Maxim MAX11040GUU+ 24-bit, 4-channel, simultaneous-sampling,cascadable,sigma-delta adc Datasheet

19-4302; Rev 2; 6/10
KIT
ATION
EVALU
E
L
B
A
AVAIL
24-Bit, 4-Channel, Simultaneous-Sampling,
Cascadable, Sigma-Delta ADC
The MAX11040 is a 24-bit, 4-channel, simultaneoussampling, sigma-delta analog-to-digital converter
(ADC). The device allows simultaneous sampling of as
many as 32 channels using a built-in cascade feature
to synchronize as many as eight devices. The serial
interface of the MAX11040 allows reading data from all
the cascaded devices using a single command. Four
modulators simultaneously convert each fully differential analog input with a programmable data output rate
ranging from 0.25ksps to 64ksps. The device achieves
106dB SNR at 16ksps and 117dB SNR at 1ksps. The
MAX11040 operates from a single +3V supply. The differential analog input range is ±2.2V when using the
internal reference, an external reference is optional.
Each input is overvoltage protected up to ±6V without
damage. The device uses an internal crystal oscillator
or an external source for clock.
The MAX11040 is compatible with SPI™, QSPI™,
MICROWIRE™, and DSP-compatible 4-wire serial interfaces. An on-board interface logic allows one serial interface (with a single chip select) to control up to eight
cascaded devices or 32 simultaneous sampling analog
input channels.
The MAX11040 is ideally suited for power-management
systems. Each channel includes an adjustable sampling phase enabling internal compensation for phase
shift due to external dividers, transformers, or filters at
the inputs. The output data rate is adjustable with a
0.065% resolution (at 16ksps or below) to track the
varying frequency of a periodic input. A SYNC input
allows periodic alignment of the conversion timing of
multiple devices with a remote timing source.
The MAX11040 is available in a 38-pin TSSOP package
specified over the -40°C to +105°C industrial temperature range.
Applications
Power Protection Relay Equipment
Multiphase Power Systems
Industrial Data-Acquisition Systems
Medical Instrumentation
Features
♦ Four Fully Differential Simultaneously Sampled
Channels
♦ Cascadable for Up to 32 Channels of
Simultaneous Sampling
♦ 106dB SNR at 16ksps
♦ 117dB SNR at 1ksps
♦ 0.25% Error Over a 1000:1 Dynamic Range,
Processed Over 16.7ms
♦ ±2.2V Full-Scale Input Range
♦ ±6V Overvoltage Protected Inputs
♦ Internal Crystal Oscillator
♦ 2.5V, 50ppm/°C Internal Reference or External
Reference
♦ Programmable Output Data Rate
0.25ksps to 64ksps Range
0.065% Resolution
♦ Programmable Sampling Phase
0 to 333µs Delay in 1.33µs Steps
♦ SPI-/QSPI-/MICROWIRE-/DSP-Compatible 4-Wire
Serial Interface
♦ Cascadable Interface Allows Control of Up to
Eight Devices with a Single CS Signal
♦ 3.0V to 3.6V Analog Supply Voltage
♦ 2.7V to VAVDD Digital Supply Voltage
♦ 38-Pin TSSOP Package
Functional Diagram
OVRFLW
AIN0+
24-BIT
ADC
AIN0-
FAULT
SYNC
DIGITAL
FILTER
DRDYIN
REF0
AIN1+
AIN1REF1
AIN2+
AIN2-
24-BIT
ADC
DIGITAL
FILTER
24-BIT
ADC
DIGITAL
FILTER
24-BIT
ADC
DIGITAL
FILTER
DRDYOUT
REGISTERS AND
DIGITAL
CONTROL
SERIAL
INTERFACE
CASCIN
CASCOUT
CS
SCLK
REF2
AIN3+
AIN3-
DIN
DOUT
REF3
Ordering Information
MAX11040
PART
TEMP RANGE
PIN-PACKAGE
MAX11040GUU+
-40°C to +105°C
38 TSSOP
+Denotes a lead(Pb)-free/RoHS-compliant package.
MICROWIRE is a trademark of National Semiconductor Corp.
SPI/QSPI are trademarks of Motorola Inc.
2.5V
REFERENCE
REFIO
AGND
CRYSTAL
OSCILLATOR
XIN
XOUT
CLKOUT
DGND
Pin Configuration appears at end of data sheet.
________________________________________________________________ Maxim Integrated Products
For pricing, delivery, and ordering information, please contact Maxim Direct at 1-888-629-4642,
or visit Maxim’s website at www.maxim-ic.com.
1
MAX11040
General Description
MAX11040
24-Bit, 4-Channel, Simultaneous-Sampling,
Cascadable, Sigma-Delta ADC
ABSOLUTE MAXIMUM RATINGS
AIN_ _ to AGND (VAVDD < 3V or VDVDD < 2.7V or
FAULTDIS = 1 or SHDN = 1 or
fXIN CLOCK < 20MHz)........................................-3.5V to +3.5V
REFIO, REF_ to AGND............................-0.3V to (VAVDD + 0.3V)
Maximum Current into Any Pin............................................50mA
Continuous Power Dissipation (TA = +70°C)
38-Pin TSSOP (derated 13.7mW/°C above +70°C) ...1096mW
Operating Temperature Range .........................-40°C to +105°C
Storage Temperature Range .............................-60°C to +150°C
Lead Temperature (soldering, 10s) .................................+300°C
Soldering Temperature (reflow) .......................................+260°C
AVDD to AGND ........................................................-0.3V to +4V
DVDD to DGND ......................................-0.3V to (VAVDD + 0.3V)
AGND to DGND.....................................................-0.3V to +0.3V
DIN, SCLK, CS, XIN, SYNC, DRDYIN,
CASCIN to DGND..............................-0.3V to (VDVDD + 0.3V)
DOUT, DRDYOUT, CASCOUT, CLKOUT,
XOUT to DGND..................................-0.3V to (VDVDD + 0.3V)
FAULT, OVRFLW to DGND ...................................-0.3V to +4.0V
AIN_+ to AIN_- ......................................................-6.0V to +6.0V
AIN_ _to AGND (VAVDD ≥ 3V, VDVDD ≥ 2.7V, FAULTDIS = 0,
SHDN = 0, fXIN CLOCK ≥ 20MHz)....................-6.0V to +6.0V
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
(VAVDD = +3.0V to +3.6V, VDVDD = +2.7V to VAVDD, fXIN CLOCK = 24.576MHz, fOUT = 16ksps, VREFIO = +2.5V (external), CREFIO =
CREF0 = CREF1 = CREF2 = CREF3 = 1µF to AGND, TA = TMIN to TMAX, unless otherwise noted. Typical values are at TA = +25°C.) (Note 1)
PARAMETER
SYMBOL
CONDITIONS
MIN
TYP
MAX
UNITS
DC ACCURACY (Note 2)
Resolution
24
Differential Nonlinearity
DNL
Integral Nonlinearity (Note 3)
INL
24-bit no missing code
Bits
0.1
TA = +25°C and +105°C
0.001
TA = -40°C
Offset Error
LSB
0.004
0.006
-1
+1
-1
mV
Gain Error
(Note 4)
Offset-Error Drift
(Note 5)
0.5
ppm/°C
Gain-Error Drift
(Note 5)
1
ppm/°C
Change in Gain Error vs. fOUT
fOUT = 0.25ksps to 64ksps
Channel-to-Channel Gain Matching
+1
% FS
% FS
< 0.025
% FS
0.03
% FS
DYNAMIC SPECIFICATIONS (62.5Hz sine-wave input, 2.17VP-P)
Signal-to-Noise Ratio
Total Harmonic Distortion
SNR
THD
Signal-to-Noise Plus Distortion
SINAD
Spurious-Free Dynamic Range
SFDR
Relative Accuracy (Note 7)
(Note 6)
103
106
dB
TA = +25°C and +105°C
-94
TA = -40°C
-90
TA = +25°C and +105°C
93
TA = -40°C
89
TA = +25°C and +105°C
94
TA = -40°C
89
98
dB
dB
100
dB
0.1% FS input
0.25
6.0% FS input
0.005
%
Bandwidth
-3dB
3.4
Latency
(Note 8)
405
µs
Passband Flatness
From DC to 1.4kHz
< 0.1
dB
Amplitude-Dependent Phase Error
FS vs. 0.1% FS
< 0.01
kHz
0.12
Deg
Channel-to-Channel Phase Matching
0.0001
Deg
Phase-Error Drift
0.001
Deg
2
_______________________________________________________________________________________
24-Bit, 4-Channel, Simultaneous-Sampling,
Cascadable, Sigma-Delta ADC
(VAVDD = +3.0V to +3.6V, VDVDD = +2.7V to VAVDD, fXIN CLOCK = 24.576MHz, fOUT = 16ksps, VREFIO = +2.5V (external), CREFIO =
CREF0 = CREF1 = CREF2 = CREF3 = 1µF to AGND, TA = TMIN to TMAX, unless otherwise noted. Typical values are at TA = +25°C.) (Note 1)
PARAMETER
SYMBOL
CONDITIONS
MIN
Channel-to-Channel Isolation
Comon-Mode Rejection
CMRR
TYP
MAX
UNITS
-130
dB
109
dB
ANALOG INPUTS (AIN_+, AIN_-)
Differential FS Input Range
VAIN_+ - VAIN_-
-2.2
+2.2
V
Positive Fault Threshold
VPFT
VIN
VAIN_+ or VAIN_- (Note 9)
2.25
2.65
V
Negative Fault Threshold
VNFT
VAIN_+ or VAIN_- (Note 9)
-2.65
Fault Pin Response Time
-2.25
2.5
Input Impedance
ZIN
DC Leakage Current
Input Sampling Rate
VNFT ≤ VIN ≤ VPFT
V
µs
130
kΩ
VIN < VNFT or VIN > VPFT
> 0.5
IIN
VAIN_+ = VAIN_-
±0.01
fS
fS = fXINCLOCK/8
3.072
Msps
4.0
pF
Input Sampling Capacitance
±1
µA
INTERNAL REFERENCE
REFIO Output Voltage
VREF
TA = TMAX
2.4
2.5
2.6
V
REFIO Output Resistance
1
kΩ
REFIO Temp Drift
50
ppm/°C
REFIO Long-Term Stability
200
ppm/
1000hr
3
µVRMS
75
dB
REFIO Output Noise
REFIO Power-Supply Rejection
PSRR
EXTERNAL REFERENCE
REFIO Input Voltage
VREF
2.3
2.7
V
REFIO Sink Current
200
µA
REFIO Source Current
200
µA
REFIO Input Capacitance
10
pF
24.576
MHz
CRYSTAL OSCILLATOR (XIN, XOUT)
Tested Resonant Frequency
(Note 10)
Maximum Crystal ESR
30
Ω
Oscillator Startup Time
<2
ms
10
ppm/°C
10
pF
Oscillator Stability
VDVDD = 3.3V, excluding crystal
Maximum Oscillator Load Capacitance
