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How to meet high input resistance and input bias current return requirements?

Question asked by standard on May 31, 2013
Latest reply on Jun 7, 2013 by standard

I am working on a circuit which needs very high input impedance; the minimum is 100 Mohm but the higher the better. The JFET input op amps or instrumentation amplifiers are ideal for that purpose. After consideration I chose the AD8626 with less than 1 pA input bias current as front-end buffers and then difference amplifier and signal amplification. My questions:

  • How can I provide return pass for the input bias current simultaneously with the required high input impedance? (I did not find solution applicable in the Application Notes).
  • A ‘simple’ 100Mohm or higher resistance between input pin and ground can satisfy both requirements? The voltage drop caused by the bias current is common mode error so it is rejected but the offset current caused drop is differential so it is amplified in the next stage.
  • I have to apply input protection as it is suggested in all the application notes which means external serial resistance and low leakage diode clamps between the positive and negative power supply lines. For instance the BAV199 has 0.003 – 5 nA reverse current. Is it possible to use these diodes –or similar- for driving the bias current into the ground?
  • May the application of the Howland Current Pump be a solution?

 

Your help will be highly appreciated because I have not found solution for this matter in the literature available.

 

Laszlo

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