AN-1271: Optimizing the ADuCM350 for Impedance Conversion (Rev. A) PDF

AN-1271
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 Impedance Conversion
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.
output voltage, the RTIA/CTIA combination, and the calibration
resistor all need to be calculated. The maximum allowed
current into the load dominates the calculation.
This application note details how to set up the ADuCM350
to optimally measure the impedance of an RC sensor using a
2-wire measurement approach. To optimize the accuracy of the
impedance measurement, the user must maximize the usage of
the16-bit ADC range. To do this, the peak-to-peak excitation
However, if there is a limitation on the load current, for
example, to meet IEC 60601 standards in 2-wire, bioimpedance applications, then the user should calculate the
maximum allowable current and use precautionary measures in
the circuitry.
If there is no limitation, then the user has the ability to
maximize the amount of signal swing into the ADC from
the transimpedance amplifier (TIA) to get the best SNR
possible.
Rev. A | Page 1 of 8
AN-1271
Application Note
TABLE OF CONTENTS
Introduction ...................................................................................... 1
Scenario 1: No Limitation on Load Current ..............................3
Revision History ............................................................................... 2
Scenario 2: Limited Load Current ..............................................4
Details................................................................................................. 3
Impedance Measurement Example in LabVIEW .....................6
Sensor Configuration ................................................................... 3
Impedance Measurement Example in Software
Development Kit ...........................................................................8
Calculate the Minimum Ideal Impedance of the Sensor......... 3
RCAL Calculation ........................................................................ 3
REVISION HISTORY
3/14—Rev. 0 to Rev. A
Changes to Figure 5 .......................................................................... 6
12/13—Revision 0: Initial Version
Rev. A | Page 2 of 8
Application Note
AN-1271
DETAILS
To calculate the magnitude of YP
SENSOR CONFIGURATION
In the example described in this application note, the user
wants to measure the impedance of an RC type sensor
with the configuration shown in Figure 1 for a 1 kHz excitation
signal.
Magnitude = R 2 + I 2
|YP| = 0.00377
Invert this to get
The sensor details are as follows:
|ZP| = 265.16 Ω
CP = 10 nF to 600 nF
Calculate the phase of YP.
RP = 10 kΩ
Phase (rads) = atan (I/R) = 1.544
RS= 1 kΩ
Phase (degrees) = Phase (rads) ×
180
= −88.48°
π
Impedance of parallel RC = ZP = 265.16 ∠ −88.48
Now, add the RS series resistor.
CP
ZT = ZP + ZS
RP
ZP = 265.16 ∠ −88.48 = 7.03 –i265.06
ZS = 1000 ∠ 0 = 1000 + i0
RS
11993-001
Add the two complex numbers.
ZT = 1007.03 – i265.06 = 1041 ∠ −14.75
Figure 1. Sensor RC Configuration
CALCULATE THE MINIMUM IDEAL IMPEDANCE OF
THE SENSOR
The first step is to calculate the lowest unknown impedance of
the sensor. This allows the user to calculate the highest current
signal into the TIA.
For the sensor in Figure 1, the impedance of the sensor is at its
minimum when CP = 600 nF.
To calculate the total impedance, ZT, the first step is to calculate
the impedance of the CP capacitor.
Z Cp
1
= −i265.26
=
2πfC P
This is the lowest impedance realized by the part and it
determines the maximum amount of current seen by the TIA.
RCAL CALCULATION
To calculate the RCAL value to calibrate the system, the lowest
unknown impedance, Z, is used. If RCAL is equal to the
magnitude of minimum impedance, the signal going into the
DFT will be large. This improves repeatability and accuracy.
Therefore, an RCAL of ~1041 Ω is used for this example.
SCENARIO 1: NO LIMITATION ON LOAD CURRENT
Where there is no limitation on current seen by load, the
maximum signal swing is used to maximize the SNR of ADC
results.
where:
f is an excitation frequency of 1 kHz.
CP is 600 nF.
•
•
Then, calculate the impedance of the parallel components
RP || CP.
•
1/ZP = 1/ZRp + 1/ZCp
•
or to simplify
YP = YRp + YCp
YRp = 1/10,000
YCp = 1/−i265.26
YP =1× 10-4 + i3.77 × 10−3 (C lags by 90°).
Maximum voltage swing is 600 mV peak.
Highest signal current into TIA = 600 mV peak/1041 Ω
= 0.576 mA 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:
750 mV/0.576 mA = 1.302 kΩ
To improve the anti-aliasing performance and stability of the
receive channel, an anti-aliasing capacitor is put in parallel with
RTIA. The 3 dB point of 80 kHz is selected (this is the maximum
bandwidth of the system).
