Datasheet

Preliminary Technical Data EVAL-AD5933EB
Rev. PrC | Page 21 of 32
Temperature <= &H1FFF Then ' msb =0. If
' 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
ds for using the
.
tion term (that is, the gain factor;
ltage range
sistor
h the system shown in Figure 27 is given by
This section outlines some of the workaroun
AD5933 to measure impedance profiles under certain conditions
Calibrating the AD5933
When calculating the calibra
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 vo
• Current-to-voltage gain setting re
• PGA gain
The gain throug
GainPGA
ResistorSettingGain
RangeVoltageExcitationOutput ××
Z
UNKNOWN
VIN
VDD
RFB
ADC
LPF
Z
UNKNOWN
V
OUT
CURRENT TO VOLTAGE
GAIN SETTING RESISTOR
PGA
(X1 OR X5)
05435-027
Figure 27. AD5933 System Voltage Gain
For example, a ion settings:
sistor = 200 kΩ
The resented to the ADC input is 2 V p-p.
However, if a programmable gain amplifier setting gain of ×5
is chosen, the voltage saturates the ADC, and, as a result, the
calculated calibration term (that is, the gain factor) is inaccurate.
s
impedance range of interest. (The reference range of the ADC is
gain settings (see Figure 27). These
settings include the output excitation voltage range (Range 1,
plifier
o correctly
calibrate the system). Then, choose a value for the I-V gain
his
e
ssume the following system calibrat
VDD = 3.3 V
Gain setting re
Z
UNKNOWN
= 200 kΩ
GA sett
P ing = ×1
Range 1 = 2 V p-p
peak-to-peak voltage p
The gain factor should be calculated when the largest response
signal is presented to the ADC while ensuring that the signal i
maintained within the linear range of the ADC over the
the supply AVDD.)
Therefore, based on your knowledge of the unknown
impedance span over the frequency range of interest, correctly
configure the system
Range 2, Range 3, or Range 4), the current-to-voltage am
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 t
setting resistor that is equal to the calibration impedance. T
results in a unity gain condition on the receive side of th
current-to-voltage amplifier.