EVAL-AD5933EBZ Datasheet, PDF (21/32 Pages) Analog Devices – Evaluation Board for the 1 MSPS 12-Bit Impedance Converter Network Analyzer

Preliminary Technical Data
EVAL-AD5933EB
If Temperature <= &H1FFF Then ' msb =0.
' Positive Temperature.
Label8.Caption = (Temperature / 32#)
Else
' Negative Temperature.
Label8.Caption = (Temperature - 16384) / 32#
End If
're-assign variables used.
TemperatureUpper = 0
TemperatureLower = 0
Temperature = 0
End Sub
IMPEDANCE MEASUREMENT TIPS
This section outlines some of the workarounds for using the
AD5933 to measure impedance profiles under certain conditions.
Calibrating the AD5933
When calculating the calibration term (that is, the gain factor;
see the AD5933 data sheet for further details), it is important
that the receive stage is operating in its linear region. This
requires careful selection of the system gain settings. The
system gain settings are
• Output excitation voltage range
• Current-to-voltage gain setting resistor
• PGA gain
The gain through the system shown in Figure 27 is given by
GainSetting Resistor
OutputExcitationVoltageRange ×
× PGAGain
Z_UNKNOWN
CURRENT TO VOLTAGE
GAIN SETTING RESISTOR
RFB
VOUT
Z_UNKNOWN
VIN
VDD
LPF
PGA
(X1 OR X5)
ADC
Figure 27. AD5933 System Voltage Gain
is chosen, the voltage saturates the ADC, and, as a result, the
calculated calibration term (that is, the gain factor) is inaccurate.
The gain factor should be calculated when the largest response
signal is presented to the ADC while ensuring that the signal is
maintained within the linear range of the ADC over the
impedance range of interest. (The reference range of the ADC is
the supply AVDD.)
Therefore, based on your knowledge of the unknown
impedance span over the frequency range of interest, correctly
configure the system gain settings (see Figure 27). These
settings include the output excitation voltage range (Range 1,
Range 2, Range 3, or Range 4), the current-to-voltage amplifier
gain setting resistor, and the programmable gain amplifier
setting (either ×1 or ×5) that precedes the ADC.
Select a calibration impedance value that is approximately
halfway between the limits of the unknown impedance
(therefore, the impedance limits must be known to correctly
calibrate the system). Then, choose a value for the I-V gain
setting resistor that is equal to the calibration impedance. This
results in a unity gain condition on the receive side of the
current-to-voltage amplifier.
For example, assume the following system calibration settings:
VDD = 3.3 V
Gain setting resistor = 200 kΩ
ZUNKNOWN = 200 kΩ
PGA setting = ×1
Range 1 = 2 V p-p
The peak-to-peak voltage presented to the ADC input is 2 V p-p.
However, if a programmable gain amplifier setting gain of ×5
Rev. PrC | Page 21 of 32