EVAL-AD5933EBZ datasheet(21/32 Pages) AD

21 / 32 page

Preliminary Technical Data

EVAL-AD5933EB

Rev. PrC | Page 21 of 32

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

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

Gain

PGA

Resistor

Setting

Gain

Range

Voltage

Excitation

Output

UNKNOWN

VIN

VDD

RFB

ADC

LPF

ZUNKNOWN

VOUT

CURRENT TO VOLTAGE

GAIN SETTING RESISTOR

PGA

(X1 OR X5)

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.

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

ssume the following system calibrat

VDD = 3.3 V

Gain setting re

ZUNKNOWN = 200 kΩ

GA sett

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.

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