ADA4891 Output circuit design (resistor RG design value)


I am making an experimental circuit and looking at the characteristics.

I have a question, please let me know.

The circuit of several hundred channels is switched by the multiplexer.
The received signal is switched by the following circuit and output to the ADC.

In the noise measurement of the receiving amplifier picked up by the ADC,
In rare cases, channels with a high noise level will occur.

At that time, if the resistor RG is changed, the noise level will change.
This resistance is thought to control ringing due to the multiplexer and the capacitive component of the subsequent stage.

Is there a specific calculation method for the RG value?
Although it is set to 33Ω, this resistance value is changed for channels with a high noise level.
It was confirmed that this reduced the noise level.
I want to know this principle.

The ICs used are ADA4891 and ADG708.

I also investigated the area around the multiplexer,
I think it is due to the characteristics of the amplifier.
Just connect a resistor in series to the output of the amplifier,
No capacitor is attached.

The peripheral circuit is attached.

Best Regards,


  • 0
    •  Analog Employees 
    on Mar 31, 2021 6:43 AM


    What is the bandwidth you are using in your application?
    What is the noise level you are seeing?

    Thanks and regards!

  • Hi, Fdelaram

    As a result of the verification, it is possible that the ADC in the latter stage is doing something wrong.
    The model number is LTC2323-16.

    The bandwidth used by the application is about 20kHz.
    The center frequency is 180kHz and the sampling frequency is 560kHz.

    A waveform is attached for the noise level.
    The vertical axis is just a digital value. It is ± 20FS.
    The horizontal axis is time (0 ~ 260ms)

    The reason why channels with high noise level are generated is
    Is it a matching between ADC and Amp?

    Replacing the ADC will eliminate the symptom.
    If you return it to the abnormal product again, it will recur.

    Certain LTC2323-16 have symptoms.

    Changing the resistor RG may improve the noise level,
    We are investigating relevance and principles.

    Best Regards,