CN-0217 Datasheet, PDF (2/6 Pages) Analog Devices – High Accuracy Impedance Measurements Using 12-Bit Impedance Converters

CN-0217
CIRCUIT DESCRIPTION
The AD5933 and AD5934 have four programmable output voltage
ranges; each range has an output impedance associated with it.
For example, the output impedance for a 1.98 V p-p output voltage
is typically 200 Ω (see Table 1).
Table 1. Output Series Resistance (ROUT) vs. Excitation Range
for VDD = 3.3 V Supply Voltage
Range
Output Excitation
Amplitude (V p-p)
Output Resistance (ROUT)
Range 1 1.98
200 Ω typical
Range 2 0.97
2.4 kΩ typical
Range 3 0.383
1.0 kΩ typical
Range 4 0.198
600 Ω typical
The output impedance affects the impedance measurement
accuracy, particularly in the low kΩ range, and must be taken
into account when calculating the gain factor. Refer to the
AD5933 or AD5934 data sheets for more details on the gain
factor calculation.
A simple buffer in the signal chain prevents the output impedance
from affecting the unknown impedance measurement. Select a
low output impedance amplifier with sufficient bandwidth to
accommodate the AD5933/AD5934 excitation frequency. An
example of the low output impedance achievable is shown in
Figure 2 for the AD8605/AD8606/AD8608 family of CMOS op
amps. The output impedance for this amplifier for an AV of 1 is
less than 1 Ω up to 100 kHz, which is the maximum operating
range of the AD5933/AD5934.
100
90 ^{VS = 2.7V}
80
70
60
A_V = 100
50
A_V = 10
40
30
A_V = 1
20
10
0
1k
10k
100k
1M
10M
FREQUENCY (Hz)
Figure 2. Output Impedance of AD8605/AD8606/AD8608
100M
Circuit Note
Matching the DC Bias of Transmit Stage to Receive Stage
The four programmable output voltage ranges in the AD5933/
AD5934 have four associated bias voltages (see Table 2). For
example, the 1.98 V p-p excitation voltage has a bias of 1.48 V.
However, the current-to-voltage (I-V) receive stage of the AD5933/
AD5934 is set to a fixed bias of VDD/2 as shown in Figure 1.
Therefore, for a 3.3 V supply, the transmit bias voltage is 1.48 V, and
the receive bias voltage is 3.3 V/2 = 1.65 V. This potential difference
polarizes the impedance under test and can cause inaccuracies in
the impedance measurement.
One solution is to add a simple high-pass filter with a corner
frequency in the low Hz range. Removing the dc bias from the
transmit stage and rebiasing the ac signal to VDD/2 keeps the dc
level constant throughout the signal chain.
Table 2. Output Levels and Respective DC Bias for VDD = 3.3 V
Supply Voltage
Range
Output Excitation
Amplitude (V p-p)
Output DC
Bias Level (V)
1
1.98
1.48
2
0.97
0.76
3
0.383
0.31
4
0.198
0.173
Selecting an Optimized I-V Buffer for the Receive Stage
The I-V amplifier stage of the AD5933/AD5934 can also add
minor inaccuracies to the signal chain. The I-V conversion
stage is sensitive to the amplifier's bias current, offset voltage,
and common-mode rejection ratio (CMRR). By selecting the
proper external discrete amplifier to perform the I-V conversion,
the user can choose an amplifier with lower bias current and
offset voltage specifications along with excellent CMRR, making
the I-V conversion more accurate. The internal amplifier can
then be configured as a simple inverting gain stage.
Selection of the RFB resistor still depends on the gain through
the system as described in the AD5933/AD5934 data sheets.
Optimized Signal Chain for High Accuracy Impedance
Measurements
Figure 1 shows a proposed configuration for measuring low
impedance sensors. The ac signal is high-pass filtered and rebiased
before buffering with a very low output impedance amplifier. The
I-V conversion is completed externally before the signal returns
to the AD5933/AD5934 receive stage. Key specifications that
determine the required buffer are very low output impedance,
the single-supply capability, low bias current, low offset voltage,
and excellent CMRR performance. Some suggested parts are the
ADA4528-1, AD8628, AD8629, AD8605, and AD8606. Depending
on board layout, use a single-channel or dual-channel amplifier.
Use precision 0.1% resistors for both the biasing resistors (50 kΩ)
and gain resistors (20 kΩ and RFB) to reduce inaccuracies.
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