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Cancelation of first harmonic

Question asked by laia on Dec 5, 2016

Good afternoon,

 

I would greatly appreciate any help on this issue, I am not an electrical engineer and I am struggling with something that may be very simple. 

[The ideal circuit is like the one attached but generating the voltage with the lock-in instead of the resistor circuit]

I am creating a current with a current source that has its reference locked with a lock-in amplifier. The voltage drop across the sample is measured and the lock-in provides the same value in order to cancel the first harmonic to read the third one. To do so, I have a PCB board with 3 AD8221 with no resistor (gain 1):

- the first one gets the voltage drop across the sample

- the second one gets the voltage from the lock-in (1 BNC cable) and the other input is connected to the ground

- the third amplifier gets the output from the other two amplifiers 

The output of the third AD8221 is connected to the lock-in amplifier (minus the ground) to measure the value (which should be 0 if we want to cancel the signal or the 1w value if nothing is generated). However, the output value from the AD8221 is not the one read with the multimeter, it is like the gain is not 1. I tried cleaning the PCB board and resoldering new amplifiers but I had no luck.

I am wondering if the capacitors showed in page 20 (http://www.analog.com/media/en/technical-documentation/data-sheets/AD8221.pdf) are required? Right now I don't have them connected because the PCB board does not have space for them, maybe this is the problem. The ground I am using is the one from the voltage source (Agilent E3631A). Is this a good ground?  

 

Thank you,

Laia. 

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