Maxim MAX147BEPP 2.7low-power, 8-channel, serial 12-bit adc Datasheet

19-0465; Rev 1; 6/97
KIT
ATION
EVALU
E
L
B
AVAILA
+2.7V, Low-Power, 8-Channel,
Serial 12-Bit ADCs
These devices provide a hard-wired SHDN pin and a
software-selectable power-down, and can be programmed to automatically shut down at the end of a conversion. Accessing the serial interface automatically
powers up the MAX146/MAX147, and the quick turn-on
time allows them to be shut down between all conversions. This technique can cut supply current to under
60µA at reduced sampling rates.
The MAX146/MAX147 are available in 20-pin DIP and
SSOP packages.
For 4-channel versions of these devices, see the
MAX1246/MAX1247 data sheet.
____________________________Features
♦ 8-Channel Single-Ended or 4-Channel
Differential Inputs
♦ Single-Supply Operation:
+2.7V to +3.6V (MAX146)
+2.7V to +5.25V (MAX147)
♦ Internal 2.5V Reference (MAX146)
♦ Low Power: 1.2mA (133ksps, 3V supply)
54µA (1ksps, 3V supply)
1µA (power-down mode)
♦ SPI/QSPI/Microwire/TMS320-Compatible
4-Wire Serial Interface
♦ Software-Configurable Unipolar or Bipolar Inputs
♦ 20-Pin DIP/SSOP Packages
______________Ordering Information
INL
(LSB)
PART†
TEMP. RANGE
MAX146ACPP
0°C to +70°C
20 Plastic DIP
±1/2
MAX146BCPP
MAX146ACAP
MAX146BCAP
0°C to +70°C
0°C to +70°C
0°C to +70°C
20 Plastic DIP
20 SSOP
20 SSOP
±1
±1/2
±1
MAX146BC/D
0°C to +70°C
Dice*
±1
PIN-PACKAGE
Ordering Information continued at end of data sheet.
† Contact factory for availability of alternate surface-mount
packages.
*Dice are specified at TA = +25°C, DC parameters only.
__________Typical Operating Circuit
________________________Applications
Portable Data Logging
+3V
Data Acquisition
CH0
Medical Instruments
Battery-Powered Instruments
Pen Digitizers
Process Control
0V TO
+2.5V
ANALOG
INPUTS
VDD
VDD
0.1µF
DGND
MAX146 AGND
CPU
CH7
VREF
4.7µF
COM
CS
SCLK
Pin Configuration appears at end of data sheet.
DIN
REFADJ
0.047µF
DOUT
I/O
SCK (SK)
MOSI (SO)
MISO (SI)
SSTRB
SHDN
VSS
SPI and QSPI are registered trademarks of Motorola, Inc.
Microwire is a registered trademark of National Semiconductor Corp.
________________________________________________________________ Maxim Integrated Products
1
For free samples & the latest literature: http://www.maxim-ic.com, or phone 1-800-998-8800
MAX146/MAX147
_______________General Description
The MAX146/MAX147 12-bit data-acquisition systems
combine an 8-channel multiplexer, high-bandwidth
track/hold, and serial interface with high conversion
speed and low power consumption. The MAX146 operates from a single +2.7V to +3.6V supply; the MAX147
operates from a single +2.7V to +5.25V supply. Both
devices’ analog inputs are software configurable for
unipolar/bipolar and single-ended/differential operation.
The 4-wire serial interface connects directly to SPI™/
QSPI™ and Microwire™ devices without external logic. A
serial strobe output allows direct connection to TMS320family digital signal processors. The MAX146/MAX147
use either the internal clock or an external serial-interface
clock to perform successive-approximation analog-todigital conversions.
The MAX146 has an internal 2.5V reference, while the
MAX147 requires an external reference. Both parts have
a reference-buffer amplifier with a ±1.5% voltageadjustment range.
MAX146/MAX147
+2.7V, Low-Power, 8-Channel,
Serial 12-Bit ADCs
ABSOLUTE MAXIMUM RATINGS
VDD to AGND, DGND................................................. -0.3V to 6V
AGND to DGND ...................................................... -0.3V to 0.3V
CH0–CH7, COM to AGND, DGND ............ -0.3V to (VDD + 0.3V)
VREF, REFADJ to AGND ........................... -0.3V to (VDD + 0.3V)
Digital Inputs to DGND .............................................. -0.3V to 6V
Digital Outputs to DGND ........................... -0.3V to (VDD + 0.3V)
Digital Output Sink Current .................................................25mA
Continuous Power Dissipation (TA = +70°C)
Plastic DIP (derate 11.11mW/°C above +70°C) ......... 889mW
SSOP (derate 8.00mW/°C above +70°C) ................... 640mW
CERDIP (derate 11.11mW/°C above +70°C) .............. 889mW
Operating Temperature Ranges
MAX146_C_P/MAX147_C_P .............................. 0°C to +70°C
MAX146_E_P/MAX147_E_P............................ -40°C to +85°C
MAX146_MJP/MAX147_MJP ........................ -55°C to +125°C
Storage Temperature Range ............................ -60°C to +150°C
Lead Temperature (soldering, 10sec) ............................ +300°C
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
(VDD = +2.7V to +3.6V (MAX146); VDD = +2.7V to +5.25V (MAX147); COM = 0V; fSCLK = 2.0MHz; external clock (50% duty cycle); 15
clocks/conversion cycle (133ksps); MAX146—4.7µF capacitor at VREF pin; MAX147—external reference, VREF = 2.500 V applied to
VREF pin; TA = TMIN to TMAX; unless otherwise noted.)
PARAMETER
SYMBOL
CONDITIONS
MIN
TYP
MAX
UNITS
DC ACCURACY (Note 1)
Resolution
12
Relative Accuracy (Note 2)
INL
MAX14_A
MAX14_B
Differential Nonlinearity
DNL
No missing codes over temperature
Bits
±0.5
±1.0
LSB
±1
LSB
MAX14_A
±0.5
±3
MAX14_B
Gain Error (Note 3)
±0.5
±0.5
±4
±4
Gain Temperature Coefficient
±0.25
ppm/°C
Channel-to-Channel Offset
Matching
±0.25
LSB
Offset Error
LSB
LSB
DYNAMIC SPECIFICATIONS (10kHz sine-wave input, 0V to 2.500Vp-p, 133ksps, 2.0MHz external clock, bipolar input mode)
Signal-to-Noise + Distortion Ratio
SINAD
Total Harmonic Distortion
THD
Spurious-Free Dynamic Range
SFDR
70
Up to the 5th harmonic
73
-88
80
dB
-80
dB
90
dB
Channel-to-Channel Crosstalk
65kHz, 2.500Vp-p (Note 4)
-85
dB
Small-Signal Bandwidth
-3dB rolloff
2.25
MHz
1.0
MHz
Full-Power Bandwidth
CONVERSION RATE
Conversion Time (Note 5)
Track/Hold Acquisition Time
tCONV
Internal clock, SHDN = FLOAT
5.5
7.5
Internal clock, SHDN = VDD
35
65
µs
External clock = 2MHz, 12 clocks/conversion
6
1.5
µs
tACQ
Aperture Delay
Aperture Jitter
Internal Clock Frequency
External Clock Frequency
2
SHDN = FLOAT
ns
<50
ps
1.8
SHDN = VDD
Data transfer only
30
MHz
0.225
0.1
2.0
0
2.0
_______________________________________________________________________________________
MHz
+2.7V, Low-Power, 8-Channel,
Serial 12-Bit ADCs
(VDD = +2.7V to +3.6V (MAX146); VDD = +2.7V to +5.25V (MAX147); COM = 0V; fSCLK = 2.0MHz; external clock (50% duty cycle); 15
clocks/conversion cycle (133ksps); MAX146—4.7µF capacitor at VREF pin; MAX147—external reference, VREF = 2.500 V applied to
VREF pin; TA = TMIN to TMAX; unless otherwise noted.)
