modulator. A single comparator within the modulator
quantizes the input signal at a much higher sample rate
than the bandwidth of the signal to be converted. The
quantizer then presents a stream of 1s and 0s to the
digital filter for processing, to remove the frequencyshaped quantization noise.
The MAX1400 modulator provides 2nd-order frequency
shaping of the quantization noise resulting from the single bit quantizer. The modulator is fully differential for
maximum signal-to-noise ratio and minimum susceptibility to power-supply noise.
The modulator operates at one of a total of eight different sampling rates (fM) determined by the master clock
frequency (fCLKIN), the X2CLK bit, the CLK bit, and the
modulator frequency control bits MF1 and MF0. Power
dissipation is optimized for each of these modes by
controlling the bias level of the modulator. Table 15
shows the input and reference sample rates.
Output Noise
Effective Resolution = (SNRdB - 1.76dB) / 6.02
The maximum possible signal divided by the noise of
the device, SNRdB, is defined as the ratio of the input
full-scale voltage (i.e., 2 · VREFIN / GAIN) to the output
rms noise. Note that it is not calculated using peak-topeak output noise numbers. Peak-to-peak noise numbers can be up to 6.6 times the rms numbers, while
effective resolution numbers based on peak-to-peak
noise can be 2.5 bits below the effective resolution
based on rms noise, as quoted in the tables.
______________________________________________________________________________________
23
MAX1400
Table 14. REXT, CEXT Values for Less than 16-Bit Gain Error in Buffered (BUFF = 1)
Mode—All Modulator Sampling Frequencies (MF1, MF0 = XX); X2CLK = 0; fCLKIN =
2.4576MHz
MAX1400
+5V, 18-Bit, Low-Power, Multichannel,
Oversampling (Sigma-Delta) ADC
Table 15. Modulator Operating Frequency, Sampling Frequency, and 16-Bit Data
Output Rates
MCLK
FREQ.
X2CLK = 0
DEFAULT
fCLKIN
(MHz)
MCLK
FREQ.
X2CLK = 1
fCLKIN
(MHz)
CLK
MF1
MF0
AIN/REFIN
SAMPLING
FREQ.
fS
(kHz)
MOD.
FREQ.
fM
(kHz)
AVAILABLE
OUTPUT
DATA RATES
AT 16-BIT
ACCURACY
(sps)
1.024
2.048
0
0
0
1.024
2.048
0
0
1
16
8
20, 25
32
16
40, 50
1.024
2.048
0
1
1.024
2.048
0
1
0
64
32
80, 100
1
128
64
160, 200
2.4576
4.9152
1
0
0
38.4
19.2
50, 60
2.4576
4.9152
2.4576
4.9152
1
0
1
76.8
38.4
100, 120
1
1
0
153.6
76.8
2.4576
4.9152
200, 240
1
1
1
307.2
153.6
400, 480
Default condition is in bold print.
Table 16a. MAX1400 Noise vs. Gain and Output Data Rate—Unbuffered Mode,
VREF = 2.5V, fCLKIN = 2.4576MHz
OUTPUT
DATA
RATE
(sps)
-3dB
FREQ.