CTIA =
Rev. A | Page 3 of 8
1
1
=
= 1.5 nF
2πfRTIA
2 × π × 80 kHz × 1.302 kΩ
AN-1271
Application Note
AFE_DAC_OFFSET_UNITY
SWIT CH
MATRIX
–900mV
RCAL1
600mV p-p
RCAL2
AFE8
AFE7
×2
CLOSED LOOP
GAIN = 2
D
EXCITATION
AMPLIFIER
LOOP
0
DAC_ATTEN_EN = “0” DAC_ATTEN_EN
DAC_ATTEN_EN
–300mV
VREFDAC 1.8V
RCF
50kHz
1.0V
DACP
12-BIT
PGA
DACN 0.6V
DAC
600mV
GAIN
1/0.025 REFNHIZ 0.6V
RP
AFE_DAC_GAIN
DAC_CODE[11:0]
DACCLK
–3584 DECIMAL
P
600mV
N
600mV p-p
–900mV
–512 DECIMAL
3072 CODES p-p DECIMAL
–300mV
AFE5
CP
1
0.2V
AFE6
1.7V
AFE_DAC_OFFSET_ATTEN
AFE4
500mV
1.2V p-p
AFE2
1.85V
DIFFERENTIAL
1.1V
AFE1
2.25V p-p
RS
1.5V p-p
0.9V
1.125V p-p
0V
0.35V
GAIN = 1.5
4Ω
T
TIA_I
RTIA
1.302kΩ
6Ω
0.9V
TIA_O
CTIA
1.5nF
4Ω
VREF 1.8V
ANTI-ALIAS
FILTERING
VBIAS = 1.1V
ADC RANGE = 3.6V
AFE3
ASSUMING
VBIAS of 1.1V
ADC
6Ω
1.1V
0.9V
1.125 V p-p
ADC
MUX
11993-002
±576µA
Figure 2. Signal Swings with No Limitation on Load Current
SCENARIO 2: LIMITED LOAD CURRENT
When there is a limitation on the load current, then a different
approach is taken. In this example, the IEC 60601 bodily
floating standard allows a maximum of 100 µA rms leakage.
Thus, for this example, it is safe to assume that 50 µA rms/
70.7 µA peak is the maximum current.
From a single fault correction perspective, with regard to the
bodily floating standard, the following is included on each leg:
•
•
1 µF dc CS blocking capacitor
A series resistor representing some form of leads (RLEAD)
Minimum impedance of the sensor remains at 1041 Ω. The
series components now add to this minimum impedance seen
by the ADuCM350 TIA.
Calculating the impedance of extra circuitry in the network
gives
200 Ω + 100 Ω +100 Ω + 1 µF + 1 µF
Assume an excitation frequency of 1 kHz.
Capacitors are in series, thus
Connect an extra current limiting series resistance of 200 Ω to
the drive leg, RLIMIT.
CT = (C1 × C2)/(C1 + C2)
where CT = 0.5 µF.
ZC = 1/(2 πfCT) = 1/(2 × π ×1 kHz × 0.5 µF) = −i318.3
RLEAD
100Ω
CS
RLIMIT
200Ω
RT = RLIMIT + RLEAD + RLEAD = 400
Total extra circuitry impedance = 400 –i318.3
1µF
The minimum impedance seen by the TIA is the minimum
sensor impedance plus the minimum extra circuitry impedance
converter.
RP
RS
RLEAD
100Ω
CS
ZT = (400 – i318.3) + (1007.03 – i265.06) = 1407 – i583.4
1µF
= 1523 ∠ −22.5
11993-003
CP
Figure 3. Sensor with Single Fault External Protection
This is the minimum impedance seen by the ADuCM350. For
safety reasons, reduce this by 20% to avoid unwanted overranging of ADC results.
Rev. A | Page 4 of 8
Application Note
AN-1271
Therefore, a minimum impedance of 1218.4 Ω is assumed.
The Cortex-M3 flags any impedance measurement below value
as an invalid result. Check the connections because the flag
indicates that the ADC has overranged or encountered another
error.
With a bigger LSB size, there is less resolution of measurement,
thus more quantization noise in creating the sinewave and
measuring the response.
Therefore, to allow a maximum of 70.7 µA peak with a
minimum impedance of 1218.4 Ω, a sinewave amplitude is
needed.
Calculate the current seen by TIA.
Continuing with this example, proceed using a 15 mV peak
sinwave with attenuation enabled (DAC_ATTEN = 1).