PARAMETER
SYMBOL
CONDITIONS
MIN
TYP
MAX
UNITS
ANALOG/COM INPUTS
Input Voltage Range, SingleEnded and Differential (Note 6)
Unipolar, COM = 0V
0 to VREF
Bipolar, COM = VREF / 2
±VREF / 2
Multiplexer Leakage Current
On/off leakage current, VCH_ = 0V or VDD
V
±0.01
Input Capacitance
±1
16
µA
pF
INTERNAL REFERENCE (MAX146 only, reference buffer enabled)
VREF Output Voltage
TA = +25°C
2.480
2.500
VREF Short-Circuit Current
VREF Temperature Coefficient
Load Regulation (Note 7)
Capacitive Bypass at VREF
2.520
V
30
mA
MAX146_C
±30
±50
MAX146_E
±30
±60
MAX146_M
±30
±80
0mA to 0.2mA output load
0.35
Internal compensation mode
0
External compensation mode
4.7
Capacitive Bypass at REFADJ
mV
µF
0.047
REFADJ Adjustment Range
ppm/°C
µF
±1.5
%
EXTERNAL REFERENCE AT VREF (Buffer disabled)
VREF Input Voltage Range
(Note 8)
VREF Input Current
VDD +
50mV
1.0
VREF = 2.500V
VREF Input Resistance
100
18
Shutdown VREF Input Current
150
25
0.01
µA
kΩ
10
VDD 0.5
REFADJ Buffer Disable Threshold
V
µA
V
EXTERNAL REFERENCE AT REFADJ
Capacitive Bypass at VREF
Reference Buffer Gain
REFADJ Input Current
Internal compensation mode
0
External compensation mode
4.7
µF
MAX146
2.06
MAX147
2.00
V/V
MAX146
±50
MAX147
±10
µA
_______________________________________________________________________________________
3
MAX146/MAX147
ELECTRICAL CHARACTERISTICS (continued)
MAX146/MAX147
+2.7V, Low-Power, 8-Channel,
Serial 12-Bit ADCs
ELECTRICAL CHARACTERISTICS (continued)
(VDD = +2.7V to +3.6V (MAX146); VDD = +2.7V to +5.25V (MAX147); COM = 0V; fSCLK = 2.0MHz; external clock (50% duty cycle); 15
clocks/conversion cycle (133ksps); MAX146—4.7µF capacitor at VREF pin; MAX147—external reference, VREF = 2.500 V applied to
VREF pin; TA = TMIN to TMAX; unless otherwise noted.)
PARAMETER
SYMBOL
CONDITIONS
MIN
TYP
MAX
UNITS
DIGITAL INPUTS (DIN, SCLK, CS, SHDN)
DIN, SCLK, CS Input High Voltage
VIH
DIN, SCLK, CS Input Low Voltage
VIL
DIN, SCLK, CS Input Hysteresis
VDD ≤ 3.6V
2.0
VDD > 3.6V, MAX147 only
3.0
VHYST
IIN
VIN = 0V or VDD
DIN, SCLK, CS Input Capacitance
CIN
(Note 9)
VSH
VDD - 0.4
SHDN Input Mid Voltage
VSM
1.1
SHDN Input Low Voltage
VSL
IS
VFLT
SHDN Maximum Allowed
Leakage, Mid Input
V
±1
µA
15
pF
V
±0.01
SHDN Input High Voltage
SHDN Voltage, Floating
0.8
0.2
DIN, SCLK, CS Input Leakage
SHDN Input Current
V
V
VDD - 1.1
SHDN = 0V or VDD
SHDN = FLOAT
V
±4.0
µA
VDD / 2
SHDN = FLOAT
V
0.4
V
±100
nA
DIGITAL OUTPUTS (DOUT, SSTRB)
Output Voltage Low
VOL
Output Voltage High
VOH
Three-State Leakage Current
Three-State Output Capacitance
IL
COUT
ISINK = 5mA
0.4
ISINK = 16mA
0.8
ISOURCE = 0.5mA
VDD - 0.5
CS = VDD
V
V
±0.01
CS = VDD (Note 9)
±10
µA
15
pF
POWER REQUIREMENTS
Positive Supply Voltage
VDD
MAX146
2.70
3.60
MAX147
2.70
5.25
Operating mode, full-scale input
Positive Supply Current, MAX146
IDD
VDD = 3.6V Fast power-down
2.0
30
70
1.2
10
VDD = 5.25V
1.8
2.5
VDD = 3.6V
0.9
1.5
VDD = 5.25V
2.1
15
VDD = 3.6V
1.2
10
Full power-down
Positive Supply Current, MAX147
IDD
Operating mode,
full-scale input
Positive Supply Current, MAX147
IDD
Full power-down
Supply Rejection (Note 10)
PSR
Full-scale input, external reference = 2.500V,
VDD = 2.7V to VDD(MAX)
4
1.2
±0.3
_______________________________________________________________________________________
V
mA
µA
mA
µA
µA
mV
+2.7V, Low-Power, 8-Channel,
Serial 12-Bit ADCs
(VDD = +2.7V to +3.6V (MAX146); VDD = +2.7V to +5.25V (MAX147); TA = TMIN to TMAX; unless otherwise noted.)
PARAMETER
SYMBOL
Acquisition Time
CONDITIONS
MIN
TYP
MAX
UNITS
tACQ
1.5
µs
DIN to SCLK Setup
tDS
100
ns
DIN to SCLK Hold
tDH
0
ns
MAX14_ _C/E
20
200
Figure 1 _M
MAX14_
20
240
SCLK Fall to Output Data Valid
tDO
Figure 1
ns
CS Fall to Output Enable
tDV
Figure 1
240
ns
CS Rise to Output Disable
tTR
Figure 2
240
ns
CS to SCLK Rise Setup
tCSS
100
ns
CS to SCLK Rise Hold
tCSH
0
ns
SCLK Pulse Width High
tCH
200
ns
SCLK Pulse Width Low
tCL
200
SCLK Fall to SSTRB
tSSTRB
ns
Figure 1
240
ns
tSDV
External clock mode only, Figure 1
240
ns
CS Rise to SSTRB Output Disable
tSTR
External clock mode only, Figure 2
240
ns
SSTRB Rise to SCLK Rise
tSCK
Internal clock mode only (Note 9)
CS Fall to SSTRB Output Enable
0
ns
Note 1: Tested at VDD = 2.7V; COM = 0V; unipolar single-ended input mode.
Note 2: Relative accuracy is the deviation of the analog value at any code from its theoretical value after the full-scale range has
been calibrated.
Note 3: MAX146—internal reference, offset nulled; MAX147—external reference (VREF = +2.500V), offset nulled.
Note 4: Ground “on” channel; sine wave applied to all “off” channels.
Note 5: Conversion time defined as the number of clock cycles multiplied by the clock period; clock has 50% duty cycle.
Note 6: The common-mode range for the analog inputs is from AGND to VDD.
Note 7: External load should not change during conversion for specified accuracy.
Note 8: ADC performance is limited by the converter’s noise floor, typically 300µVp-p.
Note 9: Guaranteed by design. Not subject to production testing.
Note 10: Measured as |VFS(2.7V) - VFS(VDD, MAX)|.
__________________________________________Typical Operating Characteristics
(VDD = 3.0V, VREF = 2.500V, fSCLK = 2.0MHz, CLOAD = 20pF, TA = +25°C, unless otherwise noted.)
0.50
0.45
0.40
0.2
0.35
0.1
0.30
0
-0.1
0.50
0.40
MAX146
0.35
0.25
0.20
MAX147
0.25
0.20
0.15
-0.3
0.10
0.10
-0.4
0.05
0.05
0
1024
2048
CODE
3072
4096
0
2.25
MAX146
0.30
-0.2
-0.5
VDD = 2.7V
0.45
INL (LSB)
0.3
INL (LSB)
INL (LSB)
0.4
MAX146/47-02
MAX146/47-01
0.5
INTEGRAL NONLINEARITY
vs. TEMPERATURE
INTEGRAL NONLINEARITY
vs. SUPPLY VOLTAGE
MAX146/47-03
INTEGRAL NONLINEARITY
vs. CODE
MAX147
0.15
0
2.75
3.25
4.25
3.75
VDD (V)
4.75
5.25
-60
-20
20
60
100
140
TEMPERATURE (°C)
_______________________________________________________________________________________
5
MAX146/MAX147
TIMING CHARACTERISTICS
____________________________Typical Operating Characteristics (continued)
(VDD = 3.0V, VREF = 2.500V, fSCLK = 2.0MHz, CLOAD = 20pF, TA = +25°C, unless otherwise noted.)
