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OPA657: OPA657 Single Supply Operation

Ajay Singhvi15
Prodigy 30 points

Part Number: OPA657

Hi,

I'm using the OPA657 as a high gain transimpedance amplifier. While it works well with split power supply rails, I want to use them with a single supply so that I can control them using TI's load switches.

To do this, I connect Vss to GND, and Vdd to +10V. However, the output I see on the scope is incorrect. In fact, I also see that the amount of power consumed is half of that with a split rail, which makes me think that I might be missing something. Is there anything else I should be doing to ensure correct operation with a single supply? (I imagine I am within the ICMR since my Vin+ is connected to gnd as it is, which would make the CM on Vin- also be 0V. )

Any help would be greatly appreciated!

Thanks,

Ajay

  • 0
    TI__Genius 12631 points
    Ajay,
    By setting the voltage at the noninverting pin at GND you are violating the OPA657s input common mode specification. The input common mode of the OPA657 is specified on page 5, section 7.5 of the datasheet at being -4V (Typ) on +/-5V supplies @ 25C. This means that for single-sided 10V/GND supplies the input common mode limit should be 1V.

    You should also account for the direction of current from the photodiode. For example if the photodiode is sourcing current (Its cathode is biased at a high positive voltage) then the output of the OPA657 will only swing in a negative direction and you will not have any room for the output to swing since the OPA657s output swing spec is also around 1V from each rail. In such a scenario you are better off setting the voltage at the noninverting pin at +9V so that you have the entire output swing range of the OPA657.

    You can use TINA TI and the OPA657 SPICE macro model to simulate this circuit. They can be downloaded from the TI web portal (TINA) and from the OPA657 product folder page (for the SPICE macro model) on the web respectively.

    Can you also please send me a schematic of the circuit you are using so I can be sure I have not misinterpreted anything.

    -Samir
  • 0 in reply to Samir Cherian
    Prodigy 30 points

    Hi Samir,

    Apologies for the delayed reply. My circuit is pretty similar to the typical application shown in Figure 34. I think you are slightly mistaken about the CM range because if I look at section 7.5 it specifies V_CM = 0V, which is the input common mode voltage, plus the non-inverting pin is set at GND in Figure 34 as well. My understanding is that as long as my input voltage is within the most positive voltage and most negative voltage specification, I should be fine? Is that the case? 

    However, you reply does clarify where I was going wrong when going for the single supply operation. If what I said is right, this means that for the single-ended case I also need to level shift the voltage at my non-inverting input to +5V to get performance identical to what I get when using +/-5V supplies (Basically a symmetric swing about the CM)?

    Thanks,

    Ajay

  • 0 in reply to Ajay Singhvi15
    TI__Genius 12631 points

    Hello,

      The Vcm = 0V in section 7.5 is when using +/- 5V supplies, so the common-mode has been set at mid-supply. If you set the common-mode less than -4V then you would be violating the amplifiers common-mode range. Figure 34 does not violate any of these rules for the aforementioned reason.

     You are correct, i.e - If you want the equivalent of Figure 34 in a single-supply (10V/GND) case then set the noninverting pin to 5V.

    -Samir