AN-1302: Optimizing the ADuCM350 for 4-Wire, Bio-Isolated Impedance Measurement Applications (Rev. 0) PDF

AN-1302
APPLICATION NOTE
One Technology Way • P.O. Box 9106 • Norwood, MA 02062-9106, U.S.A. • Tel: 781.329.4700 • Fax: 781.461.3113 • www.analog.com
Optimizing the ADuCM350 for 4-Wire, Bio-Isolated
Impedance Measurement Applications
INTRODUCTION
The ADuCM350 is an ultralow power, integrated mixed-signal
metering solution that includes a microcontroller subsystem for
processing, control, and connectivity. The processor subsystem
is based on a low power ARM® Cortex™-M3 processor, a
collection of digital peripherals, embedded SRAM and flash
memory, and an analog subsystem which provides clocking,
reset, and power management capability.
This application note details how to set up the ADuCM350 to
optimally measure the impedance of an RC type sensor, using
4-wire techniques, while targeting IEC-60601 standards.
To target the IEC-60601 standard, the ADuCM350 is used
in conjunction with an external instrumentation amplifier
(AD8226), to complete high precision absolute measurements
using a 4-wire measurement technique.
12168-101
The ADuCM350 has the ability to perform a 2048 point single
frequency discrete Fourier transform (DFT). It takes the 16-bit
ADC output as input and outputs the real and imaginary parts
of the complex impedance.
The configurable switch matrix on the ADuCM350 allows
you to choose from a 2-wire, 3-wire, or 4-wire impedance
measurement.
Figure 1. EVAL-ADuCM350EBZ Motherboard and 4-Wire Bio-configuration Daughter Board
Rev. 0 | Page 1 of 16
AN-1302
Application Note
TABLE OF CONTENTS
Introduction ...................................................................................... 1
Sensor Configuration................................................................. 10
Revision History ............................................................................... 2
4-Wire Bio-Isolated Network ................................................... 10
Configurations .................................................................................. 3
AFE Optimization .......................................................................... 11
2-Wire System ............................................................................... 3
Calculate the RLIMIT Resistor ...................................................... 11
4-Wire System ............................................................................... 3
Calculate RTIA .............................................................................. 11
4-Wire Bio-Isolated System......................................................... 3
Calculate RG of the AD8226 ...................................................... 11
Basic 4-Wire Impedance Measurement......................................... 4
Calculate RCAL .......................................................................... 12
The 4-Wire Bio-Isolated Method ................................................... 6
4-Wire Bio-Isolated Measurements ............................................. 13
Basic 4-Wire Theory .................................................................... 6
Hardware Setup For 4-Wire Bio-Configuraton Board .......... 13
4-Wire Bio-Isolated Theory in Application .............................. 6
Software Setup For 4-Wire Bio-Configuration Board............ 13
A 4-Wire Bio-Isolated Solution .................................................. 7
Schematics for the 4-Wire Bio-Configuration Board ................. 14
Example of a 4-Wire Bio-Isolated System ................................... 10
REVISION HISTORY
4/14—Revision 0: Initial Version
Rev. 0 | Page 2 of 16
Application Note
AN-1302
CONFIGURATIONS
The ADuCM350 offers three configurations for measuring the
impedance of a sensor.
2-WIRE SYSTEM
In the presence of varying access resistance to the unknown
impedance, this configuration provides relative accuracy
measurements for impedance magnitude and impedance phase.
For further details on optimizing the ADuCM350 for 2-wire
impedance measurements, refer to the AN-1271 Application
Note, Optimizing the ADuCM350 for Impedance Conversion.
The 2-wire system measures the relative accuracy of Impedance
magnitude and phase.
4-WIRE SYSTEM
This configuration provides absolute accuracy for both
impedance magnitude and impedance phase measurements
because access resistances are calibrated out. This configuration
does not operate where ac coupling capacitors are required to
isolate sensor from device, that is, capacitors in series with the
access resistances. Refer to the AN-1271 Application Note,
Optimizing the ADuCM350 for Impedance Conversion, for
information on optimizing the ADuCM350 for 4-wire
measurements.