(Hz)
50
60
TYPICAL OUTPUT NOISE IN µVRMS
BIT
STATUS
PROGRAMMABLE GAIN
13.1
x1
6.20
x2
3.27
x4
2.02
x8
1.25
x16
1.13
x32
1.10
x64
1.05
x128
0.99
MF1:MF0 = 0
FS1:FS0 = 0
15.7
7.23
3.94
2.21
1.36
1.20
1.12
1.10
1.10
FS1:FS0 = 1
300
78.6
147.60
70.73
35.10
17.91
9.57
5.05
3.54
2.75
FS1:FS0 = 2
600
157.2
844.82
417.07
216.88
107.06
50.91
26.25
13.26
7.43
FS1:FS0 = 3
MF1:MF0 = 1
100
26.2
6.98
3.61
1.94
1.24
1.10
1.04
0.98
1.00
FS1:FS0 = 0
120
31.4
7.91
3.93
2.14
1.36
1.17
1.08
1.08
1.10
FS1:FS0 = 1
600
157.2
138.79
73.86
37.69
19.19
9.34
5.15
3.29
2.76
FS1:FS0 = 2
1200
314.4
836.32
405.49
203.35
99.75
52.38
24.50
13.40
7.31
FS1:FS0 = 3
MF1:MF0 = 2
200
52.4
6.25
4.00
1.96
1.26
1.07
1.02
0.98
1.01
FS1:FS0 = 0
240
62.9
7.00
4.16
2.04
1.34
1.15
1.14
1.09
1.09
FS1:FS0 = 1
1200
314.4
141.69
71.25
35.91
18.11
9.45
5.20
3.38
2.79
FS1:FS0 = 2
2400
628.8
816.66
399.44
200.51
103.04
51.17
26.57
13.88
7.38
FS1:FS0 = 3
MF1:MF0 = 3
24
400
104.8
6.87
3.50
2.06
1.31
1.16
1.09
1.13
1.08
FS1:FS0 = 0
480
125.7
8.15
3.84
2.36
1.44
1.28
1.23
1.22
1.21
FS1:FS0 = 1
2400
628.8
150.09
69.17
36.54
18.92
9.67
5.14
3.61
3.12
FS1:FS0 = 2
4800
1258
820.73
419.17
203.74
103.78
51.42
26.47
13.96
7.68
FS1:FS0 = 3
______________________________________________________________________________________
+5V, 18-Bit, Low-Power, Multichannel,
Oversampling (Sigma-Delta) ADC
OUTPUT
DATA
RATE
(Hz)
-3dB
FREQ.
(Hz)
50
13.1
TYPICAL OUTPUT NOISE IN µVRMS
BIT
STATUS
PROGRAMMABLE GAIN
x1
6.05
x2
4.13
x4
2.35
x8
1.50
x16
1.40
x32
1.32
x64
1.37
x128
1.39
MF1:MF0 = 0
FS1:FS0 = 0
60
15.7
7.11
4.24
2.54
1.64
1.49
1.53
1.49
1.48
FS1:FS0 = 1
300
78.6
142.02
71.62
35.65
18.32
9.35
5.60
4.10
3.52
FS1:FS0 = 2
600
157.2
823.33
405.95
195.95
102.14
50.28
25.85
13.75
7.78
FS1:FS0 = 3
MF1:MF0 = 1
100
26.2
8.10
3.66
2.25
1.52
1.34
1.31
1.34
1.35
FS1:FS0 = 0
120
31.4
8.37
4.12
2.53
1.64
1.45
1.49
1.45
1.46
FS1:FS0 = 1
600
157.2
143.45
69.52
36.04
17.77
9.32
5.48
3.92
3.41
FS1:FS0 = 2
1200
314.4
830.30
408.48
201.87
101.39
52.39
26.77
13.50
7.87
FS1:FS0 = 3
MF1:MF0 = 2
200
52.4
6.55
3.21
1.92
1.35
1.24
1.16
1.16
1.10
FS1:FS0 = 0
240
62.9
7.40
3.89
2.24
1.47
1.35
1.29
1.22
1.25
FS1:FS0 = 1
1200
314.4
148.57
73.71
36.80
18.08
9.92
5.26
3.64
3.02
FS1:FS0 = 2
2400
628.8
851.32
408.09
202.57
105.18
52.98
25.71
13.33
7.97
FS1:FS0 = 3
400
104.8
6.60
3.83
2.21
1.38
1.28
1.21
1.17
1.21
FS1:FS0 = 0
480
125.7
7.58
4.14
2.28
1.58
1.40
1.34
1.30
1.31
FS1:FS0 = 1
2400
628.8
144.96
68.92
35.92
17.36
9.52
5.45
3.79
3.21
FS1:FS0 = 2
4800
1258
803.87
394.00
205.60
102.18
52.48
26.07
13.59
7.89
FS1:FS0 = 3
MF1:MF0 = 3
20
20
19
18
FS1: FS0 = 0 or 1
17
16
FS1: FS0 = 2
15
14
FS1: FS0 = 3
13
12
CLK = 1
BUFF = 0
11
10
EFFECTIVE RESOLUTION (BITS)
EFFECTIVE RESOLUTION (BITS)
19
18
FS1: FS0 = 0 or 1
17
16
FS1: FS0 = 2
15
14
FS1: FS0 = 3
13
12
CLK = 1
BUFF = 1
11
10
1
2
4
8
16
32
64
128
1
GAIN (V/V)
a) BUFF = 0
2
4
8
16
32
64
128
GAIN (V/V)
b) BUFF = 1
Figure 7. Effective Resolution vs. Gain and Notch Frequency
______________________________________________________________________________________
25
MAX1400
Table 16b. MAX1400 Noise vs. Gain and Output Data Rate—Buffered Mode,
VREF = 2.5V, fCLKIN = 2.4576MHz
Offset Correction DAC
The MAX1400 provides a coarse (3-bit plus sign) offset
correction DAC at the modulator input. Use this DAC to
remove the offset component in the input signal, allowing
the ADC to operate on a more sensitive range. The DAC
offsets up to ±116.7% of the selected range in ±16.7%
increments for unipolar mode and up to ±58.3% of the
selected range in ±8.3% increments for bipolar mode.