MAX146
1.25
1.00
CLOAD = 20pF
0.75
2.75
3.25
3.75
4.25
4.75
MAX146/47-06
2.5010
2.0
1.5
2.5005
2.5000
1.0
2.4995
0.5
0
2.25
5.25
3.25
2.75
3.75
4.25
4.75
2.4990
2.25
5.25
2.75
3.25
3.75
4.25
VDD (V)
VDD (V)
SUPPLY CURRENT vs. TEMPERATURE
SHUTDOWN CURRENT
vs. TEMPERATURE
MAX146
INTERNAL REFERENCE VOLTAGE
vs. TEMPERATURE
MAX147
VDD = 3.6V
2.499
VDD = 2.7V
1.2
VREF (V)
1.0
2.500
1.6
SHUTDOWN CURRENT (µA)
1.1
2.501
5.25
MAX146/47-09
2.0
MAX146/47-07
MAX146
1.2
0.8
2.498
2.497
2.496
0.4
0.9
2.495
RLOAD = ∞
CODE = 101010100000
-20
20
0
60
100
2.494
-60
140
-20
20
60
100
140
-20
20
60
100
TEMPERATURE (°C)
EFFECTIVE NUMBER OF BITS
vs. FREQUENCY
FFT PLOT
VDD = 2.7V
fIN = 10kHz
fSAMPLE = 133kHz
12.0
MAX146/47-10
20
0
-60
TEMPERATURE (°C)
TEMPERATURE (°C)
MAX146/47-11
-60
VDD = 2.7V
11.8
-40
ENOB
AMPLITUDE (dB)
-20
-60
11.6
11.4
-80
11.2
-100
11.0
-120
0
6
4.75
SUPPLY VOLTAGE (V)
1.3
0.8
2.5015
2.5
MAX147
0.50
2.25
2.5020
VREF (V)
1.50
FULL POWER-DOWN
3.0
MAX1247-08
SUPPLY CURRENT (mA)
1.75
CLOAD = 50pF
3.5
MAX146/47-05
RL = ∞
CODE = 101010100000
SHUTDOWN SUPPLY CURRENT (µA)
2.00
MAX146
INTERNAL REFERENCE VOLTAGE
vs. SUPPLY VOLTAGE
SHUTDOWN SUPPLY CURRENT
vs. SUPPLY VOLTAGE
MAX146/47-04
SUPPLY CURRENT
vs. SUPPLY VOLTAGE
SUPPLY CURRENT (mA)
MAX146/MAX147
+2.7V, Low-Power, 8-Channel,
Serial 12-Bit ADCs
10
20
30
40
50
FREQUENCY (kHz)
60
70
1
10
FREQUENCY (kHz)
_______________________________________________________________________________________
100
140
+2.7V, Low-Power, 8-Channel,
Serial 12-Bit ADCs
0.45
0.25
0.20
0.40
0.30
0.25
0.20
0.35
0.30
0.25
0.20
0.15
0.15
0.10
0.10
0.10
0.05
0.05
0.05
0
2.25
2.75
3.75 4.25
VDD (V)
3.25
4.75
0
2.25
5.25
2.75
0.45
0.35
GAIN ERROR (LSB)
0.35
0.20
4.75
0.15
0
2.25
5.25
0.45
0.30
0.25
0.20
0.35
0.30
0.25
0.20
0.10
0.05
0.05
0.15
0
95
0
-55
120 145
TEMPERATURE (˚C)
-30
-5
20 45 70 95
TEMPERATURE (˚C)
CHANNEL-TO-CHANNEL OFFSET MATCHING
vs. SUPPLY VOLTAGE
0.40
0.35
0.30
0.25
0.20
0.15
0.35
0.30
0.25
0.20
0.15
0.10
0.05
3.25
3.75
VDD (V)
4.25
4.75
5.25
120 145
0.40
0.05
2.75
20 45 70 95
TEMPERATURE (˚C)
0.45
0.10
0
2.25
-5
0.50
OFFSET MATCHING (LSB)
OFFSET MATCHING (LSB)
0.45
-30
CHANNEL-TO-CHANNEL OFFSET MATCHING
vs. TEMPERATURE
MAX146/47-18
0.50
-55
120 145
MAX146/47-19
70
5.25
0.40
0.05
45
4.75
0.50
0.10
20
4.25
CHANNEL-TO-CHANNEL GAIN MATCHING
vs. TEMPERATURE
0.10
-5
3.75
GAIN ERROR
vs. TEMPERATURE
0.15
-30
3.25
VDD (V)
0.15
0
-55
2.75
VDD (V)
0.45
0.40
0.25
4.25
0.50
0.40
0.30
3.75
GAIN MATCHING (LSB)
0.50
3.25
MAX146/47-16
MAX146/47-15
OFFSET vs. TEMPERATURE
OFFSET (LSB)
0.45
MAX146/47-17
0.35
0.35
0.50
GAIN MATCHING (LSB)
0.40
GAIN ERROR (LSB)
0.40
0.30
MAX146/47-13
0.45
OFFSET (LSB)
0.50
MAX146/47-12
0.50
CHANNEL-TO-CHANNEL GAIN MATCHING
vs. SUPPLY VOLTAGE
MAX146/47-14
GAIN ERROR
vs. SUPPLY VOLTAGE
OFFSET vs. SUPPLY VOLTAGE
0
-55
-30
-5
20 45 70 95
TEMPERATURE (˚C)
120 145
_______________________________________________________________________________________
7
MAX146/MAX147
____________________________Typical Operating Characteristics (continued)
(VDD = 3.0V, VREF = 2.500V, fSCLK = 2.0MHz, CLOAD = 20pF, TA = +25°C, unless otherwise noted.)
MAX146/MAX147
+2.7V, Low-Power, 8-Channel,
Serial 12-Bit ADCs
______________________________________________________________Pin Description
PIN
NAME
FUNCTION
1–8
CH0–CH7
9
COM
Ground reference for analog inputs. COM sets zero-code voltage in single-ended mode. Must be
stable to ±0.5LSB.
10
SHDN
Three-Level Shutdown Input. Pulling SHDN low shuts the MAX146/MAX147 down; otherwise, they are
fully operational. Pulling SHDN high puts the reference-buffer amplifier in internal compensation mode.
Letting SHDN float puts the reference-buffer amplifier in external compensation mode.
11
VREF
Reference-Buffer Output/ADC Reference Input. Reference voltage for analog-to-digital conversion.
In internal reference mode (MAX146 only), the reference buffer provides a 2.500V nominal output,
externally adjustable at REFADJ. In external reference mode, disable the internal buffer by pulling
REFADJ to VDD.
12
REFADJ
Input to the Reference-Buffer Amplifier. To disable the reference-buffer amplifier, tie REFADJ to VDD.
13
AGND
Analog Ground
14
DGND
Digital Ground
15
DOUT
Serial Data Output. Data is clocked out at SCLK’s falling edge. High impedance when CS is high.
16
SSTRB
Serial Strobe Output. In internal clock mode, SSTRB goes low when the MAX146/MAX147 begin the
A/D conversion, and goes high when the conversion is finished. In external clock mode, SSTRB pulses
high for one clock period before the MSB decision. High impedance when CS is high (external clock
mode).
17
DIN
Serial Data Input. Data is clocked in at SCLK’s rising edge.
18
CS
Active-Low Chip Select. Data will not be clocked into DIN unless CS is low. When CS is high, DOUT is
high impedance.
19
SCLK
20
VDD
Sampling Analog Inputs
Serial Clock Input. Clocks data in and out of serial interface. In external clock mode, SCLK also sets
the conversion speed. (Duty cycle must be 40% to 60%.)