The 4-wire system measures the absolute accuracy of
impedance magnitude and phase, however isolation capacitors
are not allowed.
4-WIRE BIO-ISOLATED SYSTEM
If isolation capacitors are required between sensor and device,
then an external instrumentation amplifier is required to
measure the differential voltage across the sensor. It is not
possible for the ADuCM350 to do this measurement as a single
chip solution because the isolation capacitors cause instability
when included on the sense (P and N channel) paths.
The 4-wire bio-isolated system measures the absolute accuracy
of impedance magnitude in the presence of isolation capacitors,
however this system is not targeted for accurate phase
measurements.
Rev. 0 | Page 3 of 16
AN-1302
Application Note
BASIC 4-WIRE IMPEDANCE MEASUREMENT
To measure the impedance of an unknown sensor, Z, a
ratiometric measurement technique is employed using the
ADuCM350.
EXTERNAL COMPONENTS
SWITCH
MATRIX
RCAL 1
RCAL
RACCESS1
RCAL 2
AFE 8
A
UNKNOWN
Z
RACCESS2
AFE 6
4.
Calculate the unknown impedance phase on the core using
the following equation:
ZUNKNOWN PHASE = ZUNK PHASE − RCALPHASE
This 4-wire measurement approach to measuring impedances
operates if there are no isolation requirements on the sensor.
However, if an isolation capacitor, such as CISO, need to be
included in series with the access resistor, RACCESS, in a 4-wire
measurement, then a single chip solution is not possible.
CLOSED LOOP
GAIN = 2
DAC_ATTEN_EN
EXCITATION
AMPLIFIER
DACP
LOOP
DACN 0.6V
RCF
50kHz
VREFDAC 1.8V
1.0V
12-BIT
DAC
PGA
D
GAIN
1/0.025
AFE 7
ZUNK MAG
× RCAL
RCAL MAG
ZUNKNOWN MAG =
0.2V
DACCLK
REFNHIZ 0.6V
P
AFE 5
RACCESS3
B
RACCESS4
AFE 4
AFE 3
N
VBIAS
AFE 2
T
ADC 16-BITS,
160kSPS
DFT
78SPS
×1.5
AFE 1
×1
AN_A
RCAL
RCAL
RTIA
33kΩ
0.1%
IMAG
=
REAL
AND RCAL IMAG
RCAL
RCAL PHASE = tan–1
Figure 2. 4-Wire Topology for ADuCM350—Measuring RCAL
Rev. 0 | Page 4 of 16
2
REAL
+ RCAL
RCAL IMAG
RCAL
REAL
2
IMAG
12168-001
2.
Measure the impedance of a known precision resistor,
RCAL, as shown in Figure 2. An excitation voltage is
applied at RCAL 1 with associated D and P switches closed
in the switch matrix. The resultant excitation current is
measured through RCAL 2, with associated T and N
switches closed in the switch matrix. This current is
converted to a voltage using the TIA amplifier, where RTIA
is optimized for the maximum current seen by the ADC,
and is converted to a voltage using the ADC. A 2048 point
Hann sample is performed on the data to give real and
imaginary components of the impedance.
Change the switch matrix configuration, as shown in
Figure 3, and excite the sensor measuring the response
MUX
1.
3.
current. The DFT engine now calculates the real and
imaginary components of the unknown impedance, Z.
