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Is it ok to adjust AD8418 output anywhere between GND and Vs?

Specifically I'd need to adjust output to +1V when Vs is +3.3V.

I would do this by tying Vref2 to GND, and pulling Vref1 to +3.3V with ~120k resistor. Assuming 100k  internal resistor in Vref1 and Vref2 that should work, but does it lead to noise etc problems?
Thanks,

Ari

  • Hi Ari,

    Apologies for the delayed reply on this. You can definitely use an additional resistor at the REF pin in order to adjust the the output voltage level. The internal resistors in the AD8418 are large enough that noise from a 120kohm resistor should not add too much. Please keep in mind that the resistors at the REF pins are matched closely to each other, but may not be accurate to 100kohms. You will need to adjust the resistor value you are adding to get to your desired output voltage level.

    Also, a very important thing to note is that the internal configuration of the AD8418 is essentially a difference amplifier. In this configuration, the matching of resistors is very important to get high common-mode rejection. Adding a resistor at the reference adds a mismatch to the network which will cause your common mode rejection to suffer. You may also see some difference in gain, but ultimately it is the common-mode rejection that is affected the most.

    Are you going for a minimum number of supplies? What is your application?

    Thanks,

    Kris

  • Hi Ari,

    I would just like to give you additional information on how much you might expect the CMRR of the device to degrade with the additional resistance at the REF pin. I simulated this on Simetrix to give you an idea.

    The green trace is the CMRR taken without the resistor, and the blue trace is taken with the resistor. Adding the 120k resistance will cause the common mode rejection to go down about 40dB.

    I hope this helps.

    Thanks,

    Kris

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