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AD5933

Data Sheet

Rev. F | Page 18 of 40

TWO-POINT CALIBRATION

Alternatively, it is possible to minimize this error by assuming

that the frequency variation is linear and adjusting the gain

factor with a two-point calibration. Figure 23 shows an

impedance profile based on a two-point gain factor calculation.

101.5

98.5

FREQUENCY (kHz)

101.0

100.5

100.0

99.5

99.0

VDD = 3.3V

CALIBRATION FREQUENCY = 60kHz

TA = 25°C

MEASURED CALIBRATION IMPEDANCE = 100k

Ω

Figure 23. Impedance Profile Using a Two-Point Gain Factor Calculation

TWO-POINT GAIN FACTOR CALCULATION

This is an example of a two-point gain factor calculation

assuming the following:

Output excitation voltage = 2 V (p-p)

Calibration impedance value, ZUNKNOWN = 100.0 kΩ

PGA gain = ×1

Supply voltage = 3.3 V

Current-to-voltage amplifier gain resistor = 100 kΩ

Calibration frequencies = 55 kHz and 65 kHz

Typical values of the gain factor calculated at the two calibration

frequencies read

Gain factor calculated at 55 kHz is 1.031224E-09

Gain factor calculated at 65 kHz is 1.035682E-09

Difference in gain factor (∆GF) is 1.035682E-09 −

1.031224E-09 = 4.458000E-12

Frequency span of sweep (∆F) = 10 kHz

Therefore, the gain factor required at 60 kHz is given by

031224

kHz

458000













The required gain factor is 1.033453E-9.

The impedance is calculated as previously described.

GAIN FACTOR SETUP CONFIGURATION

When calculating the gain factor, it is important that the receive

stage operate in its linear region. This requires careful selection

of the excitation signal range, current-to-voltage gain resistor,

and PGA gain.

VIN

VDD/2

RFB

ADC

LPF

ZUNKNOWN

VOUT

CURRENT-TO-VOLTAGE

GAIN SETTING RESISTOR

PGA

(×1 OR ×5)

Figure 24. System Voltage Gain

The gain through the system shown in Figure 24 is given by

Gain

PGA

sistor

Setting

Gain

Range

Voltage

Excitation

Ouput

UNKNOWN

For this example, assume the following system settings:

VDD = 3.3 V

Gain setting resistor = 200 kΩ

ZUNKNOWN = 200 kΩ

PGA setting = ×1

The peak-to-peak voltage presented to the ADC input is

2 V p-p. However, if a PGA gain of ×5 was chose, the voltage

would saturate the ADC.

GAIN FACTOR RECALCULATION

The gain factor must be recalculated for a change in any of the

following parameters:

•

Current-to-voltage gain setting resistor

•

Output excitation voltage

•

PGA gain

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