Calculate the unknown impedance magnitude on the core
using the following equation:
Application Note
AN-1302
EXTERNAL COMPONENTS
SWITCH
MATRIX
RCAL 1
RCAL
RACCESS1
RCAL 2
AFE 8
CLOSED LOOP
GAIN = 2
EXCITATION
AMPLIFIER
DACP
LOOP
DACN 0.6V
GAIN
1/0.025
0.2V
DACCLK
REFNHIZ 0.6V
P
AFE 5
RACCESS3
B
RACCESS4
AFE 4
AFE 3
N
VBIAS
AFE 2
T
ADC 16-BITS,
160kSPS
DFT
78SPS
×1.5
AFE 1
×1
AN_A
ZUNK
ZUNK IMAG =
RTIA
33kΩ
0.1%
ZUNK
Figure 3. 4-Wire Topology for ADuCM350—Measuring Z
Rev. 0 | Page 5 of 16
PHASE
REAL
AND ZUNK
IMAG
ZUNK REAL2 + ZUNK IMAG2
= tan–1
ZUNK IMAG
ZUNK REAL
12168-002
UNKNOWN
Z
VREFDAC 1.8V
1.0V
12-BIT
DAC
PGA
MUX
A
AFE 6
RCF
50kHz
D
AFE 7
RACCESS2
DAC_ATTEN_EN
AN-1302
Application Note
THE 4-WIRE BIO-ISOLATED METHOD
BASIC 4-WIRE THEORY
RLIMIT
In a classic 4-wire /4-terminal sensing system, a differential
current source is used to force a known current into the sensor.
This forced current generates a potential difference across the
Unknown Z, which is to be measured according to Ohm’s Law
where:
CISO1
CISO3
RACCESS3
RACCESS1
A
V
METER
V=I×R
UNKNOWN
Z
VAC
B
When the current is being forced, the access wires to Z also lead
to a drop in voltage, which causes inaccuracies in a measurement. To remove this loss from the actual measurement of Z, a
differential pair of sense lines are connected to Z at the points
labeled A and B in Figure 2.
A
UNKNOWN
Z
I
METER
B
12168-003
V
METER
Figure 4. Basic 4-Wire Topology
The differential sense lines are designed with very high input
impedance stages so that no current flows through them and
there is no voltage drop across them. The impedance Z is then
measured using the equation
Z = VMETER/IAC
4-WIRE BIO-ISOLATED THEORY IN APPLICATION
An alternative approach is to use a high precision excitation
voltage source as the force signal. Apply this voltage to Z and
measure the response current using a high accuracy current
meter (see Figure 3). The unknown impedance Z is then
measured by the equation
RACCESS4
RACCESS2
CISO2
I
METER
12168-004
CISO4
Figure 5. 4-Wire Bio-Isolated Topology
Referring back to Figure 3, it is possible to measure 4-wire
impedance using the ADuCM350.The excitation stage
excites the sensor with a known voltage, which is accurately
differentially sensed using the internal instrumentation loop.
The current response is measured through the TIA channel and
converted to a voltage.
In a real-world application, such as those governed by the
IEC-60601 standards, the Z (or sensor) allows a limited dc
voltage across it. The restrictions on ac current forced on sensor
are more relaxed. The ac voltage source is selected for the
force connection to the sensor to utilize the ADuCM350 DFT
capability.
In Figure 5, CISO1 and CISO2 are discrete isolation capacitors that
ensure that no dc voltage appears across the sensor. RACCESS1 and
RACCESS2 are access or lead resistance inherent in the connections
to the sensor. RLIMIT is an extra level of security to guarantee the
maximum allowable excitation current seen by the sensor in
a scenario where the RACCESS resistance is removed from the
measurement.
Z = VMETER/IMETER
Rev. 0 | Page 6 of 16
Application Note
AN-1302
A 4-WIRE BIO-ISOLATED SOLUTION
Referring to Figure 5, the following is required:
High Precision Current Meter
•
•
•
The ADuCM350 utilizes a TIA amplifier for current to voltage
conversion for measurement by the high precision ADC, the
gain of which is set by an external resistor, RTIA. The TIA
channel sinks the sensor excitation current and the channel is
precisely biased on a common mode of 1.1 V. Significant analog
and digital filtering is performed on measurement for rejection
of interferers and noise. The T and N channels are tied together
using the switch matrix for accurate sense capability on the
current measured (see Figure 7).