Positive Supply Voltage
VDD
DOUT
DOUT
CLOAD
50pF
6k
8
DOUT
DGND
b) High-Z to VOL and VOH to VOL
Figure 1. Load Circuits for Enable Time
6k
6k
CLOAD
50pF
DGND
a) High-Z to VOH and VOL to VOH
VDD
DOUT
CLOAD
50pF
6k
DGND
a) VOH to High-Z
Figure 2. Load Circuits for Disable Time
_______________________________________________________________________________________
CLOAD
50pF
DGND
b) VOL to High-Z
+2.7V, Low-Power, 8-Channel,
Serial 12-Bit ADCs
The MAX146/MAX147 analog-to-digital converters
(ADCs) use a successive-approximation conversion
technique and input track/hold (T/H) circuitry to convert
an analog signal to a 12-bit digital output. A flexible serial interface provides easy interface to microprocessors
(µPs). Figure 3 is a block diagram of the MAX146/
MAX147.
Pseudo-Differential Input
The sampling architecture of the ADC’s analog comparator is illustrated in the equivalent input circuit
(Figure 4). In single-ended mode, IN+ is internally
switched to CH0–CH7, and IN- is switched to COM. In
differential mode, IN+ and IN- are selected from the following pairs: CH0/CH1, CH2/CH3, CH4/CH5, and
CH6/CH7. Configure the channels with Tables 2 and 3.
In differential mode, IN- and IN+ are internally switched
to either of the analog inputs. This configuration is
pseudo-differential to the effect that only the signal at
IN+ is sampled. The return side (IN-) must remain stable within ±0.5LSB (±0.1LSB for best results) with
respect to AGND during a conversion. To accomplish
this, connect a 0.1µF capacitor from IN- (the selected
analog input) to AGND.
During the acquisition interval, the channel selected as
the positive input (IN+) charges capacitor CHOLD. The
acquisition interval spans three SCLK cycles and ends
CS
SCLK
DIN
SHDN
CH0
CH1
CH2
CH3
CH4
CH5
CH6
CH7
COM
on the falling SCLK edge after the last bit of the input
control word has been entered. At the end of the acquisition interval, the T/H switch opens, retaining charge
on CHOLD as a sample of the signal at IN+.
The conversion interval begins with the input multiplexer switching CHOLD from the positive input (IN+) to the
negative input (IN-). In single-ended mode, IN- is simply COM. This unbalances node ZERO at the comparator’s input. The capacitive DAC adjusts during the
remainder of the conversion cycle to restore node
ZERO to 0V within the limits of 12-bit resolution. This
action is equivalent to transferring a 16pF x [(VIN+) (V IN -)] charge from C HOLD to the binary-weighted
capacitive DAC, which in turn forms a digital representation of the analog input signal.
Track/Hold
The T/H enters its tracking mode on the falling clock
edge after the fifth bit of the 8-bit control word has been
shifted in. It enters its hold mode on the falling clock
edge after the eighth bit of the control word has been
shifted in. If the converter is set up for single-ended
inputs, IN- is connected to COM, and the converter
samples the “+” input. If the converter is set up for differential inputs, IN- connects to the “-” input, and the
difference of |IN+ - IN-| is sampled. At the end of the
conversion, the positive input connects back to IN+,
and CHOLD charges to the input signal.
18
19
17
10
1
2
3
4
5
6
7
8
INPUT
SHIFT
REGISTER
VREF
CONTROL
LOGIC
OUTPUT
SHIFT
REGISTER
ANALOG
INPUT
MUX
15
16
DOUT
CH0
CH1
SSTRB
CH2
CH3
CH4
T/H
CLOCK
IN 12-BIT
SAR
ADC OUT
REF
9
REFADJ 12
VREF 11
12-BIT CAPACITIVE DAC
INT
CLOCK
+1.21V
REFERENCE
(MAX146)
20k
A ≈ 2.06*
CH5
20
14
13
+2.500V
MAX146
MAX147
VDD
DGND
AGND
CH6
CH7
COMPARATOR
INPUT
CHOLD
MUX –
+
ZERO
16pF
RIN
9k
CSWITCH
TRACK
HOLD
T/H
SWITCH
COM
AT THE SAMPLING INSTANT,
THE MUX INPUT SWITCHES
FROM THE SELECTED IN+
CHANNEL TO THE SELECTED
IN- CHANNEL.
SINGLE-ENDED MODE: IN+ = CH0–CH7, IN- = COM.
DIFFERENTIAL MODE: IN+ AND IN- SELECTED FROM PAIRS OF
CH0/CH1, CH2/CH3, CH4/CH5, AND CH6/CH7.
*A ≈ 2.00 (MAX147)
Figure 3. Block Diagram
Figure 4. Equivalent Input Circuit
_______________________________________________________________________________________
9
MAX146/MAX147
_______________Detailed Description
MAX146/MAX147
+2.7V, Low-Power, 8-Channel,
Serial 12-Bit ADCs
Analog Input Protection
The time required for the T/H to acquire an input signal
is a function of how quickly its input capacitance is
charged. If the input signal’s source impedance is high,
the acquisition time lengthens, and more time must be
allowed between conversions. The acquisition time,
tACQ, is the maximum time the device takes to acquire
the signal, and is also the minimum time needed for the
signal to be acquired. It is calculated by the following
equation:
tACQ = 9 x (RS + RIN) x 16pF
where RIN = 9kΩ, RS = the source impedance of the
input signal, and tACQ is never less than 1.5µs. Note
that source impedances below 1kΩ do not significantly
affect the ADC’s AC performance.
Higher source impedances can be used if a 0.01µF
capacitor is connected to the individual analog inputs.
Note that the input capacitor forms an RC filter with the
input source impedance, limiting the ADC’s signal
bandwidth.
Internal protection diodes, which clamp the analog input
to VDD and AGND, allow the channel input pins to swing
from AGND - 0.3V to V DD + 0.3V without damage.
However, for accurate conversions near full scale, the
inputs must not exceed VDD by more than 50mV or be
lower than AGND by 50mV.
If the analog input exceeds 50mV beyond the supplies, do not forward bias the protection diodes of
off channels over 2mA.
Quick Look
To quickly evaluate the MAX146/MAX147’s analog performance, use the circuit of Figure 5. The MAX146/
MAX147 require a control byte to be written to DIN
before each conversion. Tying DIN to +3V feeds in control bytes of $FF (HEX), which trigger single-ended
unipolar conversions on CH7 in external clock mode
without powering down between conversions. In external clock mode, the SSTRB output pulses high for one
clock period before the most significant bit of the 12-bit
conversion result is shifted out of DOUT. Varying the
analog input to CH7 will alter the sequence of bits from
DOUT. A total of 15 clock cycles is required per conversion. All transitions of the SSTRB and DOUT outputs
occur on the falling edge of SCLK.
Input Bandwidth
The ADC’s input tracking circuitry has a 2.25MHz
small-signal bandwidth, so it is possible to digitize
high-speed transient events and measure periodic signals with bandwidths exceeding the ADC’s sampling
rate by using undersampling techniques. To avoid
high-frequency signals being aliased into the frequency
band of interest, anti-alias filtering is recommended.
OSCILLOSCOPE
MAX146
MAX147
0V TO
2.500V
ANALOG
INPUT 0.01µF
CH7
VDD
+3V
SCLK
0.1µF
DGND
AGND
COM
SSTRB
CS
+3V
REFADJ
DOUT*
SCLK
+3V
DIN
+3V
2.5V
VOUT
1000pF
MAX872
COMP
C1
0.1µF
OPTIONAL FOR MAX146,
REQUIRED FOR MAX147
DOUT
VREF
2MHz
OSCILLATOR
SSTRB
SHDN
N.C.
CH1
CH2
CH3
* FULL-SCALE ANALOG INPUT, CONVERSION RESULT = $FFF (HEX)
Figure 5. Quick-Look Circuit
10
CH4
______________________________________________________________________________________
+2.7V, Low-Power, 8-Channel,
Serial 12-Bit ADCs
BIT 7
(MSB)
BIT 6
BIT 5
BIT 4
BIT 3
BIT 2
BIT 1
BIT 0
(LSB)
START
SEL2
SEL1
SEL0
UNI/BIP
SGL/DIF
PD1
PD0
BIT
NAME
DESCRIPTION
7(MSB)
START
The first logic “1” bit after CS goes low defines the beginning of the control byte.
6
5
4
SEL2
SEL1
SEL0
These three bits select which of the eight channels are used for the conversion (Tables 2 and 3).