DIGITAL INPUTS (SCLK, CS, DIN, SYNC, CASCIN, DRDYIN, XIN)
Input Low Voltage
VIL
Input High Voltage
VIH
Input Hysteresis
VHYS
Input Leakage Current
Input Capacitance
0.3 x
VDVDD
0.7 x
VDVDD
VDVDD = 3.0V
V
V
100
IL
±0.01
CIN
15
mV
±1
µA
pF
_______________________________________________________________________________________
3
MAX11040
ELECTRICAL CHARACTERISTICS (continued)
MAX11040
24-Bit, 4-Channel, Simultaneous-Sampling,
Cascadable, Sigma-Delta ADC
ELECTRICAL CHARACTERISTICS (continued)
(VAVDD = +3.0V to +3.6V, VDVDD = +2.7V to VAVDD, fXIN CLOCK = 24.576MHz, fOUT = 16ksps, VREFIO = +2.5V (external), CREFIO =
CREF0 = CREF1 = CREF2 = CREF3 = 1µF to AGND, TA = TMIN to TMAX, unless otherwise noted. Typical values are at TA = +25°C.) (Note 1)
PARAMETER
SYMBOL
CONDITIONS
MIN
TYP
MAX
UNITS
CMOS DIGITAL OUTPUTS (DOUT, CASCOUT, DRDYOUT, CLKOUT)
Output Low Voltage
VOL
I SINK = 5mA
Output High Voltage
VOH
I SOURCE = 1mA
Three-State Leakage Current
Three-State Capacitance
0.15 x
VDVDD
0.85 x
VDVDD
ILT
COUT
V
V
±1
15
μA
pF
OPEN-DRAIN DIGITAL OUTPUTS (OVRFLW, FAULT)
Output Low Voltage
VOL
I SINK = 5mA
Output High Voltage
VOH
Internal pullup only
0.15 x
VDVDD
0.85 x
VDVDD
Internal Pullup Resistance
V
V
30
k
POWER REQUIREMENTS
Analog Supply Voltage
AVDD
Digital Supply Voltage
DVDD
3.0
3.6
2.7
V
VAVDD
V
35
mA
Normal operation
25
Shutdown and f XINCLOCK = 0Hz
0.1
5
μA
Normal operation
11
15
mA
Shutdown and f XINCLOCK = 0Hz
0.3
μA
AC Positive-Supply Rejection
VAVDD = 3.3V + 100mVP-P at 1kHz
70
dB
DC Positive-Supply Rejection
VAVDD = VDVDD = 3.0V to 3.6V
75
dB
Human Body Model
2.5
kV
Analog Supply Current (Note 11)
Digital Supply Current (Note 11)
IAVDD
IDVDD
ESD PROTECTION
All Pins
ESD
TIMING CHARACTERISTICS (Figures 7–10)
SCLK Clock Period
SCLK Pulse Width (High and Low)
DIN or CS to SCLK Fall Setup
SCLK Fall to DIN or CS Hold
tSCP
tPW
50
20
ns
tSU
tHD
10
ns
0
ns
SCLK Rise to DOUT Valid
tDOT
CS Fall to DOUT Enable
tDOE
tDOD
CS Rise to DOUT Disable
CS Pulse Width
CASCIN to SCLK Rise Setup
tCSW
tSC
SCLK Rise to CASCOUT Valid
tCOT
4
CLOAD = 30pF
1.5
CLOAD = 100pF
ns
10
16
< 16
ns
CLOAD = 30pF
0.3
20
ns
CLOAD = 30pF
0.7
16
ns
16
ns
16
ns
CLOAD = 100pF
_______________________________________________________________________________________
20
ns
24-Bit, 4-Channel, Simultaneous-Sampling,
Cascadable, Sigma-Delta ADC
(VAVDD = +3.0V to +3.6V, VDVDD = +2.7V to VAVDD, fXIN CLOCK = 24.576MHz, fOUT = 16ksps, VREFIO = +2.5V (external), CREFIO =
CREF0 = CREF1 = CREF2 = CREF3 = 1µF to AGND, TA = TMIN to TMAX, unless otherwise noted. Typical values are at TA = +25°C.) (Note 1)
PARAMETER
SYNC Pulse Width
SYMBOL
CONDITIONS
tSYN
XIN Clock Pulse Width
DRDYIN to DRDYOUT
XIN Clock to DRDYOUT Delay
tXPW
tDRDY
tXDRDY
XIN Clock Period
tXP
XIN Clock to SYNC Setup
tSS
tHS
SYNC to XIN Clock Hold
XIN to CLKOUT Delay
Power-On Reset Delay
MIN
TYP
MAX
UNITS
XIN
Clock
Cycles
2
16
ns
CLOAD = 30pF
DRDYIN = DGND
20
ns
40
ns
40
ns
(Note 12)
16
ns
(Note 12)
5
ns
tXCD
40
(Note 13)
<1
ns
ms
Note 1: Devices are production tested at +105°C. Specifications to -40°C are guaranteed by design.
Note 2: Tested at VAVDD = VDVDD = +3.0V.
Note 3: Integral nonlinearity is the deviation of the analog value at any code from its ideal value after the offset and gain errors are
removed.
Note 4: Offset nulled.
Note 5: Offset and gain drift defined as change in offset and gain error vs. full scale.
Note 6: Noise measured with AIN_+ = AIN_- = AGND.
Note 7: Relative accuracy is defined as the difference between the actual RMS amplitude and the ideal RMS amplitude of a 62.5Hz
sine wave, measured over one cycle at a 16ksps data rate, expressed as a fraction of the ideal RMS amplitude. The relative accuracy specification refers to the maximum error expected over 1 million measurements. Calculated from SNR. Not
production tested.
Note 8: Latency is a function of the sampling rate and XIN clock. See the Latency section in the Digital Filter section.
Note 9: Voltage levels below the positive fault threshold and above the negative fault threshold, relative to AGND on each individual AIN_+ and AIN_- input, do not trigger the analog input protection circuitry.
Note 10: Test performed using RXD MP35.
Note 11: All digital inputs at DGND or DVDD.
Note 12: SYNC is captured by the subsequent XIN clock if this specification is violated.
Note 13: Delay from DVDD exceeds 2.0V until digital interface is operational.
_______________________________________________________________________________________
5
MAX11040
ELECTRICAL CHARACTERISTICS (continued)
Typical Operating Characteristics
(VAVDD = VDVDD = 3.3V, fXIN CLOCK = 24.576MHz, fOUT = 16ksps, VREFIO = 2.5V (external), CREFIO = CREF0 = CREF1 = CREF2 =
CREF3 = 1µF, TA = +25°C, unless otherwise noted.)
400
350
INSTANCES
250
1.5
2.5
DIFFERENTIAL INPUT VOLTAGE (V)
1 MILLION 62.5Hz CYCLES
0.1
0.01
0.001
0.10030
0.5
0.10024
-0.5
0.10018
0
-1.5
0.10012
50
-0.005
-2.5
0.10006
100
-0.004
0.10000
-0.003
0.09994
150
0.09988
200
-0.002
0.09982
0
-0.001
300
0.09976
0.001
0.09970
INL (% FS)
0.002
1
MAX11040 toc03
0.003
450
MAXIMUM EXPECTED ERROR (%)
0.004
MAX11040 toc02
500
MAX11040 toc01
0.005
MAXIMUM EXPECTED ERROR OF CALCULATED
RMS AMPLITUDE vs. INPUT AMPLITUDE
HISTOGRAM OF RMS AMPLITUDE
AT 0.1% FS
INL vs. DIFFERENTIAL INPUT VOLTAGE
0.01
0.1
1
10
100
INPUT AMPLITUDE (% FS)
RMS AMPLITUDE (% FS)
100
90
-60
-80
1
10
-120
-120
-160
-180
0
0
1000 2000 3000 4000 5000 6000 7000 8000
1000 2000 3000 4000 5000 6000 7000 8000
OUTPUT DATA RATE (ksps)
FREQUENCY (Hz)
FREQUENCY (Hz)
RMS AMPLITUDE
vs. INPUT FREQUENCY
RMS AMPLITUDE GAIN ERROR
vs. OUTPUT DATA RATE
RMS AMPLITUDE
vs. SOURCE RESISTANCE
-0.1
-0.2
-0.3
-0.4
0.03
0.02
0.01
0
-0.01
-0.02
100
1000
INPUT FREQUENCY (Hz)
10,000
0
-1
-2
-3
-4
-5
-6
-0.03
-7
-0.04
-8
-0.05
-0.5
1
MAX11040 toc09
0.04
RMS AMPLITUDE (dB)
0
0.05
MAX11040 toc08
MAX11040 toc07
0.1
10
-100
-140
100
RMS AMPLITUDE GAIN ERROR (%)
0.1
-80
-100
-140
80
6
-40
60Hz SINE-WAVE INPUT
-60
AMPLITUDE (dB FS)
110
-40
MAX11040 toc05
60Hz SINE-WAVE INPUT
-20
AMPLITUDE (dB FS)
SNR (dB)
120
FFT vs. FREQUENCY AT 0.1% FULL SCALE
FFT vs. FREQUENCY AT FULL SCALE
0
MAX11040 toc04
130
MAX11040 toc06
SIGNAL-TO-NOISE RATIO
vs. OUTPUT DATA RATE
RMS AMPLITUDE (dB)
MAX11040
24-Bit, 4-Channel, Simultaneous-Sampling,
Cascadable, Sigma-Delta ADC
-9
100
1000
10,000
OUTPUT DATA RATE (Hz)
100,000
10
100
1000
10,000
SOURCE RESISTANCE (Ω)
_______________________________________________________________________________________
100,000
24-Bit, 4-Channel, Simultaneous-Sampling,
Cascadable, Sigma-Delta ADC
-120
500
0.06
0.02
0
-0.02
-0.04
1000
1500
-0.08
-0.10
3.0
3.1
3.2
3.3
3.4
GAIN ERROR vs. SUPPLY VOLTAGE
GAIN ERROR
vs. TEMPERATURE
-0.2
-0.4
0.2
0
-0.2
-0.8
-1.0
3.3
3.4
3.5
-0.08
3.6
-0.10
-40
-15
10
35
60
0 100 200 300 400 500 600 700 800 900 1000
85
TIME (h)
SUPPLY CURRENT
vs. SUPPLY VOLTAGE
SUPPLY CURRENT
vs. TEMPERATURE
SHUTDOWN SUPPLY CURRENT
vs. SUPPLY VOLTAGE
10
20
IAVDD
15
10
5
5
0
0
AVDD = DVDD
400
SUPPLY CURRENT (nA)
IDVDD
AVDD = DVDD = 3.3V
25
SUPPLY CURRENT (mA)
IAVDD
500
MAX11040 toc17
30
MAX11040 toc16
25
15
-0.06
TEMPERATURE (°C)
AVDD = DVDD
105
-0.04
SUPPLY VOLTAGE (V)
30
20
76
-0.4
-1.0
3.2
47
-0.02
-0.6
3.1
18
0
MAX11040 toc14
0.4
-0.8
3.0
-11
GAIN ERROR DRIFT
0.6
-0.6
-40
GAIN ERROR (%)
0
3.6
TEMPERATURE (°C)
AVDD = DVDD = 3.3V
0.8
GAIN ERROR (% FSR)
0.2
3.5
1.0
MAX11040 toc13
0.4
-0.04
-0.06
SUPPLY VOLTAGE (V)
0.6
0
-0.02
-0.08
INPUT FREQUENCY (Hz)
AVDD = DVDD
0.8
0.02
-0.06
2000
1.0
0.04
IDVDD
MAX11040 toc18
0
GAIN ERROR (% FSR)
0.04
-0.10
-130
SUPPLY CURRENT (mA)
MAX11040 toc11
0.06
AVDD = DVDD
0.08
MAX11040 toc15
-110
OFFSET ERROR vs. TEMPERATURE
0.10
OFFSET ERROR (% FSR)
-100
AVDD = DVDD
0.08
OFFSET ERROR (% FSR)
-90
THD (dB)
OFFSET ERROR vs. SUPPLY VOLTAGE
0.10
MAX11040 toc10
-80
MAX11040 toc12
TOTAL HARMONIC DISTORTION
vs. INPUT FREQUENCY
300
200
IDVDD
100
IAVDD
3.0
3.1
3.2
3.3
3.4
SUPPLY VOLTAGE (V)
3.5
3.6
0
-40
-11
18
47
TEMPERATURE (°C)
76
105
3.0
3.1
3.2
3.3
3.4
3.5
3.6
SUPPLY VOLTAGE (V)
_______________________________________________________________________________________
7
MAX11040
Typical Operating Characteristics (continued)
(VAVDD = VDVDD = 3.3V, fXIN CLOCK = 24.576MHz, fOUT = 16ksps, VREFIO = 2.5V (external), CREFIO = CREF0 = CREF1 = CREF2 =
CREF3 = 1µF, TA = +25°C, unless otherwise noted.)