When a DAC value of 0 is selected, the DAC is completely
disconnected from the modulator inputs and does not
contribute any noise. Figures 8 and 9 show the effect of
the DAC codes on the input range and transfer function.
Clock Oscillator
The clock oscillator may be used with an external crystal
(or resonator) connected between CLKIN and CLKOUT,
or may be driven directly by an external oscillator at
CLKIN with CLKOUT left unconnected. In normal operating mode, the MAX1400 is specified for operation with
CLKIN at either 1.024MHz (CLK = 0) or 2.4576MHz
(CLK = 1, default). When operated at these frequencies,
the part may be programmed to produce frequency
response nulls at the local line frequency (either 60Hz or
50Hz) and the associated line harmonics.
MAX CODE 262144
FULL-SCALE 259522
PGA = 3
DAC = 0
PGA = 0
DAC = 0
PGA = 3
DAC = +3
MIDSCALE 131072
V+
(VAIN-) + VREF
AGND
(VAIN-)-VREF
ZERO SCALE 2621
(VAIN-) - VREF/8 - VREF/16
(VAIN-) - VREF/8
VAIN
(VAIN-) + VREF/8 - VREF/16
(VAIN-) + VREF/8
NEGATIVE DAC
STEP SHIFTS
THE TRANSFER
FUNCTION
TOWARD THE
POSITIVE RAIL.
INPUT VOLTAGE RANGE
Figure 8. Effect of PGA and DAC Codes on the Bipolar
Transfer Function
26
In standby mode (STBY = 1) all circuitry, with the
exception of the serial interface and the clock oscillator,
is powered down. The interface consumes minimal
power with a static SCLK. Enter full power-down mode
(including the oscillator) by setting the FULLPD bit in
the special-function register. When exiting a full-power
shutdown, perform a hardware reset or a software reset
after the master clock signal is established (typically
10ms when using the on-board oscillator with an external crystal) to ensure that any potentially corrupted registers are cleared.
It is often helpful to use higher-frequency crystals or
resonators, especially for surface-mount applications
where the result may be reduced PC board area for the
oscillator component and a lower price or better component availability. Also, it may be necessary to operate the part with a clock source whose duty cycle is not
close to 50%. In either case, the MAX1400 can operate
with a master clock frequency of up to 5MHz, and
includes an internal divide-by-2 prescaler to restore the
internal clock frequency to a range of up to 2.5MHz
with a 50% duty cycle. To activate this prescaler, set
the X2CLK bit in the control registers. Note that using
CLKIN frequencies above 2.5MHz in combination with
the X2CLK mode will result in a small increase in digital
supply current.