3
UNI/BIP
1 = unipolar, 0 = bipolar. Selects unipolar or bipolar conversion mode. In unipolar mode, an
analog input signal from 0V to VREF can be converted; in bipolar mode, the signal can range
from -VREF/2 to +VREF/2.
2
SGL/DIF
1 = single ended, 0 = differential. Selects single-ended or differential conversions. In singleended mode, input signal voltages are referred to COM. In differential mode, the voltage
difference between two channels is measured (Tables 2 and 3).
1
0(LSB)
PD1
PD0
Selects clock and power-down modes.
PD1
PD0
Mode
0
0
Full power-down
0
1
Fast power-down (MAX146 only)
1
0
Internal clock mode
1
1
External clock mode
Table 2. Channel Selection in Single-Ended Mode (SGL/DIF = 1)
SEL2
0
SEL1
0
SEL0
0
1
0
0
0
0
1
1
0
1
0
1
0
1
1
0
0
1
1
1
1
1
CH0
+
CH1
CH2
CH3
CH4
CH5
CH6
CH7
COM
–
+
–
+
–
+
–
+
–
+
–
+
–
+
–
Table 3. Channel Selection in Differential Mode (SGL/DIF = 0)
SEL2
SEL1
SEL0
CH0
CH1
0
0
0
+
–
0
0
1
0
1
0
0
1
1
1
0
0
1
0
1
1
1
0
1
1
1
–
CH2
CH3
+
–
CH4
CH5
+
–
CH6
CH7
+
–
–
+
+
–
+
–
+
______________________________________________________________________________________
11
MAX146/MAX147
Table 1. Control-Byte Format
MAX146/MAX147
+2.7V, Low-Power, 8-Channel,
Serial 12-Bit ADCs
How to Start a Conversion
4) Transmit a byte of all zeros ($00 hex) and, simultaneously, receive byte RB2.
5) Transmit a byte of all zeros ($00 hex) and, simultaneously, receive byte RB3.
6) Pull CS high.
Figure 6 shows the timing for this sequence. Bytes RB2
and RB3 contain the result of the conversion, padded
with one leading zero and three trailing zeros. The total
conversion time is a function of the serial-clock frequency and the amount of idle time between 8-bit
transfers. To avoid excessive T/H droop, make sure the
total conversion time does not exceed 120µs.
Start a conversion by clocking a control byte into DIN.
With CS low, each rising edge on SCLK clocks a bit from
DIN into the MAX146/MAX147’s internal shift register.
After CS falls, the first arriving logic “1” bit defines the
control byte’s MSB. Until this first “start” bit arrives, any
number of logic “0” bits can be clocked into DIN with no
effect. Table 1 shows the control-byte format.
The MAX146/MAX147 are compatible with SPI™/
QSPI™ and Microwire™ devices. For SPI, select the
correct clock polarity and sampling edge in the SPI
control registers: set CPOL = 0 and CPHA = 0. Microwire, SPI, and QSPI all transmit a byte and receive a
byte at the same time. Using the Typical Operating
Circuit, the simplest software interface requires only
three 8-bit transfers to perform a conversion (one 8-bit
transfer to configure the ADC, and two more 8-bit transfers to clock out the 12-bit conversion result). See Figure
20 for MAX146/MAX147 QSPI connections.
Digital Output
In unipolar input mode, the output is straight binary
(Figure 17). For bipolar input mode, the output is two’s
complement (Figure 18). Data is clocked out at the
falling edge of SCLK in MSB-first format.
Clock Modes
Simple Software Interface
Make sure the CPU’s serial interface runs in master
mode so the CPU generates the serial clock. Choose a
clock frequency from 100kHz to 2MHz.
1) Set up the control byte for external clock mode and
call it TB1. TB1 should be of the format: 1XXXXX11
binary, where the Xs denote the particular channel
and conversion mode selected.
2) Use a general-purpose I/O line on the CPU to pull
CS low.
3) Transmit TB1 and, simultaneously, receive a byte
and call it RB1. Ignore RB1.
The MAX146/MAX147 may use either an external
serial clock or the internal clock to perform the successive-approximation conversion. In both clock modes,
the external clock shifts data in and out of the
MAX146/MAX147. The T/H acquires the input signal as
the last three bits of the control byte are clocked into
DIN. Bits PD1 and PD0 of the control byte program the
clock mode. Figures 7–10 show the timing characteristics common to both modes.
CS
tACQ
SCLK
1
4
SEL2 SEL1 SEL0 UNI/
BIP
DIN
8
SGL/ PD1
DIF
12
16
20
24
PD0
START
SSTRB
A/D STATE
B11
MSB
IDLE
RB3
RB2
RB1
DOUT
ACQUISITION
1.5µs
B10
B9
B8
B7
B6
B5
B4
B3
B2
B1
B0
LSB
FILLED WITH
ZEROS
CONVERSION
IDLE
(fSCLK = 2MHz)
Figure 6. 24-Clock External Clock Mode Conversion Timing (Microwire and SPI Compatible, QSPI Compatible with fSCLK ≤ 2MHz)
12
______________________________________________________________________________________
+2.7V, Low-Power, 8-Channel,
Serial 12-Bit ADCs
MAX146/MAX147
•••
CS
tCSH
tCSS
tCH
tCL
SCLK
tCSH
•••
tDS
tDH
•••
DIN
tDV
tDO
tTR
•••
DOUT
Figure 7. Detailed Serial-Interface Timing
•••
CS
•••
tSTR
tSDV
SSTRB
•••
•••
tSSTRB
SCLK
••••
tSSTRB
••••
PD0 CLOCKED IN
Figure 8. External Clock Mode SSTRB Detailed Timing
External Clock
In external clock mode, the external clock not only shifts
data in and out, but it also drives the analog-to-digital
conversion steps. SSTRB pulses high for one clock
period after the last bit of the control byte. Successive-approximation bit decisions are made and appear
at DOUT on each of the next 12 SCLK falling edges
(Figure 6). SSTRB and DOUT go into a high-impedance
state when CS goes high; after the next CS falling edge,
SSTRB outputs a logic low. Figure 8 shows the SSTRB
timing in external clock mode.
The conversion must complete in some minimum time,
or droop on the sample-and-hold capacitors may
degrade conversion results. Use internal clock mode if
the serial clock frequency is less than 100kHz, or if
serial clock interruptions could cause the conversion
interval to exceed 120µs.
Internal Clock
In internal clock mode, the MAX146/MAX147 generate
their own conversion clocks internally. This frees the µP
from the burden of running the SAR conversion clock
and allows the conversion results to be read back at the
______________________________________________________________________________________
13
MAX146/MAX147
+2.7V, Low-Power, 8-Channel,
Serial 12-Bit ADCs
Pulling CS high prevents data from being clocked into
the MAX146/MAX147 and three-states DOUT, but it
does not adversely affect an internal clock mode
conversion already in progress. When internal clock
mode is selected, SSTRB does not go into a highimpedance state when CS goes high.
Figure 10 shows the SSTRB timing in internal clock
mode. In this mode, data can be shifted in and out of
the MAX146/MAX147 at clock rates exceeding 2.0MHz
if the minimum acquisition time (tACQ) is kept above
1.5µs.
processor’s convenience, at any clock rate from 0MHz
to 2MHz. SSTRB goes low at the start of the conversion
and then goes high when the conversion is complete.
SSTRB is low for a maximum of 7.5µs (SHDN = FLOAT),
during which time SCLK should remain low for best
noise performance.
An internal register stores data when the conversion is
in progress. SCLK clocks the data out of this register at
any time after the conversion is complete. After SSTRB
goes high, the next falling clock edge produces the
MSB of the conversion at DOUT, followed by the
remaining bits in MSB-first format (Figure 9). CS does
not need to be held low once a conversion is started.