Typical Operating Characteristics (continued)
(VAVDD = VDVDD = 3.3V, fXIN CLOCK = 24.576MHz, fOUT = 16ksps, VREFIO = 2.5V (external), CREFIO = CREF0 = CREF1 = CREF2 =
CREF3 = 1µF, TA = +25°C, unless otherwise noted.)
SHUTDOWN SUPPLY CURRENT
vs. TEMPERATURE
CRYSTAL OSCILLATOR STARTUP TIME
MAX11040 toc20
MAX11040 toc19
1000
AVDD = DVDD = 3.6V
800
SUPPLY CURRENT (nA)
MAX11040
24-Bit, 4-Channel, Simultaneous-Sampling,
Cascadable, Sigma-Delta ADC
CLKOUT
500mV/div
600
IAVDD
400
IDVDD
200
0
-40
-11
18
47
76
105
40μs/div
TEMPERATURE (°C)
Pin Description
PIN
NAME
1
AIN0-
Negative Analog Input Channel 0
2
AIN0+
Positive Analog Input Channel 0
3
REF0
ADC0 Buffered Reference Voltage. Bypass REF0 with a 1µF capacitor to AGND.
4, 8, 10,
29, 31, 35
AGND
Analog Ground
5
AIN1-
Negative Analog Input Channel 1
6
AIN1+
Positive Analog Input Channel 1
7
REF1
ADC1 Buffered Reference Voltage. Bypass REF1 with a 1µF capacitor to AGND.
9
REFIO
Reference Voltage Output/Input. Reference voltage for analog-to-digital conversion. In internal reference
mode, the reference buffer provides a +2.5V nominal output. In external reference mode, overdrive REFIO
with an external reference between 2.3V to 2.7V. Bypass REFIO with a 1µF capacitor to AGND.
11, 28
DGND
Digital Ground
DVDD
Positive Digital Supply Voltage. Bypass each DVDD to DGND with a 1µF capacitor in parallel with a
0.01µF capacitor as close as possible to the device.
12, 27
13
14
15
8
CASCIN
FUNCTION
Cascade Input. A logic-low on CASCIN while CS is a logic-low during the last cycle of a byte signals the
device to perform the requested data transfer during subsequent bytes using DIN and DOUT. Once the
requested transfer is completed, the part three-states DOUT and ignores DIN until a new command is
issued. CASCIN is clocked in at the rising edge of SCLK. Connect CASCIN to DGND when not daisy
chaining multiple devices. See the Multiple Device Connection section for connection recommendations.
Cascade Output. CASCOUT is driven low during the last cycle of the last byte of a data transfer to signal
the next device in the daisy chain to begin transferring data on the next byte. CASCOUT changes after the
CASCOUT
rising edge of SCLK. Leave CASCOUT unconnected when not daisy chaining multiple devices. See the
Multiple Device Connection section.
CS
Active-Low Chip-Select Input. A falling edge on CS while CASCIN is a logic-low enables DIN and DOUT
for data transfer. A logic-high on CS prevents data from being clocked in on DIN and places DOUT in a
high-impedance state.
_______________________________________________________________________________________
24-Bit, 4-Channel, Simultaneous-Sampling,
Cascadable, Sigma-Delta ADC
PIN
NAME
16
SCLK
17
DIN
FUNCTION
Serial-Clock Input. Clocks in data at DIN on the falling edge of SCLK and clocks out data at DOUT on the
rising edge of SCLK.
Serial Data Input. Data at DIN is clocked in on the falling edge of SCLK.
18
DOUT
Serial Data Output. The drive for DOUT is enabled by a falling edge on CS while CASCIN is low or by a
falling edge on CASCIN while CS is low. DOUT is disabled/three-stated when CS is high or after the
appropriate number of data bytes have been transferred in response to the requested command. Data is
clocked out at DOUT on the rising edge of SCLK.
19
FAULT
Active-Low Overvoltage Fault Indicator Output. FAULT goes low when any analog input goes outside the fault
threshold range (between VPFT and VNFT). The FAULT output is open drain with a 30kΩ internal pullup
resistor, allowing wire-NOR functionality. See the Analog Input Overvoltage and Fault Protection section.
20
OVRFLW
Active-Low Channel Data Overflow Output. OVRFLW goes low when a conversion result goes outside the
voltage range bounded by the positive and negative full scale on one or more of the analog input
channels or when FAULT goes low. The OVRFLW output is open drain with a 30kΩ internal pullup resistor,
allowing wire-NOR functionality. See the Analog Input Overvoltage and Fault Protection section.
21
CLKOUT
Buffered Clock Output. When the XTALEN bit in the configuration register is 1 and a crystal is installed
between XIN and XOUT, CLKOUT provides a buffered version of the internal oscillator’s clock. Setting the
XTALEN bit to 0 places CLKOUT in a high-impedance state.
22
23
Active-Low Data Ready Output. When DRDYIN = 0, DRDYOUT outputs a logic-low to indicate the
DRDYOUT availability of a new conversion result. DRDYOUT transitions high at the next CS falling edge or when
DRDYIN = 1. See the Multiple Device Connection section.
DRDYIN
Active-Low Data Ready Input. A logic-high at DRDYIN causes DRDYOUT to output a logic-high. When
DRDYIN = 0, DRDYOUT outputs a logic-low when a new conversion result is available. See the Multiple
Device Connection section. Connect DRDYIN to DGND when not daisy chaining multiple devices.
24
SYNC
Sampling Synchronization Input. The falling edge of SYNC aligns sampling and output data so that
multiple devices sample simultaneously. Synchronize multiple devices running from independent crystals
by connecting DRDYOUT of the last device in the chain to the SYNC inputs of all devices in the chain.
Connect SYNC to DGND for single device operation. See the Multiple Device Connection section.
25
XOUT
Crystal Oscillator Output. Connect a 24.576MHz external crystal or resonator between XIN and XOUT
when using the internal oscillator. Leave XOUT unconnected when driving the MAX11040 with an external
frequency. See the Crystal Oscillator section.
26
XIN
Crystal Oscillator/Clock Input. Connect a 24.576MHz external crystal or resonator between XIN and XOUT
when using the internal oscillator or drive XIN with an external clock and leave XOUT unconnected. See
the Crystal Oscillator section.
30
AVDD
Positive Analog Supply Voltage. Bypass to AGND with a 1µF capacitor in parallel with a 0.01µF capacitor
as close as possible to the device.
32
REF3
ADC3 Buffered Reference Voltage. Bypass with a 1µF capacitor to AGND.
33
AIN3+
Positive Analog Input Channel 3
34
AIN3-
Negative Analog Input Channel 3
36
REF2
ADC2 Buffered Reference Voltage. Bypass with a 1µF capacitor to AGND.
37
AIN2+
Positive Analog Input Channel 2
38
AIN2-
Negative Analog Input Channel 2
_______________________________________________________________________________________
9
MAX11040
Pin Description (continued)
24-Bit, 4-Channel, Simultaneous-Sampling,
Cascadable, Sigma-Delta ADC
MAX11040
Typical Operating Circuit
1μF
3.3V
3.3V
0.01μF
20pF
DVDD
XIN
AIN0+
AIN0-
24.576MHz
REF0
XOUT
1μF
20pF
MAX11040
AIN1+
AIN1-
The MAX11040 operates from a single 3.0V to 3.6V
analog supply and a 2.7V to VAVDD digital supply. The
4-wire serial interface is SPI/QSPI/MICROWIRE and
DSP compatible.