Precision ac voltage source
High precision current meter
Precision differential voltage meter
Precision AC Voltage Source
The ADuCM350 has a high precision excitation control loop,
which drives a precision ac voltage to the sensor. An internal
differential sense configuration guarantees the accuracy of the
voltage source (see Figure 6). The positive sense, P, is tied to the
drive terminal, D, in the configurable switch matrix. A DDS
based sine wave generator is used to generate the ac stimulus
through a 12-bit DAC. For more information regarding the
transmit stage, refer to the ADuCM350 Hardware Reference
Manual (UG-587).
The ADC converts the current measurement with a 160 kSPS
ADC. A 2048 sample point DFT is performed on the data;
resulting real and imaginary components for the current
measurement are calculated.
SWITCH
MATRIX CLOED LOOP
GAIN = 2
DAC_ATTEN_EN
RCAL 1
EXCITATION
AMPLIFIER
DACP
LOOP
DACN 0.6V
RCAL 2
RCF
50kHz
VREFDAC 1.8V
1.0V
12-BIT
DAC
PGA
D
GAIN
1/0.025
AFE 8
AFE 7
0.2V
REFNHIZ 0.6V
P
DACCLK
AFE 6
AFE 5
×1.5
AFE 4
ADC 16-BITS,
160kSPS
MUX
N
AFE 3
AFE 2
12168-005
6.2kΩ
1%
VAC
22nF
CISO1
20%
RACCESS3
5kΩ
5kΩ
A
UNKNOWN Z
B
RACCESS4
5kΩ
5kΩ
22nF
20%
RLIMIT
×1.5
Figure 6. AC Voltage Source on the ADuCM350
EXCITATION
AMPLIFIER
DACP
LOOP
DACN 0.6V
RCF
50kHz
12-BIT
DAC
PGA
D
GAIN
1/0.025
AFE 8
AFE 7
CISO1
AFE 6
RACCESS1
AFE 5
RACCESS2
CISO2
AFE 4
P
ADC 16-BITS,
160kSPS
×1.5
N
AFE 3
VBIAS
AFE 2
AFE 1
DACCLK
MUX
A
UNKNOWN Z
B
0.2V
REFNHIZ 0.6V
VAC
×1.5
T
×1
RTIA
Figure 7. High Precision Current Meter
Rev. 0 | Page 7 of 16
I
METER
12168-006
RLIMIT
AN-1302
Application Note
data, and resulting real and imaginary components for the
voltage measurement are calculated.
Precision Differential Voltage Meter
To differentially sense the voltage across the sensor, a low power
instrumentation amplifier with excellent noise and commonmode rejection is required (see Figure 8). The AD8226 is
selected for this application. It is referenced off the common
mode of the system set by the VBIAS voltage on the TIA channel.
The output of the in-amp is fed back into the ADuCM350
through one of the auxiliary channels, for example, AN_A.
4-Wire, Bio-Isolated Measurement System Block
Diagram
Figure 9 shows the combination of the following:
•
•
The ADC converts the auxiliary voltage measurement with a
160 kSPS ADC. A 2048 sample point DFT is performed on the
Precision ac voltage source (ADuCM350 excitation stage)
High precision current meter (ADuCM350 TIA channel
stage)
Precision differential voltage meter (AD8226
instrumentation amplifier)
•
EXCITATION
AMPLIFIER
LOOP
EXTERNAL COMPONENTS
D
AFE 8
RLIMIT
AFE 7
CISO1
AFE 6
RACCESS3 RACCESS1
AFE 5
VBIAS
VBIAS
CISO3
RCM1
A
UNKNOWN Z
B
REF
AD8226
RG
CISO4
RCM2
RACCESS4 RACCESS2
P
AFE 4
N
AFE 3
AFE 2
CISO2
VBIAS
VAC
AFE 1
VBIAS
T
I
METER
RTIA
12168-007
V
METER
Figure 8. High Precision Differential Voltage Meter
CLOSED LOOP
GAIN = 2
SWITCH
MATRIX
RCAL 1
RCAL 2
AFE 8
VBIAS
RCM1
CISO3
REF
AD8226
RACCESS3
RG
RCM2
VBIAS
CISO4
AFE 7
CISO1
AFE 6
RACCESS1
AFE 5
A
UNKNOWN Z
B
RACCESS4 RACCESS2
