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CN0349 and AD5934

Question asked by AndreaM on May 18, 2016
Latest reply on Apr 6, 2017 by R.L

I read the real data and imaginary data.

 

Magnitude = ((RealData ^ 2) + (ImagineryData ^ 2)) ^ 0.5

 

Impedance = 1 / (Magnitude * GainFactor)

 

I followed the instructions of the datasheet

 

but at Zunknown = 1000 Ohm real data 1F9hex Imaginary data ED67hex

Magnitude = 4787 dec

 

at Zunknown = 100 Ohm real data 847hex Imaginary data B1F4 hex

Magnitude = 20092dec

 

 

at Zunknown = 200 Ohm real data 7C2hex Imaginary data b70Fhex

Magnitude = 18778dec

 

 

gain factor RFB 100ohm and Impedance 100Ohm at 500Hz frequency start and 1MHz clock connected (on the CN349 ther is an external oscillator at 1MHz)

 

Gain Factor = (1/Impedance)/Magnitude = (1/100)/20092 = 4,97576745395283E-07

 

for 1000 ohm and 200 ohm if I use this formula Impedance = 1 / (Magnitude * GainFactor)


I expect a result of 1000 ohm but bu my result is 420 ohm

I expect a result of 200 ohm but bu my result is 107ohm

 

where am I wrong?

 

I have also seen the datasheet AD5934 , AN1252 Application note,CN0217 but the result does not change.

 

you can help me

 

I have to set the following values:

Start Frequency

Frequency Increment

Number of Increments

 

 

In the initial configuration is set to a frequency of start from the formula on page 13/32 of the AD5934 datasheet (the datasheet always say 16MHz but on the board in question is a 1 MHz oscillator).

 

 

I calculate the gain factor and then the resistance.

 

 

I followed to the letter the manual

 

 

GAIN FACTOR CALCULATION

IMPEDANCE CALCULATION USING GAIN FACTOR

 

 

I tried to make a calibration for two points:

2-POINT CALIBRATION

2-POINT GAIN FACTOR CALCULATION

 

 

I saw the datasheet AD5934, AN1252 Application note, CN0217 but the result does not change (although sometimes say different things like RFB resistance as to be calculated and that of Rcal see AN1252 calibration). Things not mentioned in the datasheet AD5934 CN0349 and that even makes satisfactory tests with resistance from 400 Ω to 39 700 Ω only with calibration for one point RFB = 1K Ω and RCAL = 10K Ω.

Or with two points for calibration measurements with resistance from 1 Ω to 6120 Ω.

I not really understand why you change the frequency and to the extent that entails, because I did tests with different frequencies and the result changes a little and still linearly with respect to two different frequencies and same resistance under test.

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