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OPA335 usage

Paul Leong
Prodigy 220 points
Other Parts Discussed in Thread: DAC1220, OPA335

Hi,

Here is my scenario.  I have 2 voltages (one coming out of the TI DAC1220, the other from a voltage reference chip for 2.5V).  I want to make the 2 voltage source go into a +Sig and -Sig input of a device.  The difference of the voltage can be in the uV range (as low as 4.7 uV difference).

Can I use this op amp to make it so that I have as little noise as possible (< 4.7uV between the 2 signals)?  If so, how do i connect the 2 signals to the op-amp?

 

Thanks,

Paul

  • 0
    TI__Guru 64530 points

    Paul,

    You may NOT directly drive the inputs of an OPA335 with a differential voltage as this will cause the output to move against one of its rails.  Operational amplifiers are ONLY designed for the feedback to drive the inverting input to a virtual short with the non-inverting input terminal.  For that reason even shorting the inputs together (with no differential voltage) will push the output to one of its rails as the inherent input voltage offset gets gained up by a very high open-loop gain of 130dB (AOL~ 3,160,000)

    For your specific application, you need to use either an instrumentation amplifier (INA), programmable gain amplifier PGA) or the difference amplifier shown below (in gain of 10).   Just make sure to pay attention to the polarity of the differential voltage so that the output is not pushed against its negative rail in this single supply application.

  • 0 in reply to Marek Lis
    Prodigy 220 points

    Hi Marek,

    Thanks for your response.  I am wondering if I will gain anything by passing it through a unity gain amp?  What I am thinking is just directly connecting the output of the the DAC1220 and a voltage reference of 2.5V (both with the same ground) directly since I don't need any gain.  I want just unity gain.  This will go into the TI ADC1230 (20 bit A to D converter) AINP and AINN input.  Can you give me your opinion on this?  The 2.5V voltage reference is ultra low noise (1.2 uV) while the DAC1220 I believe is about 1uV for 0.1Hz to 10Hz.  Combining the two will give me 2.2 uV noise, which is under 4.6uV requirement I have.  I am just wondering if there are issues I didn't think of by connecting them directly as differential input to the ADC1230.  I already made sure it's not in the common mode rejection region of the ADC1230.

    Thanks,

    Paul