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AD5934

Data Sheet

Rev. E | Page 16 of 31

2-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 2-point calibration. Figure 19 shows an impedance

profile based on a 2-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 = 100kΩ

Figure 19. Impedance Profile Using a 2-Point Gain Factor Calculation

2-POINT GAIN FACTOR CALCULATION

This is an example of a 2-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 1.031224 × 10−9.

Gain factor calculated at 65 kHz is 1.035682 × 10−9.

Difference in gain factor (ΔGF) is

1.035682 × 10−9 − 1.031224 × 10−9 = 4.458000 × 10−12.

Frequency span of sweep (ΔF) is 10 kHz.

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

1.031224

kHz

4.458000E













The required gain factor is 1.033453 × 10−9.

The impedance is calculated as previously described in the

Impedance Calculation section.

GAIN FACTOR SETUP CONFIGURATION

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

stage is operating in its linear region. This requires careful selection

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

and PGA gain. The gain through the system shown in Figure 20

is given by

Output Excitation Voltage Range ×

UNKNOWN

Resistor

Setting

Gain

× PGA Gain

VIN

VDD/2

RFB

ADC

LPF

ZUNKNOWN

VOUT

CURRENT-TO-VOLTAGE

GAIN SETTING RESISTOR

PGA

(×1 OR ×5)

Figure 20. System Voltage Gain

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, had the user chosen a PGA gain of ×5, 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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