(VREF = 2.5V (VREF = 1.25V
PGA = 000)
5.00V
4.503V
4.167V
3.750V
3.333V
2.917V
2.50V
2.083V
1.667V
1.25V
0.833V
0.416V
0V
-0.416V
-0.833V
-1.25V
-1.667V
-2.083V
-2.50V
-2.917V
-3.333V
-3.750V
-4.167V
-4.503V
-5.00V
PGA = 000)
2.708V
2.50V
2.292V
2.083V
1.875V
1.667V
1.458V
1.25V
1.042V
0.833V
0.625V
0.416V
0.208V
0V
-0.208V
-0.416V
-0.625V
-0.833V
-1.042V
-1.25V
-1.458V
-1.667V
-1.875V
-2.083V
-2.292V
-2.50V
-2.708V
D3:
D2:
D1:
D0:
MA
XIM
UM
INP
UT
MI
NIM
UM
INP
UT
MI
(U
/B
NIM
UM
=1
)
INP
UT
(U
/B
=0
)
-7
1
1
1
1
-6
1
1
1
0
-5
1
1
0
1
-4
1
1
0
0
-3
1
0
1
1
-2
1
0
1
0
-1
1
0
1
0
0
0
0
0
0
+1
0
0
0
1
+2
0
0
1
0
+3
0
0
1
1
+4
0
1
0
0
+5
0
1
0
1
DAC CODE
Figure 9. Input Voltage Range vs. DAC Code
______________________________________________________________________________________
+6
0
1
1
0
+7
0
1
1
1
13/6 VREF/2PGA
2 VREF/2PGA
11/6 VREF/2PGA
10/6 VREF/2PGA
9/6 VREF/2PGA
8/6 VREF/2PGA
7/6 VREF/2PGA
VREF/2PGA
5/6 VREF/2PGA
4/6 VREF/2PGA
3/6 VREF/2PGA
2/6 VREF/2PGA
1/6 VREF/2PGA
0
-1/6 VREF/2PGA
-2/6 VREF/2PGA
-3/6 VREF/2PGA
-4/6 VREF/2PGA
-5/6 VREF/2PGA
-VREF/2PGA
-7/6 VREF/2PGA
-8/6 VREF/2PGA
-9/6 VREF/2PGA
-10/6 VREF/2PGA
-11/6 VREF/2PGA
-2 VREF/2PGA
-13/6 VREF/2PGA
INPUT VOLTAGE RANGE
The noise shown in Table 16 is composed of device noise
and quantization noise. The device noise is relatively low
but becomes the limiting noise source for high gain settings. The quantization noise is determined by the notch
frequency and becomes the dominant noise source as the
notch frequency is increased.
CODE
MAX1400
+5V, 18-Bit, Low-Power, Multichannel,
Oversampling (Sigma-Delta) ADC
+5V, 18-Bit, Low-Power, Multichannel,
Oversampling (Sigma-Delta) ADC
The SINC1 function results in a faster settling response
while retaining the same frequency response notches
as the default SINC3 filter. This allows the filter to settle
faster at the expense of resolution and quantization
noise. The SINC1 filter settles in one data word period.
With 60Hz notches (60Hz data rate), the settling time
would be 1 / 60Hz or 16.7ms whereas the SINC3 filter
would settle in 3 / 60Hz or 50ms. Toggle between these
filter responses using the FAST bit in the global setup
register. Use SINC1 mode for faster settling and switch
to SINC3 mode when full accuracy is required. Switch
from the SINC1 to SINC3 mode by resetting the FAST
bit low. The DRDY signal will go false and will be
reasserted when valid data is available, a minimum of
three data-word periods later.
The digital filter can be bypassed by setting the MDOUT
bit in the global setup register. When MDOUT = 1, the
raw output of the modulator is directly available at
DOUT.
Filter Characteristics
The MAX1400 digital filter implements both a SINC1
(sinx/x) and SINC3 (sinx/x)3 lowpass filter function. The
transfer function for the SINC3 function is that of three
cascaded SINC1 filters described in the z-domain by:





 
f


sin π
 f  
 M  

f
sin Nπ

f

M
For step changes at the input, a settling time must be
allowed before valid data can be read. The settling time
depends upon the output data rate chosen for the filter.
The settling time of the SINC3 filter to a full-scale step
0
3
fCLKIN = 2.4576MHz
MF1, 0 = 0
FS1, 0 = 1
fN = 60Hz
-20
-40
and in the frequency domain by:



1
H(f ) = 
N


Determine the cutoff frequency of the digital filter by the
value loaded into CLK, X2CLK, MF1, MF0, FS1, and
FS0 in the global setup register. Programming a different cutoff frequency with FS0 and FS1 does not alter
the profile of the filter response; it changes the frequency of the notches. For example, Figure 11 shows a cutoff frequency of 13.1Hz and a first notch frequency of
50Hz.
3
where N, the decimation factor, is the ratio of the modulator frequency fM to the output frequency fN.