15 mV peak/1218.4 Ω = 12.3 µA peak signal
Then, calculate the RTIA and CTIA to optimize the ADC range
where the RTIA resistor gives the peak 750 mV voltage signal
current.
70.7 × 10−6 × 1218.4 Ω = 86 mV peak
Note that the maximum allowed sinewave amplitude when the
DAC attenuator is enabled, DAC_ATTEN = 1, is 15 mV peak.
Because 86 mV peak exceeds this value, there are two options.
The first option is to use 15 mV peak with a reduced signal-tonoise ratio. The second option is to disable the DAC attenuator
and select 86.5 mV peak in nonattenuation mode. The
downside of this is that the LSB size increases 40 times.
750 mV/12.3 µA = 60.98 kΩ
To improve the anti-aliasing performance and improve the
stability of the receive channel, an anti-aliasing capacitor is put
in parallel to RTIA. The 3 dB point of 80 kHz is selected (the
maximum bandwidth of the system).
CTIA = 1/2πfRTIA
Nonattenuator mode
= 1/ (2π × 80 kHz × 60.98 kΩ)
LSB size = 1.6 V/2 = 390 µV p-p = 195 µV peak
12
= 32.6 pF
Attenuator mode
LSB size = 1/40 (1.6 V/2 )= 9.76 µV p-p = 4.88 µV peak
12
AFE_DAC_OFFSET_UNITY
SWIT CH
MATRIX
–607.5mV
RCAL1
30mV p-p
RCAL2
RLIMIT
RLEAD
100Ω CS 200Ω
1µF
–1.115V
AFE8
AFE7
×2
CLOSED LOOP
GAIN = 2
D
0
DAC_ATTEN_EN = “1” DAC_ATTEN_EN
DAC_ATTEN_EN
–592.5mV
VREFDAC 1.8V
RCF
50kHz
1.0V
DACP
12-BIT
PGA
DACN 0.6V
DAC
EXCITATION
AMPLIFIER
LOOP
600mV
GAIN
1/0.025 REFNHIZ 0.6V
RP
AFE_DAC_GAIN
DAC_CODE[11:0]
DACCLK
–3584 DECIMAL
P
600mV
N
600mV p-p
–900mV
–512 DECIMAL
3072 CODES p-p DECIMAL
–300mV
AFE5
–1.085V
30mV p-p
1
0.2V
AFE6
CP
AFE_DAC_OFFSET_ATTEN
AFE4
AFE2
1.85V
DIFFERENTIAL
1.1V
AFE1
1.5V p-p
0.9V
1.125V p-p
0V
0.35V
GAIN = 1.5
VBIAS = 1.1V
4Ω
T
TIA_I
RTIA
61kΩ
6Ω
0.9V
TIA_O
CTIA
32.6F
4Ω
ADC RANGE = 3.6V
1µF
AFE3
VREF 1.8V
ADC
6Ω
±12.3µA
1.1V
ADC
MUX
Figure 4. Signal Swings When Limitation On Load Current
Rev. A | Page 5 of 8
0.9V
1.125 V p-p
11993-004
RLEAD
100Ω CS
2.25V p-p
RS
ANTI-ALIAS
FILTERING
ASSUMING
VBIAS of 1.1V
AN-1271
Application Note
IMPEDANCE MEASUREMENT EXAMPLE IN LABVIEW
11993-005
The ADuCM350 LabVIEW® GUI can measure impedance and rapidly prototype sensor measurement.
Figure 5. LabVIEW Impedance Measurement AFE Control
Rev. A | Page 6 of 8
AN-1271
11993-006
Application Note
Figure 6. LabVIEW Impedance Measurement
Rev. A | Page 7 of 8
AN-1271
Application Note
IMPEDANCE MEASUREMENT EXAMPLE IN SOFTWARE DEVELOPMENT KIT
The IAR embedded workbench based software development kit includes an ImpedanceMeasurement_2Wire example. This example
verifies the performance of the impedance converter on the ADuCM350.
The sequence for the example is measuring three unknown 2-wire impedances.
•
•
•
AFE3 to AFE4
AFE4 to AFE5
AFE5 to AFE6
The code has easy programmability for excitation frequency, excitation voltage, and RCAL value. It is possible to code other changes to
the measurement into the measurement sequence. The readme section of this example project provides more details.
11993-005
Figure 7 shows an example measurement. Both real and imaginary components are calculated.
Figure 7. Signal Swings When Limited On Load Current
©2013–2014 Analog Devices, Inc. All rights reserved. Trademarks and
registered trademarks are the property of their respective owners.
AN11933-0-3/14(A)
Rev. A | Page 8 of 8
Similar pages