CS
SCLK
1
2
4
3
5
SEL2 SEL1 SEL0 UNI/
BIP
DIN
7
8
SGL/ PD1
DIF
PD0
6
9
10
11
18
12
19
20
21
22
23
24
START
SSTRB
tCONV
B11
MSB
DOUT
A/D STATE
IDLE
ACQUISITION
1.5µs
CONVERSION
7.5µs MAX
(fSCLK = 2MHz)
(SHDN = FLOAT)
B10
B9
B2
B1
B0
LSB
FILLED WITH
ZEROS
IDLE
Figure 9. Internal Clock Mode Timing
CS
tCONV
tSCK
tCSH
tCSS
SSTRB
tSSTRB
SCLK
tDO
PD0 CLOCK IN
DOUT
NOTE: FOR BEST NOISE PERFORMANCE, KEEP SCLK LOW DURING CONVERSION.
Figure. 10. Internal Clock Mode SSTRB Detailed Timing
14
______________________________________________________________________________________
+2.7V, Low-Power, 8-Channel,
Serial 12-Bit ADCs
The first high bit clocked into DIN with CS low any
time the converter is idle; e.g., after VDD is applied.
__________ Applications Information
OR
Power-On Reset
The first high bit clocked into DIN after bit 5 of a conversion in progress is clocked onto the DOUT pin.
When power is first applied, and if SHDN is not pulled
low, internal power-on reset circuitry activates the
MAX146/MAX147 in internal clock mode, ready to convert with SSTRB = high. After the power supplies stabilize, the internal reset time is 10µs, and no conversions
should be performed during this phase. SSTRB is high
on power-up and, if CS is low, the first logical 1 on DIN
is interpreted as a start bit. Until a conversion takes
place, DOUT shifts out zeros. (Also see Table 4.)
If CS is toggled before the current conversion is complete, the next high bit clocked into DIN is recognized as
a start bit; the current conversion is terminated, and a
new one is started.
The fastest the MAX146/MAX147 can run with CS held
low between conversions is 15 clocks per conversion.
Figure 11a shows the serial-interface timing necessary to
perform a conversion every 15 SCLK cycles in external
CS
1
8
15 1
8
15
1
SCLK
S
DIN
CONTROL BYTE 0
DOUT
S
S
CONTROL BYTE 1
CONTROL BYTE 2
B11 B10 B9 B8 B7 B6 B5 B4 B3 B2 B1 B0
B11 B10 B9 B8 B7 B6 B5 B4 B3 B2 B1 B0
CONVERSION RESULT 0
CONVERSION RESULT 1
SSTRB
Figure 11a. External Clock Mode, 15 Clocks/Conversion Timing
•••
CS
1
8
16
1
8
16
SCLK
DIN
DOUT
S
S
CONTROL BYTE 0
CONVERSION RESULT 0
•••
CONTROL BYTE 1
B11 B10 B9 B8 B7 B6 B5 B4 B3 B2 B1 B0
•••
B11 B10 B9 B8
•••
CONVERSION RESULT 1
Figure 11b. External Clock Mode, 16 Clocks/Conversion Timing
______________________________________________________________________________________
15
MAX146/MAX147
clock mode. If CS is tied low and SCLK is continuous,
guarantee a start bit by first clocking in 16 zeros.
Most microcontrollers (µCs) require that conversions
occur in multiples of 8 SCLK clocks; 16 clocks per conversion is typically the fastest that a µC can drive the
MAX146/MAX147. Figure 11b shows the serialinterface timing necessary to perform a conversion every
16 SCLK cycles in external clock mode.
Data Framing
The falling edge of CS does not start a conversion.
The first logic high clocked into DIN is interpreted as a
start bit and defines the first bit of the control byte. A
conversion starts on SCLK’s falling edge, after the eighth
bit of the control byte (the PD0 bit) is clocked into DIN.
The start bit is defined as follows:
MAX146/MAX147
+2.7V, Low-Power, 8-Channel,
Serial 12-Bit ADCs
Reference-Buffer Compensation
In addition to its shutdown function, SHDN selects internal or external compensation. The compensation
affects both power-up time and maximum conversion
speed. The100kHz minimum clock rate is limited by
droop on the sample-and-hold and is independent of
the compensation used.
Float SHDN to select external compensation. The
Typical Operating Circuit uses a 4.7µF capacitor at
VREF. A 4.7µF value ensures reference-buffer stability
and allows converter operation at the 2MHz full clock
speed. External compensation increases power-up
time (see the Choosing Power-Down Mode section and
Table 4).
Pull SHDN high to select internal compensation.
Internal compensation requires no external capacitor at
VREF and allows for the shortest power-up times. The
maximum clock rate is 2MHz in internal clock mode
and 400kHz in external clock mode.
Choosing Power-Down Mode
You can save power by placing the converter in a lowcurrent shutdown state between conversions. Select full
power-down mode or fast power-down mode via bits 1
and 0 of the DIN control byte with SHDN high or floating
(Tables 1 and 5). In both software power-down modes,
the serial interface remains operational, but the ADC
does not convert. Pull SHDN low at any time to shut
down the converter completely. SHDN overrides bits 1
and 0 of the control byte.
Full power-down mode turns off all chip functions that
draw quiescent current, reducing supply current to 2µA
(typ). Fast power-down mode turns off all circuitry
except the bandgap reference. With fast power-down
mode, the supply current is 30µA. Power-up time can be
shortened to 5µs in internal compensation mode.
Table 4 shows how the choice of reference-buffer compensation and power-down mode affects both power-up
delay and maximum sample rate. In external compensation mode, power-up time is 20ms with a 4.7µF compensation capacitor when the capacitor is initially fully
discharged. From fast power-down, start-up time can be
eliminated by using low-leakage capacitors that do not
discharge more than 1/2LSB while shut down. In powerdown, leakage currents at VREF cause droop on the reference bypass capacitor. Figures 12a and 12b show
the various power-down sequences in both external and
internal clock modes.
Software Power-Down
Software power-down is activated using bits PD1 and PD0
of the control byte. As shown in Table 5, PD1 and PD0
also specify the clock mode. When software shutdown is
asserted, the ADC operates in the last specified clock
mode until the conversion is complete. Then the ADC
powers down into a low quiescent-current state. In internal
clock mode, the interface remains active and conversion
results may be clocked out after the MAX146/MAX147
enter a software power-down.
The first logical 1 on DIN is interpreted as a start bit
and powers up the MAX146/MAX147. Following
the start bit, the data input word or control byte also
determines clock mode and power-down states. For
example, if the DIN word contains PD1 = 1, then the
chip remains powered up. If PD0 = PD1 = 0, a
power-down resumes after one conversion.
Table 4. Typical Power-Up Delay Times
16
REFERENCE
BUFFER
REFERENCEBUFFER
COMPENSATION
MODE
VREF
CAPACITOR
(µF)
POWER-DOWN
MODE
Enabled
Internal
—
Enabled
Internal
—
Enabled
External
4.7
POWER-UP
DELAY
(µs)
MAXIMUM
SAMPLING RATE
(ksps)
Fast
5
26
Full
300
26
Fast
See Figure 14c
133
Enabled
External
4.7
Full
See Figure 14c
133
Disabled
—
—
Fast
2
133
Disabled
—
—
Full
2
133
______________________________________________________________________________________
+2.7V, Low-Power, 8-Channel,
Serial 12-Bit ADCs
MAX146/MAX147
CLOCK
MODE
EXTERNAL
EXTERNAL
SHDN
SETS SOFTWARE
POWER-DOWN
SETS EXTERNAL
CLOCK MODE
DIN
S X X X X X 1 1
DOUT
S X X X X X 0 0
SETS EXTERNAL
CLOCK MODE
S X X X X X 1 1
VALID
DATA
12 DATA BITS
12 DATA BITS
POWERED UP
POWERED UP
MODE
SOFTWARE
POWER-DOWN
INVALID
DATA
HARDWARE
POWERDOWN
POWERED UP
Figure 12a. Timing Diagram Power-Down Modes, External Clock
CLOCK
MODE
DIN
INTERNAL
S X X X X X 1 0
S X X X X X 0 0
DOUT
SSTRB
MODE
SETS
POWER-DOWN
SETS INTERNAL
CLOCK MODE
S
DATA VALID
DATA VALID
CONVERSION
CONVERSION
POWERED UP
POWER-DOWN
POWERED UP
Figure 12b. Timing Diagram Power-Down Modes, Internal Clock
Hardware Power-Down
Pulling SHDN low places the converter in hardware
power-down (Table 6). Unlike software power-down
mode, the conversion is not completed; it stops coincidentally with SHDN being brought low. SHDN also
controls the clock frequency in internal clock mode.