GAIN (dB)
−N 

1 1− z 
H( z ) = 
−1 
N
1− z 

Figure 10 shows the filter frequency response. The
SINC3 characteristic cutoff frequency is 0.262 times the
first notch frequency. This results in a cutoff frequency
of 15.72Hz for a first filter notch frequency of 60Hz. The
response shown in Figure 10 is repeated at either side
of the digital filter’s sample frequency (fM) and at either
side of the related harmonics (2fM, 3fM, . . .).
The response of the SINC3 filter is similar to that of a
SINC1 (averaging filter) filter but with a sharper rolloff.
The output data rate for the digital filter corresponds
with the positioning of the first notch of the filter’s frequency response. Therefore, for the plot of Figure 10
where the first notch of the filter is at 60Hz, the output
data rate is 60Hz. The notches of this (sinx/x)3 filter are
repeated at multiples of the first notch frequency. The
SINC 3 filter provides an attenuation of better than
100dB at these notches.
-60
-80
-100
-120
-140
-160
0
20 40 60 80 100 120 140 160 180 200
FREQUENCY (Hz)
Figure 10. Frequency Response of the SINC3 Filter (Notch at
60Hz)
______________________________________________________________________________________
27
MAX1400
Digital Filter
The on-chip digital filter processes the 1-bit data
stream from the modulator using a SINC3 or SINC3 filter. The SINC filters are conceptually simple, efficient,
and extremely flexible, especially where variable resolution and data rates are required. Also, the filter notch
positions are easily controlled, since they are directly
related to the output data rate (1 / data word period).
input can be up to four times the output data period.
For a synchronized step input (using the FSYNC function or the internal scanning logic), the settling time is
three-times the output data period.
cause an offset error of less than 10µV). Therefore,
where any significant source impedances are required,
Maxim recommends operating the part in buffered
mode.
Analog Filtering
Calibration Channels
The digital filter does not provide any rejection close to
the harmonics of the modulator sample frequency.
However, due to the high oversampling ratio of the
MAX1400, these bands occupy only a small fraction of
the spectrum and most broadband noise is filtered.
Therefore, the analog filtering requirements in front of
the MAX1400 are considerably reduced compared to a
conventional converter with no on-chip filtering. In addition, because the part’s common-mode rejection of
90dB extends out to several kHz, common-mode noise
susceptibility in this frequency range is substantially
reduced.
Two fully differential calibration channels allow measurement of the system gain and offset errors. Connect
the CALOFF channel to 0V and the CALGAIN channel
to the reference voltage. Average several measurements on both CALOFF and CALGAIN. Subtract the
average offset code and scale to correct for the gain
error. This linear calibration technique can be used to
remove errors due to source impedances on the analog
input (e.g., when using a simple RC anti-aliasing filter
on the front end).
Depending on the application, it may be necessary to
provide filtering prior to the MAX1400 to eliminate
unwanted frequencies the digital filter does not reject. It
may also be necessary in some applications to provide
additional filtering to ensure that differential noise signals outside the frequency band of interest do not saturate the analog modulator.
If passive components are placed in front of the
MAX1400, when the part is used in unbuffered mode,
ensure that the source impedance is low enough not to
introduce gain errors in the system. This can significantly limit the amount of passive anti-aliasing filtering that
can be applied in front of the MAX1400 in unbuffered
mode. However, when the part is used in buffered
mode, large source impedances will simply result in a
small DC offset error (a 1kΩ source resistance will
0
Applications Information
SPI Interface (68HC11, PIC16C73)
Microprocessors with a hardware SPI (serial peripheral
interface) can use a 3-wire interface to the MAX1400
(Figure 12). The SPI hardware generates groups of
eight pulses on SCLK, shifting data in on one pin and
out on the other pin.
For best results, use a hardware interrupt to monitor the
INT pin and acquire new data as soon as it is available.
If hardware interrupts are not available, or if interrupt
latency is longer than the selected conversion rate, use
the FSYNC bit to prevent automatic measurement while
reading the data output register.
