High impedence opamp quesion


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  1. #1
    Member yvesmazzon's Avatar
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    Default High impedence opamp quesion

    Hello all,

    I have a question on non inverting opamp. I'm using the MCP607 and the datasheet said it has a common mode input impedance of 1 E 13 ohms or 10 tera-ohms which is very high but I need this as I use I have to measure the potential on a pH electrode which is many tens of mega ohms. If I have on the inverting input resistor Rf and R1 both 56k it will give me a DC gain of 2. My question is: does having these resistors will affect my overall impedance on my non inverting input or shall I first having it as voltage follower were the inverting input is connected directly to the output and then amplify by 2 on a second inverter which will be bummer as I use the second opamp as virtual ground? Man thank in advance.
    Regards,

    Yves

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    Senior Member tumbleweed's Avatar
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    Default Re: High impedence opamp quesion

    In the non-inverting configuration you can pretty much ignore the resistors... the input impedance will be the impedance of the non-inverting input of the opamp.

    That's one of the advantages vs the inverting configuration where the impedance is set by the input resistor Rin since the opamp input is a virtual ground.

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    Member yvesmazzon's Avatar
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    Default Re: High impedence opamp quesion

    Hi Tumbleweed,

    Many thanks for your comment.

    regards,

    Yves

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    Prolific Poster rcurl's Avatar
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    Default Re: High impedence opamp quesion

    It just so happens that I designed an amplifier for a pH probe a few months ago. A couple of things you should be aware of:
    (1) Board cleanliness is crucial. Make sure to remove any solder flux.
    (2) It may help to put a "guard ring" around the traces associated with the non-inverting input. Connect the guard ring to the inverting input.
    (3) Make sure to use a good quality PC board material, Phenolic tends to absorb water. G10 may be better.

    -Rick

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    Prolific Poster rcurl's Avatar
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    Default Re: High impedence opamp quesion

    Hi Yves-
    Here's some information that may be helpful: https://www.ti.com/lit/an/snoa529a/snoa529a.pdf

    -Rick

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