CISO2
AFE 4
D
0.2V
REFNHIZ 0.6V
DACCLK
P
×1.5
N
VBIAS
AFE 3
AFE 2
1.0V
12-BIT
DAC
PGA
GAIN
1/0.025
VREFDAC 1.8V
×1.5
T
ADC 16-BITS,
DFT
160kSPS
78SPS
AFE 1
AN_A
×1
RTIA
12168-008
VBIAS
RLIMIT
RCF
50kHz
MUX
EXTERNAL COMPONENTS
DAC_ATTEN_EN
EXCITATION
AMPLIFIER
DACP
LOOP
DACN 0.6V
Figure 9. 4-Wire Measurement System using the ADuCM350 and the Instrumentation Amplifier (AD8226)
Rev. 0 | Page 8 of 16
Application Note
AN-1302
How to Calculate the Unknown Z
After obtaining the current and voltage DFT measurements, the
part can exit the AFE Sequencer and calculate the impedance of
the sensor using the following equations:
Voltage measurement magnitude = SQRT(r2 + i2)
Voltage measurement phase = ATan(i/r)
Current measurement magnitude = SQRT(r2 + i2)
The 1.5 gain in the equation is the ratio between the gain of the
ADuCM350 current measurement channel, which is 1.5, vs. the
gain of the ADuCM350 voltage measurement channel which
is 1.
The gain of the in-amp is determined by the selection of RG.
For the AD8226, this is determined by
Current measurement phase = ATan(i/r)
where r and i are the real and imaginary components from the
voltage and current DFT measurements, respectively.
To calculate the Impedance Z, use Ohm’s law by dividing the
voltage magnitude by the current magnitude while taking into
account the gains of the signal chain.
Z(Magnitude) =
(Voltage magnitude/Current magnitude) ×
(1.5/1.494) × RTIA
The current measurement value is converted to a voltage, using
the RTIA, for measurement purposes. This gain needs to be taken
into account.
RG = (49.4 kΩ)/(G − 1)
Choosing
RG = 100 kΩ
results in a gain of 1.494.
Note that these equations are taken into account in the example
provided in the software development kit.
Rev. 0 | Page 9 of 16
AN-1302
Application Note
EXAMPLE OF A 4-WIRE BIO-ISOLATED SYSTEM
SENSOR CONFIGURATION
4-WIRE BIO-ISOLATED NETWORK
In the example described in this application note, measure the
impedance of an RC type sensor, with the configuration shown
in Figure 10, for a 30 kHz excitation signal. Note that TOL
indicates tolerance.
For this 4-wire example, select the following components:
The sensor details are as follows:
•
•
•
CS = 220 pF
RS = 20 kΩ
RP = 100 kΩ
•
A lead access resistor, RACCESS = 4.99 kΩ
•
An isolation capacitor, CISO, of 47 nF
If Z is close to or less then RACCESS, a potential divider effect
occurs which limits the bandwidth of the ADuCM350 thus
degrading accuracy (see Figure 11).
RLIMIT
CISO3
47nF
20%
A
RS = 20kΩ
TOL = 0.1%
RP = 100kΩ
TOL = 0.1%
CISO1
RACCESS3
4.99kΩ
RACCESS1
3kΩ
1%
47nF
20%
4.99kΩ
A
RC SENSOR
Figure 10. RC Sensor to be Measured
Calculate the complex sum of
RS + CS = Zs =
34962 ∠ −55.11
2.
B
RACCESS2
CISO2
4.99kΩ
47nF
20%
Figure 11. 4-Wire, Bio-Isolated Measurement Network
The total impedance of the sensor needs to be calculated to
verify the system accuracy.
1.
RACCESS4
4.99kΩ
12168-010
B
CISO4
47nF
20%
12168-009
CS = 220pF
TOL = 1%
Calculate Zs || with
Rp = ZT = 28337.15 ∠ −41.66
This is the total impedance of the RC sensor to be measured.