The example code in Figure 13 shows how to interface
with the MAX1400 using a 68HC11. System-dependent
initialization code is not shown.
fCLKIN = 2.4576MHz
MF1, 0 = 0
FS1, 0 = 0
fN = 50Hz
-20
-40
VDD
VDD
SS
GAIN (dB)
MAX1400
+5V, 18-Bit, Low-Power, Multichannel,
Oversampling (Sigma-Delta) ADC
RESET
-60
INTERRUPT
INT
-80
SCK
SCLK
-120
MISO
DOUT
-140
MOSI
DIN
-100
68HC11
-160
0
CS
20 40 60 80 100 120 140 160 180 200
FREQUENCY (Hz)
Figure 11. Frequency Response of the SINC3 Filter (Notch at
50Hz)
28
Figure 12. MAX1400 to 68HC11 Interface
______________________________________________________________________________________
MAX1400
+5V, 18-Bit, Low-Power, Multichannel,
Oversampling (Sigma-Delta) ADC
MAX1400
Figure 13. Example SPI Interface
______________________________________________________________________________________
29
MAX1400
+5V, 18-Bit, Low-Power, Multichannel,
Oversampling (Sigma-Delta) ADC
Bit Banging Interface (80C51, PIC16C54)
VDD
RESET
8051
P3.0
DOUT
DIN
P3.1
SCLK
CS
MAX1400
Any microcontroller can use general-purpose I/O pins
to interface to the MAX1400. If a bidirectional or opendrain I/O pin is available, reduce the interface pin count
by connecting DIN to DOUT (Figure 14). Figure 15
shows how to emulate the SPI in software. Use the
same initialization routine shown in Figure 13.
For best results, use a hardware interrupt to monitor the
INT pin and acquire new data as soon as it is available.
If hardware interrupts are not available, or if interrupt
latency is longer than the selected conversion rate, use
the FSYNC bit to prevent automatic measurement while
reading the data output register.
Figure 14. MAX1400 to 8051 Interface
Figure 15. Bit Banging SPI Replacement
30
______________________________________________________________________________________
+5V, 18-Bit, Low-Power, Multichannel,
Oversampling (Sigma-Delta) ADC
Temperature Measurement
Figure 17 shows a connection from a thermocouple to
the MAX1400. In this application, the MAX1400 is operated in its buffered mode to allow large decoupling
capacitors on the front end. These decoupling capacitors eliminate any noise pickup form the thermocouple
leads. When the MAX1400 is operated in buffered
mode, it has a reduced common-mode range. In order
to place the differential voltage from the thermocouple
on a suitable common-mode voltage, the AIN2 input of
the MAX1400 is biased at the reference voltage, +2.5V.
ANALOG SUPPLY
MUXOUT+
REFIN+
V+
ADCIN+
VDD
V+
RREF
CLOCK
GEN
DIVIDER
MAX1400
CLKIN
CLKOUT
REFINACTIVE
GAUGE
BUFFER
R
AIN1
AIN2
DUMMY
GAUGE
R
SWITCHING
NETWORK
PGA
ADDITIONAL
ANALOG
AND
CALIBRATION
CHANNELS
MODULATOR
DIGITAL
FILTER
BUFFER
DAC
AGND
MUXOUT-
ADCIN-
AGND
INTERFACE
AND
CONTROL
SCLK
DIN
DOUT
INT
CS
RESET
DGND
Figure 16. Strain-Gauge Application with MAX1400
______________________________________________________________________________________
31
MAX1400
Strain Gauge Operation
Connect the differential inputs of the MAX1400 to the
bridge network of the strain gauge. In Figure 16, the
analog positive supply voltage powers the bridge network and the MAX1400 along with its reference voltage.
The on-chip PGA allows the MAX1400 to handle an
analog input voltage range as low as 20mV full scale.
The differential inputs of the part allow this analog input
range to have an absolute value anywhere between
AGND and V+.
4–20mA Loop-Powered Transmitters
Low power, single-supply operation, and easy interfacing with optocouplers make the MAX1400 ideal for
loop-powered 4–20mA transmitters. Loop-powered
transmitters draw their power from the 4–20mA loop,
limiting the transmitter circuitry to a current budget of
4mA. Tolerances in the loop further limit this current
budget to 3.5mA. Since the MAX1400 consumes only
250µA, a total of 3.25mA remains to power the remaining transmitter circuitry. Figure 18 shows a block diagram for a loop-powered 4–20mA transmitter.
