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OPA330: Input Impedance

Richard Elmquist
Mastermind 45250 points
Part Number: OPA330

I am having a little difficulty understanding the impedance specifications. The data sheet for the OPA330 does not list any impedance values.

I did read a post in E2E that explained that you could calculate the common-mode impedance of an op amp (OPA330) by taking the change in the common mode voltage and dividing it by the change in the in the bias current. Many data sheets have a graph of this so calculating this is possible. I have done this calculation and it appears that the common-mode impedance of the OPA330 is (5V/5pA=) 1000Gohms. Is this the correct approach?

 The issue that I am having is that the customer is asking for the differential impedance. This is not listed, but is there a way to calculate this? I did see a post that stated that if the op amp was produced using a CMOS process that you could estimate that the differential impedance would be 100Mohms and the common-mode impedance would be 6000Gohms. I am not sure if this applies to these chopper driven op amps like the OPA330. Can you elaborate at all on this? Are there any calculations available? Are there any impedance specifications available?

 Thanks for your help with this!

Richard Elmquist

  • 0
    TI__Mastermind 45250 points
    Has anyone had a chance to look at the request above?

    Can you give a time frame as to when you might be able to provide the information?

    Thanks for your help with this!

    Richard Elmquist
  • 0
    TI__Guru 64130 points

    Richard,

    It is correct that to calculate the common-mode input resistance you could take the change in the common mode voltage and divide it by the change in the input bias current - for OPA330 this would yield around 1Teraohm (1e12).  Just be aware that since IB in OPA330 is dominated by the input current spikes coming from closing/opening of the chopper switches, the IB value is just an average integrated value of IB.

    When it comes to the question of differential input impedance, I would like to understand the reason behind the question.  If the OPA330 is used as is meant to be used in a close-loop configuration, the inputs are at virtual short and the differential input impedance is in 100's Mohms. However, if one thinks about using OPA330 in an open-loop configuraton (e.g. as a comparator), it is a completely different situation because of the parasitic back-to-back diodes between the input terminals that will gradually get turn-on when the input terminals are pulled more than couple hundred millivolts apart - see OPA330 equivalent circuit below.

  • 0 in reply to Marek Lis
    TI__Mastermind 45250 points
    Marek,
    I believe that he is just looking for the closed loop impedance. There was no mention of using the part open loop, but I will explain this when I respond to him.
    Thanks so much for your help with this!
    Richad Elmquist
  • 0 in reply to Richard Elmquist
    TI__Mastermind 45250 points

    Marek,

    This will be a closed loop application. Here is his explanation:

    We want to use it precisely as an amplifier in closed loop configuration with a gain of 100.
    We need to know the input impedance to evaluate how much it is going to charge the signal source (which has an impedance around 50 Kohms) and how much the CMRR will be deteriorate by the source imbalance.

    Can you be more specific as the input impedance that will be seen? Or will there be a lot of variance over temperature, Vin and Vcc? Please let me know so that I can explain this to the customer.

    Thanks for your help with this !

    Richard Elmquist

  • 0 in reply to Richard Elmquist
    TI__Guru 64130 points

    Richard,

    If your customer is concern about the loading effects of the OPA330 input impedance vis-a-vis 50kohm source impedance then they need to consider only the common-mode input impedance, which is around 1 Tera ohm (1E12), resulting in (50k/1e12)*100%=0.000005% error (or -146dB);  this will vary relatively little over temperature and supply voltage.  A larger effect on the gain error might be due to the fact that unlike in most op amps where input bias current, IB, has the same polarity on both inputs, in case of chopper amplifiers, like OPA330, IB's have opposite polarity (IB flows into one but comes out of the other input terminal) - this is shown in the datasheet as the input bias offset current being twice a high as the input bias current - see below.  All in all, I do not believe your customer needs to worry about the error associated with the OPA330 input impedance.

  • 0 in reply to Marek Lis
    TI__Mastermind 45250 points
    Marek,
    Thanks so much for your detailed response!
    I will let you know if the customer has any further questions.
    Have a great weekend!
    Richard Elmquist