Rev. 0 | Page 10 of 16
Application Note
AN-1302
AFE OPTIMIZATION
Optimizing the ADuCM350 consists of the following steps:
Note the following:
1.
Calculate the RLIMIT resistor.
2.
Calculate RTIA.
•
•
3.
Calculate RG of the AD8226.
4.
Calculate RCAL.
•
•
CALCULATE THE RLIMIT RESISTOR
When calculating the RLIMIT resistor, note that
Maximum output voltage from ADuCM350 = 600 mV peak
Maximum allowed ac current at 30 kHz is
To prevent overranging of the ADC, put in a safety factor of
1.2, that is, the minimum impedance is 1.2 times less than the
specified minimum impedance of
300 μA rms (targeting IEC-60601) = 424 μA peak
Being conservative, set the maximum allowable ac current to
200 μA peak (<50%).
32.985 kΩ = 27 k Ω
RTIA with safety factor included = 41.2 kΩ/1.2
RTIA = 34.3 kΩ
RLIMIT ~= 600 mV peak/200 μA peak = 3 kΩ.
This calculation ignores CISO due to its small size.
Note that 33 kΩ is used for this example.
CALCULATE RTIA
RTIA is the feedback resistor on the TIA to convert the current to
a voltage.
CALCULATE RG OF THE AD8226
The maximum impedance of sensor
Minimum impedance/maximum current seen by the TIA is
ZUNKNOWN MAX
Z PNMIN = (Real) 2 + (Sumofimaginary) 2
(R
(XC
LIMIT
+ R ACCESS1MIN + ZUNKNOWN MIN + R ACCESS 2 MIN
ISO1MIN
+ XC ISO 2 MIN
)
2
Maximum voltage swing is 600 mV peak.
Highest signal current into TIA = 600 mV peak/32.98 kΩ
= 18.19 μA peak.
Peak voltage at output of TIA (maximum allowed by the
ADuCM350) = 750 mV peak.
RTIA resistor to give peak 750 mV voltage for peak signal
current is as follows:
RTIA = 750 mV/18.19 μA
RTIA = 41.2 kΩ
)
2
+
= 28.337 kΩ
A safety factor is incorporated on the RTIA to prevent the ADC
from overranging. The same needs to be done here thus the
maximum peak current is divided across differential inputs of
AD8226 by a factor of 1.2.
Peak current seen at VIN(AD8226) =
Assume 20 kΩ is the minimum impedance of ZUNKNOWN.
(18.19 μA peak)/1.2 = 15.16 μA peak
RACCESS1 = RACCESS2 = 4.99 kΩ
RLIMIT = 3 kΩ
VIN(AD8226) =
15.16 μA peak × 28.337 kΩ = 439.6 mV peak
XCISO1 = XCISO2 = 289.37 Ω
AD8226 G = 750 mV peak/(439.6 mV peak) = 1.706
ZPNMIN = 32.985 kΩ
If a further safety factor of 1.1 is used on the peak-to-peak of
the voltage (this may be unnecessary for the application), then
AD8226 G = 750 mV peak/(1.1 × 439.6 mV peak) = 1.55
AD8226 G = 1 + (49.4 kΩ/RG) = 1.55
RG = (49.4K)/(1.55 − 1) = 89.8 kΩ
Select an RG of 100 kΩ since it is a standard value.
AD8226G = 1 + (49.4 kΩ/RG) =
1 + (49.4 kΩ/100 kΩ) = 1.494
Note that the AD8226 has bandwidth limitations. For a
frequency of 50 kHz, the gain is limited to 10 (see Figure 12).
Rev. 0 | Page 11 of 16
AN-1302
Application Note
CALCULATE RCAL
70
60
VS = 2.7V
GAIN = 1000
The calibration of the auxiliary channel and TIA channel must
take into account the gain through the system.
50
GAIN = 100
•
For the voltage measurement channel, the auxiliary
channel is calibrated.