1μF
0.01μF
AVDD
AIN0+
AIN0-
AIN1+
AIN1REF1
CLKOUT
CASCOUT
CASCIN
1μF
AIN2+
AIN2-
AIN2+
AIN2-
OVRFLW
FAULT
REF2
1μF
CS
AIN3+
AIN3-
the effective sample rate of the ADC, is software programmable.
AIN3+
AIN3REF3
1μF
SCLK
DIN
DOUT
MICROCONTROLLER
OR DSP
ADC Modulator
Each channel of the MAX11040 performs analog-todigital conversion on its input using a dedicated
switched-capacitor sigma-delta modulator. The modulator converts the input signal into low-resolution digital data
for which the average value represents the digitized signal information at 3.072Msps for a 24.576MHz XIN clock.
This data stream is then presented to the digital filter for
processing to remove the high-frequency noise that creates a high-resolution 24-bit output data stream.
The input sampling network of the analog input consists
of a pair of 4pF capacitors (C SAMPLE ), the bottom
plates of which are connected to AIN_+ and AIN_- during the track phase and then shorted together during
the hold phase (see Figure 1). The internal switches
have a total series resistance of 400Ω. Given a
24.576MHz XIN clock, the switching frequency is
3.072MHz. The sampling phase lasts for 120ns.
SYNC
DRDYOUT
REFIO
1μF
AGND
DRDYIN
DGND
MAX11040
TRACK
AIN_+
CSAMPLE+
Detailed Description
The MAX11040 is a 24-bit, simultaneous-sampling,
4-channel, sigma-delta ADC including support for synchronized sampling and daisy chaining of the serial
interface across multiple (up to eight) MAX11040
devices. The serial interface of the set of synchronized
devices behaves as one device. Each channel includes
a differential analog input, a sigma-delta modulator, a
digital decimation filter, an independent programmable
sampling delay, and a buffered reference signal from
the internal or an external reference. The device contains an internal crystal oscillator. The output data rate,
10
HOLD
TO ADC
CSAMPLE-
AIN_TRACK
RON
RON
AVDD/2
Figure 1. Simplified Track/Hold Stage
______________________________________________________________________________________
24-Bit, 4-Channel, Simultaneous-Sampling,
Cascadable, Sigma-Delta ADC
Since the transfer function of a digital filter is repeatable
and predictable, it is possible to correct for frequencydependent attenuation in downstream software. See
the Compensating for the Rolloff of the Digital Filter in a
Typical FFT Analysis section. The transfer function is
defined by the following equation:
⎛
⎛
fAIN ⎞ ⎞
⎟ ⎟
⎜ fSAMPLE × sin⎜ π × f
⎝
SAMPLE ⎠ ⎟
Gain(fAIN ) = ⎜
⎜
⎛
⎞⎟
fAIN
⎟
⎜ fXINCLOCK × sin⎜ π ×
fXINCLOCK ⎟⎠ ⎠
⎝
⎝
3
where:
MAX11040 fig02
1
0
GAIN (dB)
-1
-2
-3
-4
-5
-6
0
0.4
0.8
0.12
0.16
0.20
0.24
0.28
fAIN/fSAMPLE
Gain is the filter gain.
fAIN is the analog input frequency.
fSAMPLE is the programmed output data rate, nominally 16kHz.
fXINCLOCK is the clock frequency at XIN, nominally
24.576MHz.
FIR_Gain (fAIN) is the normalized gain of the FIR filter with the following filter coefficients, as a function
of the analog input frequency fAIN. These coefficients are applied at the output data rate:
+ 0.021484375
- 0.072265625
- 0.035156250
+ 0.304687500
+ 0.539062500
+ 0.304687500
Figure 2. Digital Filter Response
Table 1. Bandwidth vs. Output Data Rate
OUTPUT DATA
RATE (ksps)
-3dB BANDWIDTH
(kHz)
-0.1dB BANDWIDTH
(kHz)
0.5
0.11
0.05
1
0.21
0.11
2
0.42
0.22
4
0.85
0.43
8
1.69
0.87
10
2.11
1.09
12
2.54
1.31
16
3.38
1.74
32
6.78
3.48
64
13.5
6.96
- 0.035156250
- 0.072265625
+ 0.021484375
Latency
The digital filter determines the latency of the
MAX11040. Latency is defined as the time between the
effective point in time that a sample is taken and when
the resulting digital data is available for reading.
The latency of the converter is specified by the following equation:
Latency = (6 x tDOUT) + (PHI x 1.3µs) + 30µs
where tDOUT is the data output period (inverse of the
programmed sample rate), determined by fXINCLOCK
and the selected output data rate, and PHI is the programmed sampling instant delay for the channel in
question (0 ≤ PHI ≤ 255). The latency is approximately
405µs at 16ksps.
______________________________________________________________________________________
11
MAX11040
Digital Filter
The MAX11040 contains an on-chip digital lowpass filter that processes the data stream from each modulator
and generates the high-resolution output data. The lowpass filter frequency response is determined by the
programmable output data rate. At the nominal 16ksps
output data rate, the -3dB bandwidth of the filter is
3.4kHz. The passband flatness is better than ±0.1dB
from 0 to 1.74kHz. The notches are located at 5.75kHz
and 7.195kHz. These frequencies scale linearly with the
output data rate. See Figure 2 and Table 1 for the frequency response at different data rates.
MAX11040
24-Bit, 4-Channel, Simultaneous-Sampling,
Cascadable, Sigma-Delta ADC
Modulator Clock
The modulator clock is created by dividing the frequency at the XIN input by a factor of 8. The XIN input is driven either directly by an external clock or by the
on-chip crystal oscillator.
Crystal Oscillator
The on-chip oscillator requires an external crystal (or
resonator) with a 24.576MHz operating frequency connected between XIN and XOUT, as shown in Figure 3.
As in any crystal-based oscillator circuit, the oscillator
frequency is sensitive to the capacitive load (CL). CL is
the capacitance that the crystal needs from the oscillator circuit and not the capacitance of the crystal. The
input capacitance across XIN and XOUT is 1.5pF.
Choose a crystal with a 24.576MHz oscillation frequency and an ESR less than 30Ω, such as the MP35 from
RXD Technologies. See Figure 3 for the block diagram
of the crystal oscillator. Set XTALEN = 1 in the configuration register to enable the crystal oscillator. The
CLKOUT output provides a buffered version of the
clock that is capable of driving eight MAX11040
devices, allowing synchronized operation from a single
crystal. See the Multiple Device Synchronization section in the Applications Information section.
External Clock
To use an external clock, set XTALEN = 0 in the
Configuration register and connect an external clock
source (20MHz–25MHz) to XIN. CLKOUT becomes
high impedance.
MAX11040
20pF
XIN
24.576MHz
OSCILLATOR
24.576MHz
20pF
XOUT
Analog Input Overvoltage
and Fault Protection
The full-scale differential input range of the MAX11040
is ±0.88VREF. The converter accurately represents any
input for which the positive and negative analog inputs
are separated by a magnitude of less than 0.88VREF.
The device includes special circuitry that protects it
against voltages on the analog inputs up to +6V.
Setting FAULTDIS = 1 disables the protection circuitry.
There are two mechanisms of overvoltage detection
and protection: full-scale overflow and overvoltage
fault. Full-scale overflow occurs if the magnitude of the
applied input voltage on any one or more channels is
greater than 0.88VREF. In this case, the digital output is
clipped to positive or negative full scale and the OVRFLW
flag goes low. Overvoltage fault occurs if the magnitude of an applied input voltage on any one or more
channels goes outside the fault-detection thresholds.
The reaction to an overvoltage fault is dependent on
whether the fault-protection circuitry is enabled. If
enabled, the input-protection circuits engage and the
FAULT flag goes low. A full-scale overflow or an overvoltage fault condition on any one channel does not
affect the output data for the other channels.
The input protection circuits allow up to ±6V relative to
AGND on each input, and up to ±6V differentially
between AIN+ and AIN-, without damaging the device
only if the following conditions are satisfied: power is
applied, the device is not in shutdown mode, a clock
frequency of at least 20MHz is available at XIN, and
FAULTDIS = 0. The analog inputs allow up to ±3.5V relative to AGND when either the MAX11040 is placed in
shutdown mode, the clock stops, or FAULTDIS = 1.
During an overvoltage fault condition the impedance
between AIN_+ and AIN_- reduces to as low as 0.5kΩ.
The output structure and cascading features of FAULT
and OVRFLW are discussed in the Multiple Device
Digital Interface section.
Analog Input Overflow
Detection and Recovery (OVRFLW)
The OVRFLW flag is set based on the ADC conversion
result. When the applied voltage on one or more analog
inputs goes outside the positive or negative full scale
(±0.88VREF), OVRFLW asserts after a delay defined by
the latency of the converter, coincident with the DRDYOUT
of the full-scale clamped conversion result (see Figure
4). The specifics of the latency are discussed earlier in
the data sheet in the Latency section.
Figure 3. Crystal Oscillator Input
12
______________________________________________________________________________________
24-Bit, 4-Channel, Simultaneous-Sampling,
Cascadable, Sigma-Delta ADC
Overvoltage Fault Detection and Recovery (FAULT)
With overvoltage fault protection enabled (FAULTDIS =
0), FAULT immediately transitions from a high to low
when any of the analog inputs go outside the voltage
range bounded by the fault-detection thresholds VPFT
and VNFT.
Once the analog inputs return back within the fault
thresholds, the FAULT interrupt output goes high after a
delay called the fault-recovery time. The fault-recovery
time is:
20 x tDOUT < fault-recovery time < 25 x tDOUT
where tDOUT is the data output period determined by
fXINCLOCK and the selected output data rate.
In the event the analog input voltage changes between
the ADC full scale and the fault threshold faster than
the latency of the converter, the ADC conversion result
prematurely jumps to the full-scale value when a fault is
detected (see Detection Discontinuity in Figure 4).
During a fault condition and the subsequent faultrecovery time, the ADC conversion result remains at full
scale. This creates a discontinuity in the digital conversion result only if the fault recovery time is greater than
the latency plus the time that the input changes
between the fault threshold and the ADC full scale (see
Recovery Discontinuity in Figure 4). Neither of these
steps occur if the fault-protection circuitry is disabled
(FAULTDIS = 1), or if the input is slow relative to the
above descriptions (see Figure 5).
For data rates faster than 32ksps (FSAMPC = 111), the
converter output may contain invalid data for up to
188µs after FAULT returns high. To prevent this behavior, disable the overvoltage fault protection by setting
the FAULTDIS bit in the configuration register to 1 when
using FSAMPC = 111, and limit the analog input swing
to ±3.5V.
