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OPA2355 Buffer

Denver Walling
Prodigy 20 points
Other Parts Discussed in Thread: OPA2355, THS4531, ADS8354, THS4531A

Hello!

I am currently trying to implement the OPA2355 as a buffer.  Since the OPA2355 has two amps (A & B) in the chip, it will have two inputs, as well as two outputs that will feed into the THS4531 differential amplifier, which then feeds into the ADS8354 ADC.

I have already verified that my diff. amp works correctly, however, my issue arises with the OPA2355 in the fact that when I apply a small input voltage to the inputs of OPA2355, lets say 2mV, the differential output does not coincide with unity gain. The circuit uses simple negative feedback.  Below is a TINA screen shot of the basic buffer circuit I am trying to implement.  The documentation says that the enable pins can be left open to assume TTL high and therefore fully enable the amps. Is there something else I may be missing?


Thank you!

Denver

  • 0
    TI__Genius 12631 points

    Hello Denver,

    Looks like your TINA schematic screenshot did not come through.  In any case please go to the link below and find the "OPA2355 TINA-TI Reference Design" file. Please try that circuit. I tried the circuit with the enable pin floating and that doesn't seem to be an issue.

    Also, do you really need to use the OPA2355 buffer. Can you previous stage not drive a 1-2kOhm load? If it can you can using the circuit in Figure 85 of the THS4531A datasheet. You can remove the 49.9ohm input resistor and the 100pF capacitor. In that case your driving stage will see a 2kohm single-ended load (or a 4kohm differential load).

  • 0 in reply to Samir Cherian
    Prodigy 20 points

    Samir,

    We have tried that circuit in the design reference and it still did not work.

    The sensors that will be used will not output larger than 40mV and will have varying internal resistances (from 500Ohm-5kOhm) and the buffer is being used to decouple the sensor impedance so that it does not affect the gain on the differential amplifier.

    We would like to keep with dual supply for the buffer but that also seems to cause some issue because when we used a positive single supply, it seemed to also be a little closer to unity gain.


    I attempted to attach a snap shot of the circuit again, below.

    Thank you

  • 0 in reply to Denver Walling
    TI__Genius 12631 points

    Hi,

     I do not generally use floating voltage sources, but instead use an ideal transformer to convert a single-ended signal to differential. Please see attached schematic. The center tap is used to level shift the dc input to the amplifier.

    -SamirOPA2355_diff.TSC