CN-0217 Datasheet, PDF (5/6 Pages) Analog Devices – High Accuracy Impedance Measurements Using 12-Bit Impedance Converters

Circuit Note
120
R0
100
80
C6
60
40
R1
C5
20
R2
R3
0
30 32 34 36 38 40 42 44 46 48 50
FREQUENCY (kHz)
Figure 10. Magnitude Results for Example 2: R1, R2, R3, C5, C6
Example 3: Parallel R-C (R||C) Measurement
An R||C type measurement was also made using the configuration,
using an RCAL of 1 kΩ, an R of 10 kΩ, and a C of 10 nF, measured
across a frequency range of 4 kHz to 100 kHz. The magnitude
and phase results vs. ideal are plotted in Figure 11 and Figure 12.
Table 5. R||C Impedance Range Setup for VDD = 3.3 V Supply
Voltage
Parameter
Value
Voltage Peak-to-Peak (V p-p)
0.383 V (Range 3)
Number of Settling Time Cycles 15
MCLK
16 MHz
RCAL
RFB
Excitation Frequency Range
1 kΩ
1 kΩ
4 kHz to 100 kHz
Unknown Impedance R||C
R = 10 kΩ, C = 10 nF
4000
3500
3000
2500
2000
1500
1000
IDEAL
MEASURED
500
0
4
24
44
64
84
104
FREQUENCY (kHz)
Figure 11. Magnitude Results for ZC = 10 kΩ||10 nF, RCAL = 1 kΩ
CN-0217
–60
–65
–70
–75
–80
IDEAL
MEASURED
–85
–90
–95
4
24
44
64
84
104
FREQUENCY (kHz)
Figure 12. Phase Results for ZC = 10 kΩ||10 nF, RCAL = 1 kΩ
Setup and Test
The evaluation board software is the software used on the
EVAL-AD5933EBZ. Refer to the technical note available on the
CD provided with the evaluation board for details on the board
setup. Note that there are alterations to the schematic. Link
connections on the EVAL-AD5933EBZ board are listed in Table 4.
In addition, note that the location for RFB is located at R3 on the
evaluation board, and the location for ZUNKNOWN is C4.
Table 6. Link Connections for EVAL-AD5933EBZ
Link Number
Default Position
LK1
Open
LK2
Open
LK3
Insert
LK4
Open
LK5
Insert
LK6
A
Complete setup and operation for the hardware and software
for the evaluation board can be found in User Guide UG-364.
COMMON VARIATIONS
Other op amps can be used in the circuit, such as the ADA4528-1,
AD8628, AD8629, AD8605, and AD8608.
Switching Options for System Applications
For this particular circuit, the ZUNKNOWN and RCAL were interchanged
manually. However, in production, use a low on-resistance switch.
The choice of the switch depends on how large the unknown
impedance range is and how accurate the measurement result
needs to be. The examples in this circuit note use just one
calibration resistor, and so a low on-resistance switch, such as
the ADG849, can be used as shown in Figure 13. Multichannel
switch solutions, such as the quad ADG812, can also be used.
The errors caused by the switch resistance on the ZUNKNOWN are
removed during calibration, but by choosing a very low RON
switch, the effects can be further minimized.
Rev. A | Page 5 of 6