CN-0217 Datasheet, PDF (3/6 Pages) Analog Devices – High Accuracy Impedance Measurements Using 12-Bit Impedance Converters

Circuit Note
CIRCUIT EVALUATION AND TEST
The schematic in Figure 1 was developed to improve impedance
measurement accuracy, and some example measurements were
taken. The AD8606 dual-channel amplifier buffers the signal on
the transmit path and converts the receive signal from current to
voltage. For the three examples shown, the gain factor is calculated
for each frequency increment to remove frequency dependent
errors. A complete design package including schematics, bill of
materials, layout, and Gerber files is available for this solution at
www.analog.com/CN0217-DesignSupport. The software used is
the same software that is available with evaluation boards and is
accessible from the AD5933 and AD5934 product pages.
Example 1: Low Impedance Range
Table 3. Low Impedance Range Setup for VDD = 3.3 V Supply
Voltage
Parameter
Value
Voltage Peak-to-Peak (V p-p)
1.98 V (Range 1)
Number of Settling Time Cycles 15
MCLK
16 MHz
RCAL
20.1 Ω
RFB
20.0 Ω
Excitation Frequency Range
30 kHz to 30.2 kHz
Unknown Impedances
R1 = 10.3 Ω, R2 = 30.0 Ω,
C3 = 1 µF (ZC = 5.3 Ω at
30 kHz)
The results of the low impedance measurements are shown in
Figure 3, Figure 4, and Figure 5. Figure 5 is for the 10.3 Ω
measurement and is shown on an expanded vertical scale.
The accuracy achieved is very much dependent on how large the
unknown impedance range is relative to the calibration resistor,
RCAL. Therefore, in this example, the unknown impedance of
10.3 Ω measured 10.13 Ω, an approximate 2% error. Choosing
an RCAL closer to the unknown impedance achieves a more accurate
measurement; that is, the smaller the unknown impedance range is
centered on RCAL is the more accurate the measurement.
Consequently, for large unknown impedance ranges, it is possible
to switch in various RCAL resistors to break up the unknown
impedance range using external switches. The RON error of the
switch is removed by calibration during the RCAL gain factor
calculation. Using a switch to select various RFB values can
optimize the dynamic range of the signal seen by the ADC.
In addition, note that to achieve a wider range of measurements
a 200 mV p-p range was used. If the unknown Z is a small range, a
larger output voltage range can be used to optimize the ADC
dynamic range.
CN-0217
35
30Ω
30
25
20
15
10.3Ω
10
1µF
5
0
29.95
30.00
30.05 30.10 30.15
FREQUENCY (kHz)
30.20 30.25
Figure 3. Measured Low Impedance Magnitude Results
20
10.3Ω, 30Ω
0
–20
–40
–60
–80
1µF
–100
29.95
30.00
30.05 30.10 30.15
FREQUENCY (kHz)
30.20
Figure 4. Measured Low Impedance Phase Results
10.22
30.25
10.20
10.18
10.16
10.14
10.12
10.10
10.08
10.06
10.04
29.50
30.00
30.05 30.10 30.15
FREQUENCY (kHz)
30.20
30.25
Figure 5. Measured 10.3 Ω Magnitude Results (Expanded Scale)
Rev. A | Page 3 of 6