Gain Factor Temperature Variation; Impedance Error - Analog Devices AD5934 Manual

GAIN FACTOR TEMPERATURE VARIATION

The typical impedance error variation with temperature is in
the order of 30 ppm/°C. Figure 21 shows an impedance profile
with a variation in temperature for 100 kΩ impedance using a
2-point gain factor calibration.
101.5
101.0
100.5
100.0
99.5
99.0
VDD = 3.3V
CALIBRATION FREQUENCY = 60kHz
MEASURED CALIBRATION IMPEDANCE = 100kΩ
98.5
54 56 58
FREQUENCY (kHz)
Figure 21. Impedance Profile Variation with Temperature Using a
2-Point Gain Factor Calibration

IMPEDANCE ERROR

Minimizing the impedance range under test optimizes the
AD5934 measurement performance. Following are the examples of
the AD5934 performance when operating in the six different
impedance ranges. The gain factor is calculated with a precision
resistor in each case. Note that R
for 2 V p-p. R was calibrated out in the gain factor calculations.
OUT
In Figure 22 to Figure 26, the 10 kHz excitation frequency was
generated using a 4 MHz clock.
Impedance Range 1 (0.1 kΩ to 1 kΩ)
The following conditions were used to conduct the tests shown
in Figure 22:
Output excitation voltage = 2 V p-p
Calibration impedance value, Z
PGA gain = ×1
Supply voltage = 3.3 V
Current-to-voltage amplifier gain resistor = 100 Ω
+125°C
+25°C
–40°C
60 62 64 66
was measured to be 200 Ω
OUT
= 100 Ω
CALIBRATION
7
R
FB
CALIBRATION IMPEDANCE = 0.1kΩ
6 T
A
5
4
3
2
1
0
Figure 22. Impedance Range 1 Typical % Impedance Error over Frequency
Impedance Range 2 (1 kΩ to 10 kΩ)
The following conditions were used to conduct the tests shown
in Figure 23:
Output excitation voltage = 2 V p-p
Calibration impedance value, Z
PGA gain = ×1
Supply voltage = 3.3 V
Current-to-voltage amplifier gain resistor = 1 kΩ
2.0
R
FB
CALIBRATION IMPEDANCE = 1kΩ
1.8
T
A
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0
Figure 23. Impedance Range 2 Typical % Impedance Error over Frequency
Rev. A | Page 17 of 40
= 0.1kΩ
= 25°C
10 35 60
FREQUENCY (kHz)
= 1 kΩ
CALIBRATION
= 1kΩ
= 25°C
10 35 60
FREQUENCY (kHz)
AD5934
0.5kΩ
1kΩ
100
5kΩ
10kΩ
100