•
For the current measurement channel, the temperature
sensor is calibrated and the results are loaded to the offset
and gain registers of the TIA channel. This ensures that the
difference between the voltage and current gain is exactly
1.5.
30
20
GAIN = 10
10
0
GAIN = 1
–10
–20
–30
100
1k
10k
100k
FREQUENCY (Hz)
1M
10M
12168-111
GAIN (dB)
40
All this is done for the user in the 4-wire bio-isolated example
code in the software development kit.
Figure 12. Gain vs. Frequency of AD8226 at 2.7 V
Rev. 0 | Page 12 of 16
Application Note
AN-1302
4-WIRE BIO-ISOLATED MEASUREMENTS
HARDWARE SETUP FOR 4-WIRE BIOCONFIGURATON BOARD
1.
When setting up the EVAL-ADuCM350EBZ motherboard
2.
3.
4.
•
•
For the voltage measurement, insert LK1 (Auxiliary
Channel A).
Open LK6.
For the ADuCM350 4-wire bio-configuration board
•
•
Insert LK7, LK8, LK9, and LK10.
To measure the network shown in Figure 10 and Figure 11,
insert LK16, LK17, and LK21. The result should appear as
shown in Figure 13.
By default, the 4-wire configuration seen in Figure 10 and
Figure 11 is setup on the 4-wire bio-configuration board.
After downloading the software development kit, go to
C:\Analog Devices\ADuCM350BBCZ\EVALADUCM350EBZ\examples.
Click the BioImpedanceMeasurement_4Wire folder.
Click the .eww file in IAR.
During the download and debug stage, open the Terminal
I/O window to read the returned results.
Measurement Results
Impedance Magnitude
Measured result = 28405 Ω
Theoretical value measured ZT = 28337 Ω
However, the Cs of 220 pF used in the calculation had a
tolerance of 1%.
Upon analysis, the capacitor measured closer to 221 pF.
SOFTWARE SETUP FOR 4-WIRE BIOCONFIGURATION BOARD
In theory, a Cs of 221 pF would give a ZT of 28416 Ω vs. the
measured result of 28405 Ω.
Firmware Example
Code available in the ADuCM350 software development kit is
designed to be used with the 4-wire bio-configuration board to
validate the solution discussed in this application note.
Impedance Phase
The current 4-wire-bio-isolated configuration is not capable of
measuring accurate phase measurements.
If an absolute phase measurement is required, use a single chip
ADuCM350 4-wire measurement configuration. Note that this
configuration does not have isolation capacitors (CISO).
12168-011
The Readme.txt in the example folder provides more details on
the measurement.
For more details, refer to the Sensor Configuration section.
Figure 13. 4-Wire Measurements Display on Terminal I/O
Rev. 0 | Page 13 of 16
AN-1302
Application Note
SCHEMATICS FOR THE 4-WIRE BIO-CONFIGURATION BOARD
MOTHERBOARD CONNECTOR - ALIGN J8 AND J9.