Letting SHDN float sets the internal clock frequency to
1.8MHz. When returning to normal operation with SHDN
floating, there is a tRC delay of approximately 2MΩ x CL,
where CL is the capacitive loading on the SHDN pin.
Pulling SHDN high sets internal clock frequency to
225kHz. This feature eases the settling-time requirement
for the reference voltage. With an external reference, the
MAX146/MAX147 can be considered fully powered up
within 2µs of actively pulling SHDN high.
______________________________________________________________________________________
17
Power-Down Sequencing
The MAX146/MAX147 auto power-down modes can
save considerable power when operating at less than
maximum sample rates. Figures 13, 14a, and 14b show
the average supply current as a function of the sampling rate. The following discussion illustrates the various power-down sequences.
Lowest Power at up to 500
Conversions/Channel/Second
The following examples show two different power-down
sequences. Other combinations of clock rates, compensation modes, and power-down modes may give lowest
power consumption in other applications.
Figure 14a depicts the MAX146 power consumption for
one or eight channel conversions utilizing full powerdown mode and internal-reference compensation. A
0.047µF bypass capacitor at REFADJ forms an RC filter
with the internal 20kΩ reference resistor with a 0.9ms
time constant. To achieve full 12-bit accuracy, 10 time
constants or 9ms are required after power-up. Waiting
this 9ms in FASTPD mode instead of in full power-up
can reduce power consumption by a factor of 10 or
more. This is achieved by using the sequence shown in
Figure 15.
AVERAGE SUPPLY CURRENT
vs. CONVERSION RATE
WITH EXTERNAL REFERENCE
100
8 CHANNELS
1 CHANNEL
1
0.1
0.1
1
10
100
1k
10k
100k
1 CHANNEL
CONVERSION RATE (Hz)
0.1
1
10
100
1k
CONVERSION RATE (Hz)
Figure 13. Average Supply Current vs. Conversion Rate with
External Reference
Figure 14a. MAX146 Supply Current vs. Conversion Rate,
FULLPD
AVERAGE SUPPLY CURRENT
vs. CONVERSION RATE
(USING FASTPD)
TYPICAL REFERENCE-BUFFER POWER-UP
DELAY vs. TIME IN SHUTDOWN
2.0
MAX146/47-Fig14b
10,000
RLOAD = ∞
CODE = 101010100000
POWER-UP DELAY (ms)
1000
8 CHANNELS
100
1 CHANNEL
1.5
1.0
0.5
10
1
0.1
1
10
100
1k
10k
100k
1M
CONVERSION RATE (Hz)
Figure 14b. MAX146 Supply Current vs. Conversion Rate,
FASTPD
18
8 CHANNELS
10
1
0.01
1M
MAX146/47-Fig14a
RLOAD = ∞
CODE = 101010100000
MAX146/47-Fig14c
10
100
AVERAGE SUPPLY CURRENT (µA)
VREF = VDD = 3.0V
RLOAD = ∞
CODE = 101010100000
1000
AVERAGE SUPPLY CURRENT
vs. CONVERSION RATE
(USING FULLPD)
MAX146/47-13
AVERAGE SUPPLY CURRENT (µA)
10,000
AVERAGE SUPPLY CURRENT (µA)
MAX146/MAX147
+2.7V, Low-Power, 8-Channel,
Serial 12-Bit ADCs
0
0.001
0.01
0.1
1
10
TIME IN SHUTDOWN (sec)
Figure 14c. Typical Reference-Buffer Power-Up Delay vs. Time
in Shutdown
______________________________________________________________________________________
+2.7V, Low-Power, 8-Channel,
Serial 12-Bit ADCs
MAX146/MAX147
COMPLETE CONVERSION SEQUENCE
9ms WAIT
DIN
CH1
(ZEROS)
1
00
FULLPD
1
01
FASTPD
(ZEROS)
CH7
1
11
1
00
NOPD
1
FULLPD
01
FASTPD
1.21V
REFADJ
0V
2.50V
τ = RC = 20kΩ x CREFADJ
VREF
0V
tBUFFEN ≈ 200µs
Figure 15. MAX146 FULLPD/FASTPD Power-Up Sequence
Lowest Power at Higher Throughputs
Figure 14b shows the power consumption with
external-reference compensation in fast power-down,
with one and eight channels converted. The external
4.7µF compensation requires a 200µs wait after
power-up with one dummy conversion. This graph
shows fast multi-channel conversion with the lowest
power consumption possible. Full power-down mode
may provide increased power savings in applications
where the MAX146/MAX147 are inactive for long periods of time, but where intermittent bursts of high-speed
conversions are required.
Internal and External References
The MAX146 can be used with an internal or external
reference voltage, whereas an external reference is
required for the MAX147. An external reference can be
connected directly at VREF or at the REFADJ pin.
An internal buffer is designed to provide 2.5V at
VREF for both the MAX146 and the MAX147. The
MAX146’s internally trimmed 1.21V reference is buffered with a 2.06 gain. The MAX147’s REFADJ pin is
also buffered with a 2.00 gain to scale an external 1.25V
reference at REFADJ to 2.5V at VREF.
Internal Reference (MAX146)
The MAX146’s full-scale range with the internal reference is 2.5V with unipolar inputs and ±1.25V with bipolar inputs. The internal reference voltage is adjustable
to ±1.5% with the circuit in Figure 16.
External Reference
With both the MAX146 and MAX147, an external reference can be placed at either the input (REFADJ) or the
output (VREF) of the internal reference-buffer amplifier.
The REFADJ input impedance is typically 20kΩ for the
MAX146, and higher than 100kΩ for the MAX147. At
+3.3V
24k
MAX146
510k
100k
12
REFADJ
0.047µF
Figure 16. MAX146 Reference-Adjust Circuit
Table 5. Software Power-Down and
Clock Mode
PD1
PD0
DEVICE MODE
0
0
Full Power-Down
0
1
Fast Power-Down
1
0
Internal Clock
1
1
External Clock
Table 6. Hard-Wired Power-Down and
Internal Clock Frequency
SHDN
STATE
DEVICE
MODE
REFERENCE
BUFFER
COMPENSATION
INTERNAL
CLOCK
FREQUENCY
1
Enabled
Internal
225kHz
Floating
Enabled
External
1.8MHz
0
Power-Down
N/A
N/A
______________________________________________________________________________________
19
MAX146/MAX147
+2.7V, Low-Power, 8-Channel,
Serial 12-Bit ADCs
VREF, the DC input resistance is a minimum of 18kΩ.
During conversion, an external reference at VREF must
deliver up to 350µA DC load current and have 10Ω or
less output impedance. If the reference has a higher
output impedance or is noisy, bypass it close to the
VREF pin with a 4.7µF capacitor.
Using the REFADJ input makes buffering the external
reference unnecessary. To use the direct VREF input,
disable the internal buffer by tying REFADJ to VDD. In
power-down, the input bias current to REFADJ is typically 25µA (MAX146) with REFADJ tied to VDD. Pull
REFADJ to AGND to minimize the input bias current in
power-down.
OUTPUT CODE
FULL-SCALE
TRANSITION
11 . . . 111
11 . . . 110
11 . . . 101
FS = VREF + COM
ZS = COM
VREF
1LSB =
4096
00 . . . 011
Transfer Function
Table 7 shows the full-scale voltage ranges for unipolar
and bipolar modes.
The external reference must have a temperature coefficient of 4ppm/°C or less to achieve accuracy to within
1LSB over the 0°C to +70°C commercial temperature
range.
Figure 17 depicts the nominal, unipolar input/output
(I/O) transfer function, and Figure 18 shows the bipolar
input/output transfer function. Code transitions occur
halfway between successive-integer LSB values.
Output coding is binary, with 1LSB = 610µV (2.500V /
4096) for unipolar operation, and 1LSB = 610µV
[(2.500V / 2 - -2.500V / 2) / 4096] for bipolar operation.
Layout, Grounding, and Bypassing
For best performance, use printed circuit boards.