FAULT-DETECTION
THRESHOLD
(VPFT OR |VNFT|)
|AIN+ - AIN-|
DETECTION
DISCONTINUITY
RECOVERY
DISCONTINUITY
FULL SCALE
(|0.88VREF|)
LATENCY
RECOVERY TIME
DIGITAL OUTPUT
DATA AT DOUT
LATENCY
LATENCY
LATENCY
FAULT
OVRFLW
Figure 4. High-Frequency Analog Input Overvoltage Detection and Recovery
______________________________________________________________________________________
13
MAX11040
When the analog input voltage changes between the
ADC full scale and the fault threshold faster than the
latency of the converter, OVRFLW goes low with the
FAULT output. OVRFLW remains invalid until a valid
clock frequency is available at XIN.
MAX11040
24-Bit, 4-Channel, Simultaneous-Sampling,
Cascadable, Sigma-Delta ADC
FAULT DETECTION
THRESHOLD
(VPFT OR |VNFT|)
|AIN+ - AIN-|
LATENCY
LATENCY
LATENCY
FULL SCALE
(|0.88VREF|)
RECOVERY TIME
DIGITAL OUTPUT
DATA AT DOUT
LATENCY
FAULT
OVRFLW
Figure 5. Low-Frequency Analog Input Overvoltage Detection and Recovery
Reference
The MAX11040 operates with either a +2.5V internal
bandgap reference or an external reference source
between +2.3V and +2.7V applied at REFIO. Bypass
REFIO and each REF_ to AGND with a 1µF capacitor.
The reference voltage sets the positive and negative
full-scale voltage according to the following formula:
±FS = ±0.88 VREFIO
The reference voltage at REFIO (external or internal) is
individually buffered to generate the reference voltages
at REF0 to REF3 (see Figure 6.) These independent
buffers minimize the potential for crosstalk between
each of the internal ADCs.
Serial Interface
The MAX11040 interface is fully compatible with
SPI/DSP standard serial interfaces (compatible with SPI
modes CPOL = 0, CPHA = 0 and CPOL = 1, and CPHA
= 1). The serial interface provides access to four onchip registers: Sampling Instant Control register (32
bits), Data Rate Control register (16 bits), Configuration
register (8 bits), and Data register (96 bits). All serialinterface commands begin with a command byte,
which addresses a specific register, followed by data
bytes with a data length that depends on the specific
14
REF0
REF1
REF2
REF3
REFIO
+2.5V
REFERENCE
Figure 6. REFIO Input
register addressed and the number of devices cascaded (see Figures 7, 8, and the Registers section).
The serial interface consists of eight signals: CS,
SCLK, DIN, DOUT, CASCIN, CASCOUT, DRDYIN,
and DRDYOUT. CASCIN, CASCOUT, DRDYIN, and
DRDYOUT are used for daisy chaining multiple devices
together. See the Multiple Device Connection section
for details on how to connect CASCIN, CASCOUT,
______________________________________________________________________________________
24-Bit, 4-Channel, Simultaneous-Sampling,
Cascadable, Sigma-Delta ADC
ferred MSB first. Drive CS high to disable the interface
and place DOUT in a high-impedance state.
An interface operation with the MAX11040 takes effect
on the last rising edge of SCLK. If CS goes high before
the complete transfer, the write is ignored. Every data
transfer is initiated by the command byte. The command byte consists of an R/W bit and 7 address bits
(see Table 2.) Figures 7 and 8 show the timing for read
and write operations, respectively.
tCSW
tSU
CS
tPW
tSCP
tPW
tDOD
SCLK
tSU
DIN
tHD
DOUT
tHD
R/W A6 A5 A4 A3 A2 A1 A0
tDOT
COMMAND ADDRESS
HIGH-Z
B7 B6 B5 B4 B3 B2 B1 B0
HIGH-Z
tDOE
DATA LENGTH (NUMBER OF BYTES) DEPENDS
ON THE REGISTER BEING READ (SEE TABLE 2)
DRDYIN
tDRDY
DATA READY
DRDYOUT
Figure 7. General Read-Operation Timing Diagram
tCSW
tSU
CS
tHD
COMMAND ADDRESS
DIN
R/W A6 A5 A4 A3 A2 A1 A0
B7 B6 B5 B4 B3 B2 B1 B0
tHD
SCLK
tPW
tPW
DOUT
tSU
HIGH-Z
tSCP
DATA LENGTH (NUMBER OF BYTES) DEPENDS ON
THE REGISTER BEING WRITTEN (SEE TABLE 2)
HIGH-Z
Figure 8. General Write-Operation Timing Diagram
______________________________________________________________________________________
15
MAX11040
DRDYIN, and DRDYOUT. For single-device applications,
connect CASCIN and DRDYIN to DGND and drive CS
low to transfer data in and out of the MAX11040. With
DRDYIN low, a falling edge at the data ready signal output (DRDYOUT) indicates that new conversion results
are available for reading in the 96-bit data register. A
falling edge on SCLK clocks in data at DIN. Data at
DOUT changes on the rising edge of SCLK and is valid
on the falling edge of SCLK. DIN and DOUT are trans-
24-Bit, 4-Channel, Simultaneous-Sampling,
Cascadable, Sigma-Delta ADC
MAX11040
Registers
The MAX11040 includes four registers accessible by 7
command bytes. The command bytes provide read
and write access to the Data Rate Control register, the
Sampling Instant Control register, and the Configuration
register and read access to the Data register. See
Table 2. Figure 9 shows the CASCIN and CASOUT
timing diagram. Figure 10 is the XIN clock, CLKOUT,
SYNC, and DRDYOUT timing diagram.
SCLK
tCOT
CASCOUT
(DEVICE n)
CASCIN
(DEVICE n+1)
Table 2. Command Bytes
tSC
Figure 9. CASCIN and CASCOUT Timing Diagram
R/W
ADDRESS
[A6:A0]
DATA
LENGTH*
0
1000000
32 x n** bits
Write Sampling Instant
Control Register
1
1000000
32 x n bits
Read Sampling Instant
Control Register
0
1010000
16 bits
Write Data Rate Control
Register
1
1010000
16 bits
Read Data Rate Control
Register
0
1100000
8 x n bits
Write Configuration
Register
1
1100000
8 x n bits
Read Configuration
Register
1
1110000
96 x n bits
Read Data Register
tXPW
tXP
XIN CLOCK
tXCD
CLKOUT
tHS
tSS
SYNC
tSYN
tXDRDY
DRDYOUT
Figure 10. XIN Clock, CLKOUT, SYNC, and DRDYOUT Timing
Diagram
16
FUNCTION
*All data lengths are proportional to the number of cascaded
devices except for reads and writes to the Data Rate Control
register. When accessing the Data Rate Control register, the
data length is fixed at 16 bits. These 16 bits are automatically written to all cascaded devices.
**n is the total number of cascaded devices.
______________________________________________________________________________________
24-Bit, 4-Channel, Simultaneous-Sampling,
Cascadable, Sigma-Delta ADC
which is 1.3µs to 333µs with fXINCLOCK at 24.576MHz
(see Table 3.)
Configuration Register
The Configuration register contains 5 bits that control the
functionality of the MAX11040. The default state is 0x00.
The data length of the Configuration register is 8 bits
per cascaded device (see Table 4).
Table 3. Sampling Instant Control Register
BIT
NAME
DESCRIPTION
[31:24]
Channel 0 sample instant adjust. PHI0 delays sampling instant on channel 0 by 32 XIN clock cycles per
PHI0[7:0]
LSB, up to 8192 cycles total (1.3µs resolution; 333µs range at XIN of 24.576MHz).
[23:16]
PHI1[7:0]
Channel 1 sample instant adjust. PHI1 delays sampling instant on channel 1 by 32 XIN clock cycles per
LSB, up to 8192 cycles total (1.3µs resolution; 333µs range at XIN of 24.576MHz).
[15:8]
PHI2[7:0]
Channel 2 sample instant adjust. PHI2 delays sampling instant on channel 2 by 32 XIN clock cycles per
LSB, up to 8192 cycles total (1.3µs resolution; 333µs range at XIN of 24.576MHz).
[7:0]
PHI3[7:0]
Channel 3 sample instant adjust. PHI3 delays sampling instant on channel 3 by 32 XIN clock cycles per
LSB, up to 8192 cycles total (1.3µs resolution; 333µs range at XIN of 24.576MHz).
Table 4. Configuration Register
BIT
NAME
7
SHDN
6
RST
5
EN24BIT
Enable 24-bit resolution bit. Set EN24BIT high to enable the 24-bit data output. Set EN24BIT low to enable
19-bit data output with device address and channel address tags. Tables 5 and 6 specify the Data register
for both states of this bit.
XTALEN
Internal oscillator enable bit. When using the on-chip crystal oscillator as the clock source, set XTALEN high
to enable the crystal oscillator and provide a buffered version of the crystal clock at the CLKOUT output.
When using an external clock source, set XTALEN low to disable the internal crystal oscillator and tri-state
the CLKOUT output. Connect the external clock source to the XIN input.
4
3
[2:0]
DESCRIPTION
Shutdown bit. Set SHDN high to place the device in shutdown mode. In shutdown mode, the internal
oscillator, fault circuitry, and internal bandgap reference are turned off. Set SHDN low for normal operation.
Reset bit. Set RST high to reset all registers to the default states except for the RST bit, and realign
sampling clocks and output data.
Overvoltage fault-protection disable bit. Set FAULTDIS high to disable the overvoltage fault-protection
circuits. For FAULTDIS = 0, the absolute maximum input range is ±6V. Analog inputs beyond the faultdetection threshold range trip the fault-protection circuits. The output remains clipped for a fault-recovery
FAULTDIS
time (typically < 1.57ms) after the inputs return within the fault-detection threshold range. For FAULTDIS =
1, the absolute maximum input range is only ±3.5V, but there is no fault-recovery delay. See the
Overvoltage Fault Detection and Recovery section.
Don’t Care Don’t-care bits.
______________________________________________________________________________________
17
MAX11040
Sampling Instant Control Register
By default, the MAX11040 samples all 4 input channels
simultaneously. To delay the sampling instant on one or
more channels, program the appropriate byte in the
Sampling Instant Control register. The delay of the
actual sampling instant of each individual channel from
the default sampling instant (PHI_[7:0] = 0x00) is
adjustable between 32 to 819,121 XIN clock cycles,
MAX11040
24-Bit, 4-Channel, Simultaneous-Sampling,
Cascadable, Sigma-Delta ADC
Data Register
The Data register contains the results of the ADC conversion. The result is reported in two’s complement format. The register contains one or two pieces of
information, depending on the state of EN24BIT in the
Configuration register. When EN24BIT is set to zero, the
Data register contains the ADC data truncated to 19
bits, followed by the device and channel addresses
(see Table 5). When EN24BIT is set to one, the data
contained in the Data register represents the 24-bit
conversion (see Table 6). The data length of the Data
register is 96 bits for each cascaded device. Figure 11
shows the sequence of the conversion result output of
all channels for two cascaded devices.