J8-21
J8-2
J8-41
AFE6
J8-22
P4.2
J8-3
J8-43
J8-23
J8-4
J8-45
J8-25
J8-6
AFE7
J8-26
P0.10
J8-7
J8-27
J8-8
AFE3
REF_EXCITE_1
J8-29
J8-10
AFE8
J8-30
J8-11
J8-31
J8-12
J8-32
AFE4
J8-13
RCAL1
J8-33
J8-14
RCAL2
J8-34
J8-15
J8-35
J8-46
TIA_I
J9-4
J9-24
J9-5
J9-25
J9-45
J9-6
J9-26
J9-46
J9-7
J9-27
J9-47
J9-8
J9-28
J9-48
J9-44
J9-49
J8-50
J9-50
J8-51
J9-11
J9-31
J9-51
J8-52
J9-12
J9-32
J9-52
J8-53
J9-13
J9-33
J9-53
J8-54
J9-14
J9-34
J9-54
J8-55
J9-15
J9-35
J9-55
J8-56
J9-16
J9-36
J9-56
J9-17
J9-37
J9-57
VBIAS_1
J9-18
J9-38
J9-58
J9-19
J9-39
J9-59
VREF_1
J9-20
J9-40
J9-60
J8-58
TIA_O
J9-43
J9-29
J8-38
J8-40
VCCM_ANA J9-23
3.3V_BOARD J9-30
J8-57
J8-39
J9-42
J9-3
J9-41
J9-9
J8-37
J8-20
J9-21
J9-22
J9-10
J8-17
J8-19
J9-1
J9-2
J8-49
J8-36
AFE5
AN_D_1
J8-48
J8-16
J8-18
AN_C_1
J8-47
J8-28
J8-9
AN_B_1
J8-44
J8-24
AFE2
J8-5
AN_A_1
J8-42
3.3V_BOARD
J8-59
J8-60
12168-012
AFE1
J8-1
Figure 14. Motherboard Connector
4-WIRE BIO IMPEDANCE CONFIGURATION
ISOLATION + LEAD
LK8
AFE8
R32
RLIMIT (3.0kΩ)
R24
R25
47nF
4.99kΩ
UNCOMITTED CONFIGURATION
ISOLATION +LEAD
R1A
DNI
R1
DNI
R2A
AFE2
LK2
DNI
R2
DNI
SENSOR
R3A
AFE3
18nF
3
R17
R30
DNI
R4
2K
R15
DNI
LK4
R5A
AFE5
LK20
LK19
VBIAS_1
4
R12
100kΩ
INAMP_M
R9
20kΩ
R31
10MΩ
1
2
100kΩ
–IN
8
+VS RG
U3
7 OUT
AD8226
REF
RG
6
–VS
+IN
5
AFE4
LK5
DNI
R5
10µF
10MΩ
R6A
LK21
R16
DNI
0Ω
R13
R10
INAMP_P
220pF
AFE6
LK6
DNI
R6
6.8K
LK22
ISOLATION +LEAD
LK10
INAMP_M
R26
R29
47nF
4.99kΩ
ISOLATION + LEAD
LK7
AFE7
R19
R27
47nF
4.99kΩ
Figure 15. 4-Wire and Uncommitted Schematics
Rev. 0 | Page 14 of 16
12168-013
VBIAS_1
DNI
R3
R4A
C3
0.1µF
AN_A_1
LK17
LK16
VBIAS_1
LK3
LK18
VCCM_ANA
LK13
4.99kΩ
47nF
LK1
LK11
AFE1
LK12
R28
R23
LK14
LK9
LK15
INAMP_P
Application Note
AN-1302
RCAL
RCAL2
1kΩ
R18
TEST POINTS
RCAL1
TP5
TP1
TP4
TP6
TP12
M20-9991246 HARWIN
3.3V_BOARD
J1-4
M20-9991246 HARWIN
REF_EXCITE_1
J1-1
M20-9991246 HARWIN
VBIAS_1
J1-5
M20-9991246 HARWIN
VREF_1
J1-6
M20-9991246 HARWIN
VCCM_ANA
J1-12
TP2
TP3
M20-9991246 HARWIN
P0.10
J1-3
M20-9991246 HARWIN
P4.2
TP10
TP9
TP8
J1-2
TP7
IV GAIN
M20-9991246 HARWIN
AN_A_1
33kΩ
J1-10
M20-9991246 HARWIN
AN_B_1
J1-9
M20-9991246 HARWIN
AN_C_1
TIA_I
AN_D_1
R7
J1-8
M20-9991246 HARWIN
J1-7
TP11
DNI
M20-9991246 HARWIN
J1-11
TIA_O
Figure 16. Miscellaneous Schematics
Rev. 0 | Page 15 of 16
12168-014
R11
AN-1302
Application Note
NOTES
©2014 Analog Devices, Inc. All rights reserved. Trademarks and
registered trademarks are the property of their respective owners.
AN12168-0-4/14(0)
Rev. 0 | Page 16 of 16
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