CN-0217

Circuit Note

Rev. A | Page 2 of 6

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).

Range

Output Excitation

Amplitude (V p-p)

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

less than 1 Ω up to 100 kHz, which is the maximum operating

range of the AD5933/AD5934.

FREQUENCY (Hz)

100

90

0

1k

100M

10k

100k

1M

10M

80

70

20

60

50

30

10

40

Figure 2. Output Impedance of AD8605/AD8606/AD8608

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/

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

level constant throughout the signal chain.

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.

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)