If the results are not read back prior to completion of
the next conversion, the data is overwritten.
Table 5. Data Register (EN24BIT = 0)
BIT
NAME
[95:77]
CH0DATA[18:0]
[76:74]
IC[2:0]
[73:72]
00
[71:53]
CH1DATA[18:0]
[52:50]
IC[2:0]
[49:48]
01
[47:29]
CH2DATA[18:0]
[28:26]
IC[2:0]
[25:24]
10
[23:5]
CH3DATA[18:0]
[4:2]
IC[2:0]
[1:0]
11
DESCRIPTION
Channel 0 19-bit conversion result (two’s complement)
Device address tag. IC[2:0] starts with 000 for the device nearest the master.
Channel 0 address tag = 00
Channel 1 19-bit conversion result (two’s complement)
Device address tag. IC[2:0] starts with 000 for the device nearest the master.
Channel 1 address tag = 01
Channel 2 19-bit conversion result (two’s complement)
Device address tag. IC[2:0] starts with 000 for the device nearest the master.
Channel 2 address tag = 10
Channel 3 19-bit conversion result (two’s complement)
Device address tag. IC[2:0] starts with 000 for the device nearests the master.
Channel 3 address tag = 11
Table 6. Data Register (EN24BIT = 1)
BIT
NAME
DESCRIPTION
[95:72]
CH0DATA[23:0]
Channel 0 24-bit conversion result (two’s complement)
[71:48]
CH1DATA[23:0]
Channel 1 24-bit conversion result (two’s complement)
[47:24]
CH2DATA[23:0]
Channel 2 24-bit conversion result (two’s complement)
[23:0]
CH3DATA[23:0]
Channel 3 24-bit conversion result (two’s complement)
18
______________________________________________________________________________________
24-Bit, 4-Channel, Simultaneous-Sampling,
Cascadable, Sigma-Delta ADC
MAX11040
CS
DIN
SCLK
24 CYCLES
24 CYCLES
24 CYCLES
24 CYCLES
24 CYCLES
24 CYCLES
24 CYCLES
24 CYCLES
DOUT
CHANNEL 0,
DEVICE 0
CHANNEL 1,
DEVICE 0
CHANNEL 2,
DEVICE 0
CHANNEL 3,
DEVICE 0
CHANNEL 0,
DEVICE 1
CHANNEL 1,
DEVICE 1
CHANNEL 2,
DEVICE 1
CHANNEL 3,
DEVICE 1
CASCOUT0
(CASCIN0 = 0)
DEVICE 1 TAKES OVER SPI BUS
CASCOUT1
DRDYOUT0
(DRDYIN0 = 0)
DRDYOUT1
DEVICE 0 DATA READY
DEVICE 0 AND DEVICE 1 DATA READY
Figure 11. 192-Bit Data Read Operation Diagram for Two Cascaded Devices
Data Rate Control Register
The Data Rate Control register controls the output data
period, which corresponds to the output data rate of
the ADC. The data period is controlled by both a
coarse (FSAMPC[2:0]) and a fine (FSAMPF[10:0])
adjustment (see Table 7).
The final data rate is derived by dividing the XIN clock
frequency by a divider value. The divider value is a
function of FSAMPC[2:0] and FSAMPF[10:0]:
Data Rate = fXINCLOCK/Divider
Divider = Coarse Cycle Factor x 384 + Fine Cycle
Factor x FSAMPF[10:0]
Note: Fractional results for the divider are rounded
down to the nearest integer. Coarse cycle factor and
fine cycle factor come from Table 7. The effect of
FSAMPF[10:0] in the formula has limitations as noted in
the table.
Examples of output data rate vs. FSAMPC[2:0] and
FSAMPF[10:0] are shown in Table 8. Table 9 shows typical device performance for various data rate settings.
The data length of the Data Rate Control register is 16
bits total for writes and reads (see Table 2). Changes to
the Data Rate Control register take effect within one conversion period.
______________________________________________________________________________________
19
MAX11040
24-Bit, 4-Channel, Simultaneous-Sampling,
Cascadable, Sigma-Delta ADC
Table 7. Data Rate Control Register
BITS
NAME
DESCRIPTION
Output data rate coarse adjust bits. FSAMPC[2:0] sets the coarse cycle factor.
FSAMPC
[15:13]
FSAMPC[2:0]
[12:11]
Don’t Care
Coarse Cycle
Factor
Sample Rate in ksps
(fXIN CLOCK = 24.576MHz)
000
4
16
001
128
0.5
010
64
1
011
32
2
100
16
4
101
8
8
110
2
32
111
1
64
Don’t-care bits.
Output data rate fine adjust bits. FSAMPF[10:0] increases the output data period by a number of
XIN clock cycles. This number is the value of the register times the fine cycle factor. Values of
FSAMPF greater than 1535 have no additional effect.
[10:0]
FSAMPF[10:0]
FSAMPC
XIN Fine Cycle Factor
000
1 cycle
001
32 cycles
010
16 cycles
011
8 cycles
100
4 cycles
101
2 cycles
110
1/2 cycle
111
1/4 cycle
Table 8. Examples of Output Data Rate as a Function of FSAMPC[2:0] and FSAMPF[10:0]
FSAMPC[2:0]
001
010
011
20
FSAMPF[10:0]
OUTPUT DATA
RATE (sps)
OUTPUT DATA PERIOD
(24.576MHz CLOCK CYCLES)
11xxxxxxxxx
250.1
98272
10111111111
250.1
98272
00000000001
499.7
49184
00000000000
500.0
49152
11xxxxxxxxx
500.2
49136
10111111111
500.2
49136
00000000001
999.3
24592
00000000000
1000.0
24576
11xxxxxxxxx
1000.3
24568
10111111111
1000.3
24568
00000000001
1998.7
12296
00000000000
2000.0
12288
FSAMPF OUTPUT DATA PERIOD
RESOLUTION
(24.576MHz CLOCK CYCLES)
______________________________________________________________________________________
32
16
8
24-Bit, 4-Channel, Simultaneous-Sampling,
Cascadable, Sigma-Delta ADC
FSAMPC[2:0]
100
101
000
110
111
FSAMPF[10:0]
OUTPUT DATA
RATE (sps)
OUTPUT DATA PERIOD
(24.576MHz CLOCK CYCLES)
11xxxxxxxxx
2000.7
12284
10111111111
2000.7
12284
00000000001
3997.4
6148
00000000000
4000.0
6144
11xxxxxxxxx
4001.3
6142
10111111111
4001.3
6142
00000000001
7994.8
3074
00000000000
8000.0
3072
11xxxxxxxxx
8002.6
3071
10111111111
8002.6
3071
00000000001
15990
1537
00000000000
16000
1536
11xxxxxxxxx
16010
1535
1011111111x
16010
1535
0000000001x
31958
769
0000000000x
32000
768
11xxxxxxxxx
32042
767
101111111xx
32042
767
000000001xx
63834
385
000000000xx
64000
384
FSAMPF OUTPUT DATA PERIOD
RESOLUTION
(24.576MHz CLOCK CYCLES)
4
2
1
1
1
Table 9. Typical Performance vs. Output Data Rate
OUTPUT
DATA RATE
(ksps)
-3dB
-0.1dB
BANDWIDTH BANDWIDTH
(kHz)
(kHz)
FAULT
LATENCY
SNR OF 24-BIT
RECOVERY
(µs)
DATA (dB)
TIME (µs)
RELATIVE
ACCURACY OF
256 DATA
POINTS (%)
RELATIVE
ACCURACY OF
SINGLE CYCLE
AT 60Hz (%)
0.5
0.11
0.05
12030
16375
117
0.04
0.23
1
0.21
0.11
6030
8375
115
0.05
0.20
2
0.42
0.22
3030
4375
113
0.06
0.17
4
0.85
0.43
1530
2375
111
0.08
0.16
8
1.69
0.87
780
1375
108
0.11
0.16
10
2.11
1.09
630
1175
107
0.13
0.16
12
2.54
1.31
530
1042
106
0.14
0.16
16
3.38
1.74
405
875
105
0.16
0.16
32
6.78
3.48
218
625
97
0.40
0.28
64
13.5
6.96
124
500
81
2.51
1.26
______________________________________________________________________________________
21
MAX11040
Table 8. Examples of Output Data Rate as a Function of FSAMPC[2:0] and
FSAMPF[10:0] (continued)
MAX11040
24-Bit, 4-Channel, Simultaneous-Sampling,
Cascadable, Sigma-Delta ADC
Multiple Device Connection
Daisy chain up to eight MAX11040 devices for applications that require up to 32 simultaneously sampled inputs
over a single SPI-/DSP-compatible serial interface with a
single chip-select signal, and single interface commands
that apply to all devices in the chain. The eight
MAX11040 devices effectively operate as one device.
There are two aspects to cascading multiple devices:
the digital interface and the mechanism for keeping
multiple devices sampling simultaneously.
There are many configurations for connecting multiple
devices; one is described in the next section, others
are described in the Synchronizing Multiple Devices
section within the Applications Information section.
Multiple Device Digital Interface
Figure 12 shows the most common way to daisy chain
the digital interface of multiple devices.
SPI bus arbitration is performed using CASCIN and
CASCOUT. A falling edge at the CASCIN input of device
n, which is driven by the CASCOUT of device n-1,
allows device n to take over the SPI bus until all expected data is written or read; at this point, device n pulls its
CASCOUT output low. Similarly, CASCOUT of device n
drives CASCIN of device n+1. Figures 11 and 13 show
read operations, including CASCIN and CASCOUT timings, for two cascaded devices and eight cascaded
devices, respectively. The operation described above
applies to all register operations except for writes to the
Data Rate Control register. A fixed 16-bit word is written
to the Data Rate Control registers of all devices in the
chain, independent of the number of cascaded devices
(see Figure 14). Reading from the Data Rate Control
register returns 16 bits per cascaded device.
