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Measurement of Ground Referenced Impedance

Question asked by conorgray on Jun 17, 2015
Latest reply on Jul 3, 2015 by conorgray

Hi,

 

I want to measure a load impedance that is referenced to ground.

 

I have suggested a possible solution below, that places a current sense resistor in series with load to measure current. 

It is placed in the circuit in such a way that its impedance will not affect the measurement.

 

The internal current sense amplifier is not used , as it requires that the load have a virtual potential of Vbias on one terminal,

which is not possible in our case as it cannot be explicitly isolated (one terminal is grounded and we cannot change that).

 

The output of the differential external current amplifier is fed directly to the ADC.

 

Is it possible to do what is suggested below and get results on a par with the normal method of measuring impedance?

 

The reason I ask is because I did tests with the 4 wire bio configuration board and with the 2 wire option the results were pretty good.

However the 4 wire results were not so good particularly the phase results were completely.

Is this because of the phase response of the external amp used on this eval. Module or is there some other reason?

 

 

Analog switches set to use Calibration Resistor:Figure1.png

Analog switches set to use Unknown Load Impedance:

 

Figure2.png

 

Thanks in Advance,

 

Kind regards,

 

Conor.

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