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3 wire LVDT sensor_AD698 excitation and output

Question asked by jim.luo@longjingtech.com on Jan 19, 2015

Hello

I met a issue during I tuning AD698. I refer to the figure 7 on Page 6 in AD698 SPEC to do our design.

If I mounting the circuit follow the figure 7, the output of AD698 didn't changed with the changes of LVDT sensor.

original.JPG

But, if I connect -AIN to GND, the output of AD698 would be changed with the changes of LVDT sensor.

-AIN_GND.JPG

I set R1=4.7K, C1=6800pF, C2=C3=C4=200nF, R2=119K, Vs= +/- 15V.

our sensor's sensitivity = 83.006mV/V/mm, d= +/-1mm.

Base on these R and C setting, the excitation voltage is 3.5Vrms, the F_system = 5KHz. the output range is +/-5V.

 

According to AD698 SPEC,

the R1=12.7k, excitation voltage is 2.6V ~ 4.35V, why did I set R1=4.7k, my excitation voltage is 3.5Vrms?

Why did I should connect -AIN to GND to let AD698 working?

 

Could you point out my mistakes?

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