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THP210: How to measure fully differential amplifier input impedance in pspice?Result interpretation

Electronx Electronx
Intellectual 285 points
Part Number: THP210


Other Parts Discussed in Thread: ADS131E08S, , OPA2210, OPA2192, PGA855, INA851

Before the differential ADC, I have a fully differential amplifier as the front end. I want to simulate and measure differential input impedance.

Why am I using the 500/2k=1/4 ratio? because I want to include the incoming signal in the range.

My working range is max 20 khz

ADS131E08S is the adc I use and in the datasheet I read that the input impedance is 200MΩ. For this reason, I installed a 200MΩ resistor as a load.



(V(Vin+)-V(Vin-) ) / (I(E2:3)- I(E1:3))

Generally, there is a concept of high input impedance at the input of cards that read ADC. Let's assume the differential opamp input goes to the connectors. In this case, is the opamp input really a value like 2k ohm?

here are the possibilities.

  1. The conclusion is correct, you should do something to increase the input impedance
  2. I misunderstand the concept of differential impedance.
  3. I make an input calculation error
  4. Everything is wrong Can someone help sort out the confusion in my mind?
  • 0
    TI__Guru 51626 points

    Hi Electronx,

    Yes, the input impedance of any fully-differential amplifier (FDA) is a direct function of the input resistors. 

    The simulation result shows the correct result, with 2kΩ input resistors, providing a 2kΩ input impedance.  If you need a higher input impedance, for example 20kΩ, you could scale the resistors higher with RIN=20kΩ, RFB = 5kΩ.  The THP210 has relatively low input bias current, so depending on the BW requirement, you may able to increase the resistors higher as long as the circuit is properly compensated for stability and the circuit meets your noise and accuracy requirements.

    There are limitations on how high the resistances on the feedback/gain resistor network of the FDA can be due to stability, noise, BW and input bias current considerations.  If you a very high input impedance in the ~100s ΜΩ, or increasing the impedances is impractical due to these considerations, you can always buffer the THP210 circuit inputs with a dual op-amp in the follower configuration.  For example, depending on your input bias current requirement, you could buffer the THP210 with a dual OPA2210 op-amp.  The OPA2210 is a bipolar input op-amp offering nanoamps of input bias current offering very low voltage noise.  The OPA2192 is a precision CMOS input amplifier offering picoamps of input bias current.    

    Also consider the INA851, this is an instrumentation amplifier with differential outputs.  This device offers very high-input impedance inputs, and can be set on attenuation of 0.2-V/V (1/5).  The PGA855 is also a fully-differential output programmable gain amplifier that offers binary gains, which would meet your 1/4 gain requirement.  The PGA855 offers binary programmable gains of  0.125, 0.25, 0.5, 1, 2, 4, 8, 16 V/V.  The PGA855 and INA851 input impedance is very high, in the ~100 GΩ range.

    Thank you and Regards,

     Luis