Connecting the open-drain OVRFLW output of all
devices together creates one signal that summarizes
the overflow information of all devices. This is also true
of the FAULT output. Connecting together these outputs from multiple devices has the effect of a “wire
NOR.” Any device that has an active condition on these
outputs is allowed to pull the line low.
SYNC
SYNC
SYNC
CS
CS
CS
SCLK
SCLK
SCLK
DIN
DIN
DIN
MAX11040
DSP OR
MICROCONTROLLER
CASCIN
MAX11040
CASCOUT
DRDYIN
CASCIN
DRDYIN
DEVICE 0
CASCIN
DEVICE n
DEVICE n+1
FAULT
FAULT
OVRFLW
OVRFLW
OVRFLW
XOUT CLKOUT
DOUT
XIN
DOUT
XIN
CASCADE UP TO 8 DEVICES
Figure 12. Daisy Chaining Multiple Devices
22
DRDYOUT
DRDYIN
FAULT
DOUT
XIN
MAX11040
CASCOUT
______________________________________________________________________________________
24-Bit, 4-Channel, Simultaneous-Sampling,
Cascadable, Sigma-Delta ADC
ent XIN clock, connect DRDYIN of device 0 to ground,
and connect DRDYIN of device n to the DRDYOUT of
device n-1 for all devices. DRDYOUT does not go low
until DRDIN is low and the conversion of the device is
complete. In this configuration, DRDYOUT of the last
device goes low only when all devices in the chain have
their data ready.
tCSW
CS
DIN
SCLK
DOUT
DEVICE 0
DEVICE 1
DEVICE 2
DEVICE 3
DEVICE 4
DEVICE 5
DEVICE 6
DEVICE 7
CASCOUT0
(CASCIN0 = 0)
CASCOUT1
CASCOUT2
CASCOUT3
CASCOUT4
CASCOUT5
CASCOUT6
CASCOUT7
Figure 13. Configuration Register Read Operation Timing Diagram for Eight Cascaded Devices
______________________________________________________________________________________
23
MAX11040
There are two ways to use a single line to indicate that
all MAX11040s have their data ready, depending on
whether or not they are clocked synchronously. If all
MAX11040s have the same XIN clock and have been
synchronized using SYNC or reset commands, the
DRDYOUT of any device in the chain is used to represent all of them. Alternatively, if the devices use a differ-
MAX11040
24-Bit, 4-Channel, Simultaneous-Sampling,
Cascadable, Sigma-Delta ADC
CS
DIN
B15 B14 B13 X
X B10 B9 B8
B7 B6 B5 B4 B3 B2 B1 B0
SCLK
DOUT
HIGH-Z
CASCOUT0
(CASCIN0 = 0)
CASCOUT1
CASCOUT2
CASCOUT3
CASCOUT4
CASCOUT5
CASCOUT6
CASCOUT7
X = RESERVED
Figure 14. Data Rate Controller Register Write Operation Timing Diagram for Eight Cascaded Devices
24
______________________________________________________________________________________
HIGH-Z
24-Bit, 4-Channel, Simultaneous-Sampling,
Cascadable, Sigma-Delta ADC
falling edge as shown in Figure 15 is described in
sequence below:
1) A SYNC falling edge is issued two XIN clock cycles
after the DRDYOUT event 2.
2) The converter remembers the two XIN clock cycles,
and completes the current sample, issuing DRDYOUT
event 3 a period of tS after DRDYOUT event 2.
3) Then, the converter pauses for the remembered time
period, two XIN clock cycles for this example.
4) Correspondingly, DRDYOUT event 4 is issued two
XIN cycles later than it would have without the
SYNC falling edge.
5) The process continues as normal with DRDYOUT
event 5 appearing tS after DRDYOUT event 4.
NOTE: THE LATENCY IS NOT TO SCALE.
tS
1
tS
DELAY 2
CYCLES
tS
2
tS
3
AIN_
4
tS
5
6
tS
tS
tS
tS
tS
XIN
DRDYOUT
1
2
3
4
5
SYNC
MEASURE
PAUSE
Figure 15. Effect of a SYNC Falling Edge
______________________________________________________________________________________
25
MAX11040
SYNC for Simultaneous
Sampling with Multiple Devices
The SYNC input permits multiple devices to sample
simultaneously. The mismatch between the power-up
reset of multiple devices causes the devices to begin
conversion at different times. After a falling edge on the
SYNC input, the device completes the current conversion and then synchronizes subsequent conversions
(see Figure 15).
Upon a SYNC falling edge, the MAX11040 measures the
time between the SYNC falling edge to the preceding
DRDYOUT falling edge, waits until the next DRDYOUT
falling edge, then pauses the ADC for the measured
amount of time. Figure 15 shows an example where the
converter is regularly sampling the input and producing
a DRDYOUT with a period tS. The effect of a SYNC
MAX11040
24-Bit, 4-Channel, Simultaneous-Sampling,
Cascadable, Sigma-Delta ADC
Referring back to the analog input, since the entire sampling section of the converter also paused for two clock
cycles, the sampling point for sample 5 is also paused
by two clock cycles, possibly creating a small disturbance at the SYNC falling edge. This disturbance is filtered with the digital filter, which makes it less distinct.
If the SYNC falling edge occurred during the same XIN
clock period as the DRDYOUT signal, the disturbance
does not effect the periodic timing since the SYNC
falling edge would demand a pause of zero XIN clock
cycles. Hence, connecting the DRDYOUT of one converter to the SYNC inputs of many converters, as illustrated in Figure 12, aligns the sampling of the
converters on the first SYNC falling edge, but does not
disturb a regular sampling process for future samples.
OUTPUT CODE
FS = +0.88 x VREFIO
ZS = 0
011..110
-FS = -0.88 x VREFIO
000...011
000...010
000...001
000...000
111...110
Transfer Function
100...001
Power-On Reset
The serial interface, logic, digital filter, and modulator
circuits reset to zero at power-up. The power-on reset
circuit releases this reset no more than 1ms after
VDVDD rises above 2V.
26
1 LSB =
2(0.88 x VREFIO)
2N*
*N = 19 FOR 19-BIT TRANSFER
FUNCTION,
N = 24 FOR 24-BIT TRANSFER
FUNCTION
111...111
See the Multiple Device Synchronization section for different ways to use the SYNC input.
Figures 16 shows the bipolar I/O transfer function.
Code transitions occur halfway between successiveinteger LSB values. Output coding is binary, with 1 LSB
= (0.88 x VREFIO) x 2/524,288 in 19-bit mode and 1 LSB
= (0.88 x VREFIO) x 2/16,777,216 in 24-bit mode.
FULL-SCALE
TRANSITION
011...111
100...000
-FS
0
DIFFERENTIAL INPUT VOLTAGE (LSB)
Figure 16. ADC Transfer Function
______________________________________________________________________________________
FS
FS - 3/2 LSB
24-Bit, 4-Channel, Simultaneous-Sampling,
Cascadable, Sigma-Delta ADC
Multiple Device Synchronization
Synchronizing Multiple Devices
Using a Shared XIN Clock Source
To synchronize multiple devices sharing a single XIN
clock source, transition the SYNC input that is shared
by all devices high to low. When an external sync
source is not available, connect DRDYOUT of one
MAX11040 to the SYNC input of all devices in the
chain. The MAX11040 ignores any SYNC transitions
applied during the power-on reset.
Synchronizing Multiple Devices
Using Independent XIN Clock Sources
If it is undesirable to connect the XIN clock sources
together, due to EMI or other reasons, use DRDYIN,
DRDYOUT, and SYNC to align the conversion timing as
shown in Figure 17. This minimizes the effects of drift
between the clock sources by resynchronizing after
each conversion when DRDYOUT transitions low. In
this configuration, the maximum correction caused by a
SYNC edge is one XIN clock cycle.
The resulting sampling rate is determined by the sampling
frequency of the device with the slowest clock source,
plus the delay through the DRDYIN to DRDYOUT chain
between this slowest device and the end of the chain.
Synchronizing Multiple Devices
to an Independent Clock Source
To periodically synchronize multiple devices to an independent timing source, connect the timing source to
the SYNC inputs of the devices. If minimal jitter is
important in the application, program the MAX11040’s
to a frequency slightly slower than the external frequency, such that SYNC falling edges only occur a short
time after the DRDYOUT signals.
SYNC
SYNC
SYNC
CS
CS
CS
SCLK
SCLK
SCLK
DIN
DIN
DIN
MAX11040
DSP OR
MICROCONTROLLER
MAX11040
MAX11040
CASCIN
CASCOUT
CASCIN
CASCOUT
CASCIN
DRDYIN
DRDYOUT
DRDYIN
DRDYOUT
DRDYIN
DEVICE 0
DEVICE n
DEVICE n+1
FAULT
FAULT
FAULT
OVRFLW
OVRFLW
OVRFLW
DOUT
XIN
DRDYOUT
XOUT CLKOUT
DOUT
XIN
XOUT
DOUT
XIN
XOUT
CASCADE UP TO 8 DEVICES
Figure 17. One Crystal per MAX11040 and All SYNC Inputs Driven by DRDYOUT of the Last Device in the Chain
______________________________________________________________________________________
27
MAX11040
Applications Information
MAX11040
24-Bit, 4-Channel, Simultaneous-Sampling,
Cascadable, Sigma-Delta ADC
Signal Distortion at SYNC Falling Edges
Each SYNC falling edge causes a disruption in the digital filter timing proportional to the delay from the previous falling edge of DRDYOUT to the falling edge of
SYNC. Any analysis of the output data that assumes a
uniform sampling period sees an error proportional to
that delay, with a maximum value determined by the
maximum derivative of the analog input. Figure 18
shows the effect of this discontinuity at output sample 5.
Assuming a 60Hz ±2.2V sine wave, the maximum possible error on any given sample caused by a SYNC
falling edge is:
VERROR_MAX = 2.2V x 2π x 60Hz x tDRDYOUT_TO_SYNC
= 0.83µV/ns x tDRDYOUT_TO_SYNC
The delay from DRDYOUT to SYNC is quantized to
within one cycle of the 24.576MHz clock. SYNC pulses
that are asynchronous to DRDYOUT may cause large
errors. To eliminate this error, use a single clock source
for all devices and avoid disrupting the output data timing with SYNC pulses while making high-precision
measurements. Alternately, minimize the delay from
DRDYOUT to SYNC to minimize the error.
Example:
Assume f AIN_ = 60Hz, f S = 16ksps, and eight total
devices in the chain.
Device 1 has the longest tDRDYOUT_TO_SYNC delay,
therefore the worst-case SYNC error.
