Advice on selecting amplifier components for an autobalancing bridge (impedance meter)

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Joined Oct 19, 2013
I am currently building an impedance meter using the auto balancing technique. I intend to operate it from 1Hz to 25MHz powered by a 5V supply. The attached picture gives a summary of how the bridge should work high level but a more detailed explanation could be obtained from

From my understanding and calculations, i believe that the slew rate should be at least 44V/us and the Gain Bandwidth Product be at about 25MHz (i dont think the CMRR needs to be too high). The results i have obtained from simulation would be attached as "Simulation circuit.jpg". It shows both the circuit i intend to use and the bode plot for that circuit. The Vin corresponds to 'Vx', Vout corresponds to 'Vr', R2 the DUT and R1 would be 'Rf' in the high level auto balancing bridge. For the real circuit, i intend to use the AD5930 to produce a frequency sweep up to the 25MHz instead of the voltage source in the diagram. I also intend on connecting ADCs to Vout and Vin.I tried different set amplifiers for the best results but there are just too many to try (It would be nice if multisim had all).

The outputs, Vout and Vin should not exceed 2Vpp according to the specifications

I have a couple of questions:
  • Why is the output voltage (Vout) 180 degrees off even though i inverted the wave twice and the DUT was purely resistive?
  • What else should i look out for when choosing the amplifiers?
  • Could you suggest some amplifiers? (my budget varies but i am willing to put out depending on the quality of the results )
  • Do you have any recommendations or any suggestions that would make this a bit practically easier?
  • Do you have any questions for me? (ie, more info on the circuitry perhaps)

Thanks alot.


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