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[SOLVED] Current Transducer - Full Diferential Output Project (problem with amplifier)

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evandro.bubiak

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Hi, I am trying to make a current transducer with full differential output. It will be used for data acquisition.
The transducer uses a isolation amplifier optocoupler (U1) and a full differential instrumentation amplifier with a integrated ADC driver (IC1).

Everything is OK to the isolation amplifier, when I put 8 A on the input I get the correct output on the TP2 and TP3. But, when I connect the isolation amplifier output (U1) to the input of the differential amplifier (IC1) the output TP2 and TP3 goes down to the half of the value.

I tried to put a 1Mohm resistor on the inputs of the amplifier and even so the output of the opto drops to the half. As I know the current on the input of the amplifier was supposed to be insignificant (correct?) why could this be happening?

Need some help to understand whats going on. The voltage supplies are ok, the board is ok, I can't figure out whats happening.

Thanks.

upload_2015-8-4_11-10-43.png
 

AMS012

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As per the datasheet of U1, Rout of the optocoupler is about 21 ohms. So, loading should not be an issue here.
Did you ensure that the routing capacitance (as well as inductance) at the output on the board is not significant at your operating frequency?
 

evandro.bubiak

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As per the datasheet of U1, Rout of the optocoupler is about 21 ohms. So, loading should not be an issue here.
Did you ensure that the routing capacitance (as well as inductance) at the output on the board is not significant at your operating frequency?

Thanks. The test I am doing is a DC signal, so I think this is not the problem.
 

AMS012

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Ok, well. Then, have you checked whether the common mode operating points when both ICs are connected (and no signal) are good? Could it be because of common modes being different?
 

evandro.bubiak

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Ok, well. Then, have you checked whether the common mode operating points when both ICs are connected (and no signal) are good? Could it be because of common modes being different?

Yes, when there is no signal and opto and amplifier are disconnected the commom mode output of the opto is about 1,3V (exactly the value said on the datasheet) and when they are connected is almost the same value.

Here is a table with results in both cases, look at the differential voltage how they are different.

results.PNG

- - - Updated - - -

Sorry, the second part of the table is with "Connected Opto and Amplifier" not "Isolated".
 

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I solved the problem. It was a short circuit on a connector pin to the power plane.
 

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