Using differential Amplifier on LTC1968, and the input voltage changes; please help

I am currently using LTC1968 to output DC RMS value from a sine wave signal input, and the output of the LTC1968 is connected to the differential amplifier. The issue is after connecting the LTC1968 output to the differential amplifier, the LTC1968 signal gets affected (the voltage decreases). Can anyone please explain me why this is happening, and how can I fix this issue?

LTC1968: https://www.analog.com/media/en/technical-documentation/data-sheets/1968f.pdf

I hope to hear your response.

Thank you



fixed mistakes.
[edited by: shurp at 7:10 PM (GMT -4) on 21 Sep 2021]
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  • +1
    •  Analog Employees 
    on Sep 21, 2021 8:58 PM

    Hi Shurp:  Yes, this happens simply because the LTC1966/7/8 output is not buffered.  The data sheet spec for output impedance of LTC1968 is 12.5k Ohms typically.   So you need to buffer the outputs before doing the difference amplifier.  Or, you could use a much higher impedance difference amplifier (such as LT1991), except that the + and - inputs of a difference amplifier present different loads upstream.  The +input presents 2R to ground, while the -input presents 1R to +input.

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  • +1
    •  Analog Employees 
    on Sep 21, 2021 8:58 PM

    Hi Shurp:  Yes, this happens simply because the LTC1966/7/8 output is not buffered.  The data sheet spec for output impedance of LTC1968 is 12.5k Ohms typically.   So you need to buffer the outputs before doing the difference amplifier.  Or, you could use a much higher impedance difference amplifier (such as LT1991), except that the + and - inputs of a difference amplifier present different loads upstream.  The +input presents 2R to ground, while the -input presents 1R to +input.

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