Wire-wrap boards are not recommended. Board layout
should ensure that digital and analog signal lines are
separated from each other. Do not run analog and digital (especially clock) lines parallel to one another, or
digital lines underneath the ADC package.
Figure 19 shows the recommended system ground
connections. Establish a single-point analog ground
(star ground point) at AGND, separate from the logic
ground. Connect all other analog grounds and DGND
to the star ground. No other digital system ground
should be connected to this ground. For lowest-noise
operation, the ground return to the star ground’s power
00 . . . 010
00 . . . 001
00 . . . 000
0 1
(COM)
2
3
FS
INPUT VOLTAGE (LSB)
FS - 3/2LSB
Figure 17. Unipolar Transfer Function, Full Scale (FS) = VREF
+ COM, Zero Scale (ZS) = COM
supply should be low impedance and as short as
possible.
High-frequency noise in the VDD power supply may
affect the high-speed comparator in the ADC. Bypass
the supply to the star ground with 0.1µF and 1µF
capacitors close to pin 20 of the MAX146/MAX147.
Minimize capacitor lead lengths for best supply-noise
rejection. If the power supply is very noisy, a 10Ω resistor can be connected as a lowpass filter (Figure 19).
High-Speed Digital Interfacing with QSPI
The MAX146/MAX147 can interface with QSPI using
the circuit in Figure 20 (fSCLK = 2.0MHz, CPOL = 0,
CPHA = 0). This QSPI circuit can be programmed to do a
conversion on each of the eight channels. The result is
stored in memory without taxing the CPU, since QSPI
incorporates its own microsequencer.
The MAX146/MAX147 are QSPI compatible up to the
maximum external clock frequency of 2MHz.
Table 7. Full Scale and Zero Scale
UNIPOLAR MODE
20
BIPOLAR MODE
Full Scale
Zero Scale
Positive
Full Scale
Zero
Scale
Negative
Full Scale
VREF + COM
COM
VREF / 2
+ COM
COM
-VREF / 2
+ COM
______________________________________________________________________________________
+2.7V, Low-Power, 8-Channel,
Serial 12-Bit ADCs
MAX146/MAX147
OUTPUT CODE
011 . . . 111
FS = VREF + COM
2
011 . . . 110
ZS = COM
000 . . . 010
000 . . . 001
-FS =
+3V
-VREF
+ COM
2
1LSB =
000 . . . 000
SUPPLIES
+3V
GND
+3V
DGND
VREF
4096
R* = 10Ω
111 . . . 111
111 . . . 110
111 . . . 101
VDD
AGND
100 . . . 001
100 . . . 000
MAX146
MAX147
- FS
COM*
COM DGND
DIGITAL
CIRCUITRY
+FS - 1LSB
*OPTIONAL
INPUT VOLTAGE (LSB)
*COM ≤ VREF / 2
Figure 18. Bipolar Transfer Function, Full Scale (FS) =
VREF / 2 + COM, Zero Scale (ZS) = COM
TMS320LC3x Interface
Figure 21 shows an application circuit to interface the
MAX146/MAX147 to the TMS320 in external clock mode.
The timing diagram for this interface circuit is shown in
Figure 22.
Use the following steps to initiate a conversion in the
MAX146/MAX147 and to read the results:
1) The TMS320 should be configured with CLKX
(transmit clock) as an active-high output clock and
CLKR (TMS320 receive clock) as an active-high
input clock. CLKX and CLKR on the TMS320 are
tied together with the MAX146/MAX147’s SCLK
input.
2) The MAX146/MAX147’s CS pin is driven low by the
TMS320’s XF_ I/O port to enable data to be clocked
into the MAX146/MAX147’s DIN.
3) An 8-bit word (1XXXXX11) should be written to the
MAX146/MAX147 to initiate a conversion and place
the device into external clock mode. Refer to Table
1 to select the proper XXXXX bit values for your
specific application.
Figure 19. Power-Supply Grounding Connection
4) The MAX146/MAX147’s SSTRB output is monitored
via the TMS320’s FSR input. A falling edge on the
SSTRB output indicates that the conversion is in
progress and data is ready to be received from the
MAX146/MAX147.
5) The TMS320 reads in one data bit on each of the
next 16 rising edges of SCLK. These data bits represent the 12-bit conversion result followed by four
trailing bits, which should be ignored.
6) Pull CS high to disable the MAX146/MAX147 until
the next conversion is initiated.
______________________________________________________________________________________
21
MAX146/MAX147
+2.7V, Low-Power, 8-Channel,
Serial 12-Bit ADCs
+3V
0.1µF
ANALOG
INPUTS
+3V
1µF
(POWER SUPPLIES)
1
CH0
2
CH1
SCLK 19
3
CH2
CS 18
PCS0
4
CH3
DIN 17
MOSI
5
CH4
6
CH5
DOUT 15
7
CH6
DGND 14
8
CH7
AGND 13
9
COM
REFADJ 12
10 SHDN
VREF 11
VDD
MAX146
MAX147
20
SCK
MC683XX
SSTRB 16
MISO
(GND)
0.1µF
+2.5V
Figure 20. MAX146/MAX147 QSPI Connections, External Reference
XF
CLKX
CS
SCLK
TMS320LC3x
MAX146
MAX147
CLKR
DX
DIN
DR
DOUT
FSR
SSTRB
Figure 21. MAX146/MAX147-to-TMS320 Serial Interface
22
______________________________________________________________________________________
+2.7V, Low-Power, 8-Channel,
Serial 12-Bit ADCs
MAX146/MAX147
CS
SCLK
DIN
START
SEL2
SEL1
SEL0
UNI/BIP SGL/DIF
PD1
PD0
HIGH
IMPEDANCE
SSTRB
DOUT
MSB
B10
B1
HIGH
IMPEDANCE
LSB
Figure 22. TMS320 Serial-Interface Timing Diagram
_Ordering Information (continued)
PART†
TEMP. RANGE
PIN-PACKAGE
INL
(LSB)
MAX146AEPP
-40°C to +85°C
20 Plastic DIP
±1/2
MAX146BEPP
MAX146AEAP
MAX146BEAP
-40°C to +85°C
-40°C to +85°C
-40°C to +85°C
20 Plastic DIP
20 SSOP
20 SSOP
±1
±1/2
±1
MAX146AMJP
-55°C to +125°C
20 CERDIP**
±1/2
MAX147BCPP
-55°C to +125°C
0°C to +70°C
0°C to +70°C
MAX147ACAP
0°C to +70°C
20 SSOP
±1/2
MAX147BCAP
0°C to +70°C
20 SSOP
±1
MAX147BC/D
MAX147AEPP
MAX147BEPP
0°C to +70°C
-40°C to +85°C
-40°C to +85°C
Dice*
20 Plastic DIP
20 Plastic DIP
±1
±1/2
±1
MAX147AEAP
-40°C to +85°C
20 SSOP
±1/2
MAX147BEAP
MAX147AMJP
MAX147BMJP
-40°C to +85°C
-55°C to +125°C
-55°C to +125°C
20 SSOP
20 CERDIP**
20 CERDIP**
±1
±1/2
±1
MAX146BMJP
MAX147ACPP
20 CERDIP**
20 Plastic DIP
20 Plastic DIP
±1
±1/2
±1
Contact factory for availability of alternate surface-mount
packages.
* Dice are specified at TA = +25°C, DC parameters only.
** Contact factory for availability of CERDIP package, and for
processing to MIL-STD-883B.
†
__________________Pin Configuration
TOP VIEW
CH0 1
20 VDD
CH1 2
19 SCLK
18 CS
CH2 3
CH3 4
CH4 5
MAX146
MAX147
17 DIN
16 SSTRB
CH5 6
15 DOUT
CH6 7
14 DGND
CH7 8
13 AGND
COM 9
12 REFADJ
11 VREF
SHDN 10
DIP/SSOP
___________________Chip Information
TRANSISTOR COUNT: 2554
______________________________________________________________________________________
23
________________________________________________________Package Information
SSOP.EPS
MAX146/MAX147
+2.7V, Low-Power, 8-Channel,
Serial 12-Bit ADCs
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.
24 __________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 (408) 737-7600
© 1997 Maxim Integrated Products
Printed USA
is a registered trademark of Maxim Integrated Products.