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Data Sheet

AD5933

Rev. E | Page 31 of 40

The key point is that the output impedance of the external

amplifier in Figure 35 (which is also in series with ZUNKNOWN)

has a far less significant effect on gain factor calibration and

subsequent impedance readings in comparison to connecting

the small impedance directly to the VOUT pin (and directly in

series with ROUT). The external amplifier buffers the unknown

impedance from the effects of ROUT and introduces a smaller

output impedance in series with ZUNKNOWN.

For example, if the user measures ZUNKNOWN that is known to

have a small impedance value within the range of 90 Ω to

110 Ω over the frequency range of 30 kHz to 32 kHz, the

user may not be in a position to measure ROUT directly in

the factory/lab. Therefore, the user may choose to add on

an extra amplifier circuit like that shown in Figure 35 to the

signal path of the AD5933. The user must ensure that the

chosen external amplifier has a sufficiently low output series

resistance over the bandwidth of interest in comparison to the

impedance range under test (for an op amp selection guide, see

www.analog.com/opamps). Most amplifiers from Analog

Devices have a curve of closed loop output impedance vs.

frequency at different amplifier gains to determine the output

series impedance at the frequency of interest.

The system settings are

VDD = 3.3 V

VOUT = 2 V p-p

R2 = 20 kΩ

R1 = 4 kΩ

Gain setting resistor = 500 Ω

ZUNKNOWN = 100 Ω

PGA setting = ×1

To attenuate the excitation voltage at VOUT, choose a ratio

of R1/R2. With the values of R1 = 4 kΩ and R2 = 20 kΩ,

attenuate the signal by 1/5th of 2 V p-p = 400 mV. The

maximum current flowing through the impedance is 400 mV/

90 Ω = 4.4 mA.

The system is subsequently calibrated using the usual method

with a midpoint impedance value of 100 Ω, a calibration

resistor, and a feedback resistor at a midfrequency point in the

sweep. The dynamic range of the input signal to the receive side

of the AD5933 can be improved by increasing the value of the

I-V gain resistor at the RFB pin. For example, increasing the I-V

gain setting resistor at the RFB pin increases the peak-to-peak

signal presented to the ADC input from 400 mV (RFB = 100 Ω)

to 2 V p-p (RFB = 500 Ω).

The gain factor calculated is for a 100 Ω resistor connected

between VOUT and VIN, assuming the output series resistance

of the external amplifier is small enough to be ignored.

When biasing the circuit shown in Figure 35, note that the

receive side of the AD5933 is hard-biased about VDD/2 by

design. Therefore, to prevent the output of the external

amplifier (attenuated AD5933 Range 1 excitation signal) from

saturating the receive side amplifiers of the AD5933, a voltage

equal to VDD/2 must be applied to the noninverting terminal

of the external amplifier.

AD5933 Datasheet(PDF) 31 Page - Analog Devices

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