If device 1 has the fastest XIN clock in the chain, and
device 2 has the slowest XIN clock in the chain, and
they differ by 0.1%, device 1 completes its conversion
as much as 0.1% earlier than device 2. Hence, the
delay of device 2 is:
0.1% x (1/16kHz ) = 62.5ns
The signal then propagates down the chain at a time
delay of nominally 20ns for each device.
The total delay back to the SYNC falling edge after
going through six additional delays is:
tDELAY = 62.5ns + 6 x 20ns = 182.5ns
Maximum % Error = 2π x fIN x tDRDYOUT_TO_SYNC x
100% = 2 x π x 60Hz x 182.5ns x 100% = 0.007%
The above error is relative to the signal level, not to the
full scale of the data converter.
NOTE: THE LATENCY IS NOT TO SCALE.
tS
tS
1
tS
2
tS
3
AIN_
tDRYOUT_TO_SYNC
4
tS
5
tS
tS
6
tS
tS
DRDYOUT
1
2
3
4
SYNC
PAUSE FOR
tDRYOUT_TO_SYNC
MEASURE
tDRYOUT_TO_SYNC
1
RECONSTRUCTED
DIGITAL OUTPUT
2
3
4
DISCONTINUITY DUE TO SYNC EVENT
5
6
Figure 18. Example of Discontinuity in Reconstructed Digital Output Due to SYNC Falling Edge with a Large DRDYOUT-to-SYNC Delay
28
______________________________________________________________________________________
24-Bit, 4-Channel, Simultaneous-Sampling,
Cascadable, Sigma-Delta ADC
The source impedance that drives the analog inputs
affects the sampling period.
Low-Impedance Sources
Minimize the source impedance to ensure the input
capacitor fully charges during the sampling phase.
The required source resistance is defined by the
equation below:
RSOURCE _ MAX <
=
tSAMP
⎛ 1 ⎞
K x CSAMP x In⎜
⎟
⎝ Error ⎠
120ns
⎛ 1 ⎞
1.5 x 4pF x In⎜
⎟
⎝ Error ⎠
− RINT
− 2600Ω
where K = 1.5 and RINT = 2600Ω.
For example, the required source resistance to achieve
0.1% accuracy is:
120ns
RSOURCE _ MAX <
− 2600Ω
⎛ 1 ⎞
1.5 x 4pF x ln⎜
⎟
⎝ 0.1% ⎠
=
=
120ns
− 2600Ω
1.5 x 4pF x ln(1000)
120ns
− 2600Ω = 294Ω
1.5 x 4pF x 6.91
High-Impedance Sources
If the source impedance is greater than
R SOURCE_MAX , as defined in the Low-Impedance
Sources section, place a 0.1µF bypass capacitor
between AIN_+ and AIN_- to provide transient charge.
The average switched-capacitor load with a proper
bypass capacitor and XIN clock frequency =
24.576MHz is equivalent to a 130kΩ resistor connected between AIN_+ and AIN_-. This resistance is independent of the value of the 0.1µF bypass capacitor. If
another XIN clock frequency is chosen, this resistance
is directly proportional to the XIN clock period.
Although the addition of a bypass capacitor helps charge
the MAX11040 input capacitor, some gain error due to
resistive drop across the source resistance still remains.
Calculate this gain error using the following equation:
ΔGain =
RSOURCE
RSOURCE
=
RSOURCE + RLOAD
RSOURCE + 130kΩ
Analog Filtering
The analog filtering requirements in front of the
MAX11040 are considerably reduced compared to a
conventional converter with no on-chip filtering. The
internal digital filter has significant rejection of signals
higher than the Nyquist frequency of the output data
rate that would alias back into the sampled signal.
The internal digital filter does not provide rejection
close to the harmonics of the 3.072MHz modulator frequency. For example, assuming an output data rate of
16ksps if the XIN clock is set to 24.576MHz, then the
band between 3.0686MHz and 3.0750MHz is not
explicitly filtered. Since this unfiltered band is very
small compared to its actual frequency, very little
broadband noise enters through this mechanism. If
focused narrowband noise in this band is present, a
simple analog filter can create significant attenuation at
this frequency because the ratio of passband to stopband frequency is large.
In addition, because the device’s common-mode rejection extends out to several 100kHz, the common-mode
noise susceptibility in this frequency range is substantially reduced.
Providing additional filtering in some applications
ensures that differential noise signals outside the frequency band of interest do not saturate the analog
modulator.
The modulator saturates if the input voltage exceeds its
full scale (±2.2V). The digital filter does not prevent a
large signal in the filter stopband from saturating the
modulator. If signals outside the band of interest cause
violation of this full scale while accurate conversion of
passband signals is desired, then additional analog filtering is required to prevent saturation.
Compensating for the Rolloff of
the Digital Filter in Typical FFT Analysis
To calculate FIR_GAIN(fAIN_):
1) Decide the number of evenly spaced frequencies
between DC and the Nyquist frequency of the output
data rate at which correction factors are desired,
which is usually the same as the FFT result.
2) Create an array with a length that is 2x the number of
the desired frequencies. (Again, the result is likely to
correlate with the time domain array that is loaded
into an FFT algorithm.)
3) Fill this array with the filter coefficients provided in
the Digital Filter section. Fill the rest of the array with
zeros.
4) Take an FFT of this array. The result represents the
response of the MAX11040 built-in FIR filter.
______________________________________________________________________________________
29
MAX11040
Source Impedance and
Input Sampling Network
MAX11040
24-Bit, 4-Channel, Simultaneous-Sampling,
Cascadable, Sigma-Delta ADC
To compensate the result of an FFT for the MAX11040
output data:
1) Calculate the inverse (1/x) of the equation provided
in the Digital Filter section for each frequency in the
FFT.
2) Multiply the FFT of the MAX11040 output data by the
result of the above step.
Layout the traces in perpendicular directions when a digital line and an analog line cross each other.
Bypass AVDD to the analog ground plane with a 0.1µF
capacitor in parallel with a 1µF to 10µF low-ESR capacitor. Keep capacitor leads short for best supply-noise
rejection. Bypass REF+ and REF- with a 0.1µF capacitor to GND. Place all bypass capacitors as close as
possible to the device for optimum decoupling.
Power Supplies
AVDD and DVDD provide power to the MAX11040. The
AVDD powers up the analog section, while the DVDD
powers up the digital section. The power supply for
AVDD and DVDD ranges from +3.0V to +3.6V and 2.7V
to VAVDD, respectively. Bypass AVDD to AGND with a
1µF electrolytic capacitor in parallel with a 0.1µF ceramic
capacitor and bypass DVDD to DGND with a 1µF electrolytic capacitor in parallel with a 0.1µF ceramic capacitor. For improved performance, place the bypass
capacitors as close as possible to the device.
Layout, Grounding, and Bypassing
The best layout and grounding design always comes
from a thorough analysis of the complete system. This
includes the signal source’s dependence and sensitivity on ground currents, and knowledge of the various
currents that could travel through the various potential
grounding paths.
Use PCBs with separate analog and digital ground
planes. Connect the two ground planes together only at
the MAX11040 GND input. Isolate the digital supply
from the analog with a low-value resistor (10Ω) or ferrite
bead when the analog and digital supplies come from
the same source.
Ensure that digital return currents do not pass through
the analog ground and that return-current paths are low
impedance. A 5mA current flowing through a PCB
ground trace impedance of only 0.05Ω creates an error
voltage of approximately 250µV.
Ensure that digital and analog signal lines are kept separate. Do not run digital (especially the SCLK and DOUT)
lines parallel to any analog lines or under the MAX11040.
Crystal Layout
Follow these basic layout guidelines when placing a
crystal on a PCB with a MAX11040 to avoid coupled
noise:
1) Place the crystal as close as possible to XIN and
XOUT. Keeping the trace lengths between the crystal and inputs as short as possible reduces the
probability of noise coupling by reducing the length
of the “antennae.” Keep the XIN and XOUT lines
close to each other to minimize the loop area of the
clock lines. Keeping the trace lengths short also
decreases the amount of stray capacitance.
2) Keep the crystal solder pads and trace width to XIN
and XOUT as small as possible. The larger these
bond pads and traces are, the more likely it is that
noise will couple from adjacent signals.
3) Place a guard ring (connect to ground) around the
crystal to isolate the crystal from noise coupled from
adjacent signals.
4) Ensure that no signals on other PCB layers run
directly below the crystal or below the traces to XIN
and XOUT. The more the crystal is isolated from
other signals on the board, the less likely for noise to
couple into the crystal.
5) Place a local ground plane on the PCB layer immediately below the crystal guard ring. This helps to
isolate the crystal from noise coupling from signals
on other PCB layers.
Note: Keep the ground plane in the vicinity of the
crystal only and not on the entire board.
Chip Information
PROCESS: BiCMOS
30
______________________________________________________________________________________
24-Bit, 4-Channel, Simultaneous-Sampling,
Cascadable, Sigma-Delta ADC
TOP VIEW
+
Package Information
For the latest package outline information and land patterns, go
to www.maxim-ic.com/packages. Note that a “+”, “#”, or “-” in
the package code indicates RoHS status only. Package drawings may show a different suffix character, but the drawing pertains to the package regardless of RoHS status.
AIN0- 1
38 AIN2-
AIN0+ 2
37 AIN2+
REF0 3
36 REF2
PACKAGE TYPE
PACKAGE CODE
DOCUMENT NO.
AGND 4
35 AGND
38 TSSOP
U38+2
21-0081
AIN1- 5
34 AIN3-
AIN1+ 6
33 AIN3+
REF1 7
MAX11040
32 REF3
AGND 8
31 AGND
REFIO 9
30 AVDD
AGND 10
29 AGND
DGND 11
28 DGND
DVDD 12
27 DVDD
CASCIN 13
26 XIN
CASCOUT 14
25 XOUT
CS 15
24 SYNC
SCLK 16
23 DRDYIN
DIN 17
22 DRDYOUT
DOUT 18
21 CLKOUT
FAULT 19
20 OVRFLW
TSSOP
______________________________________________________________________________________
31
MAX11040
Pin Configuration
MAX11040
24-Bit, 4-Channel, Simultaneous-Sampling,
Cascadable, Sigma-Delta ADC
Revision History
REVISION
NUMBER
REVISION
DATE
0
5/09
Initial release
—
1
4/10
Corrected equation in Digital Filter section
11
2
6/10
Changed gain-error drift in Electrical Characteristics section
2
DESCRIPTION
PAGES
CHANGED
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.
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© 2010 Maxim Integrated Products
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