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How can I convert +-5V to a 4 to 20mA loop current signal?

Question asked by Tychoaussie on Jun 2, 2015
Latest reply on Jun 15, 2015 by JGunao

Hi,

I have an electromechanical seismometer with a +-5V voice coil signal output representing velocity, and I want to signal condition the line by converting the signal to a 4 - 20mA output. I recently experimented with the EVAL-CN0314-EB1Z circuit eval board that features the AD8420 amplifier and it looks to be fairly promising. However it's only configured for a single-ended input. My signal is bidirectional. Is there a good way to add some offset to the circuit ? I was thinking of substituting a 6.8K resistor into R4 to add some DC offset from the reference onto the input signal.

A concern I have is how these low resistance values affect my sensor. I have not seen anything within the literature for the AD8420 on how input resistance affects noise on the input, but the voltage divider used on this board is going to load my sensor. What are the tradeoffs of moving R3/R4 and R5 into the Mohm range? Such as setting R3 = 1.1M, R5 = 100K, R4 = 680K. This would reduce the loading of my signal coil to a manageable level. However I am unsure of the compromise in terms of noise or accuracy.

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