Circuit Note

CN-0217

Rev. A | Page 3 of 6

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

Voltage

Parameter

Value

Voltage Peak-to-Peak (V p-p)

1.98 V (Range 1)

Number of Settling Time Cycles

15

MCLK

16 MHz

20.1 Ω

20.0 Ω

Excitation Frequency Range

30 kHz to 30.2 kHz

Unknown Impedances

R1 = 10.3 Ω, R2 = 30.0 Ω,

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,

10.3 Ω measured 10.13 Ω, an approximate 2% error. Choosing

measurement; that is, the smaller the unknown impedance range is

Consequently, for large unknown impedance ranges, it is possible

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.

35

30

25

20

15

10

5

0

29.95

30.00

30.05

30.10

30.15

30.20

10.3Ω

30Ω

1µF

30.25

FREQUENCY (kHz)

Figure 3. Measured Low Impedance Magnitude Results

20

0

–20

–40

–60

–80

–100

29.95

30.00

30.05

30.10

30.15

30.20

1µF

30.25

FREQUENCY (kHz)

10.3Ω, 30Ω

Figure 4. Measured Low Impedance Phase Results

10.22

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

30.20

30.25

FREQUENCY (kHz)

Figure 5. Measured 10.3 Ω Magnitude Results (Expanded Scale)