AD5933 datasheet(30/40 Pages) AD

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AD5933

Data Sheet

Rev. F | Page 30 of 40

TYPICAL APPLICATIONS

MEASURING SMALL IMPEDANCES

The AD5933 is capable of measuring impedance values up to

10 MΩ if the system gain settings are chosen correctly for the

impedance subrange of interest.

If the user places a small impedance value (≤500 Ω over the

sweep frequency of interest) between the VOUT and VIN pins,

it results in an increase in signal current flowing through the

impedance for a fixed excitation voltage in accordance with

Ohm’s law. The output stage of the transmit side amplifier

available at the VOUT pin may not be able to provide the

required increase in current through the impedance. To have a

unity gain condition about the receive side I-V amplifier, the

user needs to have a similar small value of feedback resistance

for system calibration as outlined in the Gain Factor Setup

Configuration section. The voltage presented at the VIN pin is

hard biased at VDD/2 due to the virtual earth on the receive

side I-V amplifier. The increased current sink/source

requirement placed on the output of the receive side I-V

amplifier may also cause the amplifier to operate outside of

the linear region. This causes significant errors in subsequent

impedance measurements.

The value of the output series resistance, ROUT, (see Figure 35)

at the VOUT pin must be taken into account when measuring

small impedances (ZUNKNOWN), specifically when the value of

the output series resistance is comparable to the value of the

impedance under test (ZUNKNOWN). If the ROUT value is unac-

counted for in the system calibration (that is, the gain factor

calculation) when measuring small impedances, there is an

introduced error into any subsequent impedance measurement

that takes place. The introduced error depends on the relative

magnitude of the impedance being tested compared to the value

of the output series resistance.

PGA

I-V

VDD/2

RFB

VIN

AD8531

AD820

AD8641

AD8627

VDD

20kΩ

1µF

VDD/2

VOUT

ROUT

RFB

DDS

2V p-p

ZUNKNOWN

TRANSMIT SIDE

OUTPUT AMPLIFIER

Figure 35. Additional External Amplifier Circuit for Measuring Small

Impedances

The value of the output series resistance depends upon the

selected output excitation range at VOUT and has a tolerance

from device to device like all discrete resistors manufactured in

a silicon fabrication process. Typical values of the output series

resistance are outlined in Table 17.

Table 17. Output Series Resistance (ROUT) vs. Excitation Range

Parameter

Value (Typ)

Output Series Resistance Value

Range 1

2 V p-p

200 Ω typ

Range 2

1 V p-p

2.4 kΩ typ

Range 3

0.4 V p-p

1.0 kΩ typ

Range 4

0.2 V p-p

600 Ω typ

Therefore, to accurately calibrate the AD5933 to measure small

impedances, it is necessary to reduce the signal current by

attenuating the excitation voltage sufficiently and also account

for the ROUT value and factor it into the gain factor calculation

(see the Gain Factor Calculation section).

Measuring the ROUT value during device characterization is

achieved by selecting the appropriate output excitation range at

VOUT and sinking and sourcing a known current at the pin

(for example, ±2 mA) and measuring the change in dc voltage.

The output series resistance can be calculated by measuring the

inverse of the slope (that is, 1/slope) of the resultant I-V plot.

A circuit that helps to minimize the effects of the issues

previously outlined is shown in Figure 35. The aim of this

circuit is to place the AD5933 system gain within its linear

range when measuring small impedances by using an additional

external amplifier circuit along the signal path. The external

amplifier attenuates the peak-to-peak excitation voltage at

VOUT by a suitable choice of resistors (R1 and R2), thereby

reducing the signal current flowing through the impedance and

minimizing the effect of the output series resistance in the

impedance calculations.

In the circuit shown in Figure 35, ZUNKNOWN recognizes the

output series resistance of the external amplifier which is

typically much less than 1 Ω with feedback applied depending

upon the op amp device used (for example, AD820, AD8641,

AD8531) as well as the load current, bandwidth, and gain.

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