Datasheet

AD5933 Data Sheet
Rev. E | 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
54
66
FREQUENCY (kHz)
IMPEDANCE (kΩ)
101.0
100.5
100.0
99.5
99.0
56 58 60 62
64
VDD = 3.3V
CALIBR
A
TION FREQUENCY
= 60kHz
T
A
= 25°C
MEASURED CALIBRA
TION IMPEDANCE = 100
kΩ
05324-023
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, Z
UNKNOWN
= 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
9-
10031224.1kHz5
kHz10
12-E458000.4
×+








×
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
Z
UNKNOWN
VOUT
CURREN
T
-
T
O-VO
LT
AGE
GAIN SETTING RESIS
TOR
PGA
(×1 OR ×5)
05324-024
Figure 24. System Voltage Gain
The gain through the system shown in Figure 24 is given by
GainPGA
Z
sistorSettingGain
RangeVoltageExcitationOuput
UNKNOWN
×
×
Re
For this example, assume the following system settings:
VDD = 3.3 V
Gain setting resistor = 200 kΩ
Z
UNKNOWN
= 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