MAX1400
THERMOCOUPLE
JUNCTION
R
AIN1
SWITCHING
NETWORK
R
PGA
BUFFER
AIN2
C
Power Supplies
C
No specific power sequence is required for the
MAX1400; either the V+ or the VDD supply can come
up first. While the latchup performance of the MAX1400
is good, it is important that power be applied to the
MAX1400 before the analog input signals (AIN_) or the
CLKIN inputs, to avoid latchup. If this is not possible,
then the current flow into any of these pins should be
limited to 50mA. If separate supplies are used for the
MAX1400 and the system digital circuitry, then the
MAX1400 should be powered up first.
+5V
+2.5V
REFIN+
REFINAGND
DGND
Figure 17. Thermocouple Application with MAX1400
V+
ISOLATION
BARRIER
V+
VOLTAGE
REGULATOR
VIN+
ROFST
RX
RGAIN
SENSOR
MAX1400
MAX1400
+5V, 18-Bit, Low-Power, Multichannel,
Oversampling (Sigma-Delta) ADC
4
4
SPI
SPI
µP/µC
3
RY
DAC
SPI
4–20mA LOOP
INTERFACE
CC
GND
GND
RFDBK
RSENSE
VIN-
Figure 18. 4–20mA Transmitter
32
______________________________________________________________________________________
+5V, 18-Bit, Low-Power, Multichannel,
Oversampling (Sigma-Delta) ADC
Optical Isolation
For applications that require an optically isolated
interface, refer to Figure 19. With 6N136-type optocouplers, maximum clock speed is 4MHz. Maximum clock
speed is limited by the degree of mismatch between
the individual optocouplers. Faster optocouplers allow
faster signaling at a higher cost.
Design the printed circuit board so that the analog and
digital sections are separated and confined to different
areas of the board. Join the digital and analog ground
planes at only one point. If the MAX1400 is the only
device requiring an AGND to DGND connection, then
the ground planes should be connected at the AGND
and DGND pins of the MAX1400. In systems where
multiple devices require AGND to DGND connections,
the connection should still be made at only one point.
Make the star ground as close to the MAX1400 as possible.
Avoid running digital lines under the device, because
these may couple noise onto the die. Run the analog
ground plane under the MAX1400 to minimize coupling
of digital noise. Make the power-supply lines to the
MAX1400 as wide as possible to provide low-impedance paths and reduce the effects of glitches on the
power-supply line.
Shield fast switching signals, such as clocks, with digital ground to avoid radiating noise to other sections of
the board. Avoid running clock signals near the analog
inputs. Avoid crossover of digital and analog signals.
Traces on opposite sides of the board should run at
right angles to each other. This will reduce the effects
of feedthrough on the board. A microstrip technique is
best, but is not always possible with double-sided
boards. In this technique, the component side of the
board is dedicated to ground planes while signals are
placed on the solder side.
Good decoupling is important when using high-resolution ADCs. Decouple all analog supplies with 10µF tantalum capacitors in parallel with 0.1µF HF ceramic
capacitors to AGND. Place these components as close
to the device as possible to achieve the best decoupling.
See the MAX1402 evaluation kit manual for recommended layout. The evaluation board package
includes a fully assembled and tested evaluation
board.
ISO
+5V
+VDD
2k
VCC
6N136
DIN
470Ω
MOSI
2k
VCC
6N136
MAX1400
SCLK
470Ω
SCK
VCC
2k
470Ω
6N136
MISO
DOUT
VCC
2k
470Ω
INT
INT
6N136
Figure 19. Optically Isolated Interface
Chip Information
TRANSISTOR COUNT: 34,648
SUBSTRATE CONNECTED TO AGND
______________________________________________________________________________________
33
MAX1400
Grounding and Layout
For best performance, use printed circuit boards with
separate analog and digital ground planes. Wire-wrap
boards are not recommended.
+5V, 18-Bit, Low-Power, Multichannel,
Oversampling (Sigma-Delta) ADC
SSOP.EPS
MAX1400
Package Information
34
______________________________________________________________________________________
+5V, 18-Bit, Low-Power, Multichannel,
Oversampling (Sigma-Delta) ADC
______________________________________________________________________________________
MAX1400
NOTES
35
MAX1400
+5V, 18-Bit, Low-Power, Multichannel,
Oversampling (Sigma-Delta) ADC
NOTES
Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product. No circuit patent licenses are
implied. Maxim reserves the right to change the circuitry and specifications without notice at any time.
36 ____________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600
© 1999 Maxim Integrated Products
Printed USA
is a registered trademark of Maxim Integrated Products.