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OPA857EVM-978: Increase output swing

Part Number: OPA857EVM-978
Other Parts Discussed in Thread: OPA857,

To evaluate the OPA857 I purchased a OPA857EVM-978 board but I am a bit confused by the output configuration of the board. First of all my board had R15 installed and not R14. So there was no output at VOUTP but there was at VOUTN. Not a big deal but took me some time to figure out.

After shining some light onto the PIN photodiode (Hamamatsu S5972) I was wondering about the low output level. I could clearly see my pulses but the amplifier was saturating at about 30mV peak (with 3.3V VCC). Measuring with a probe directly at R6 and R7 revealed a voltage swing of around 500mV. What is the purpose of R6 and R7? It is increasing the output load of the amplifier while sacrificing some gain?

Is it safe to short R6,R7 and remove R8? The two 25 Ohms in series with the output pins should make up for a total of 50 Ohms differential. Which the 1:1 BALUN will transform to 50 Ohms single ended. Do I miss something here?

Thank you,Tobias

  • Hi Tobias,

    The output transformer is there to convert the pseudo-differential output of the OPA857 into a fully differential output if required. Installing R15 just turns it back into a single ended signal centered at ground.

    You are correct. R6, R7, and R8 give the amplifier a 1kΩ load, to improve its characteristics. They also give it a 50Ω source resistance to drive a long cable at a high frequency without reflections. Then, you can back-calculate what the output would be in your circuit with a short trace and no impedance matching attenuation or loading.

    If you are using the EVM itself as a stand-alone system, you can remove R8 and set R6=R7=25Ω. The amplifier will have a higher load and might have worse specs that with a light load..

    Best regards,


  • Hi Sean,

    Thank you for your explanations. To keep the output impedance of the TIA high without loosing much signal when interfacing to 50 Ohms single-ended I then see only two options:

    1. Use a higher impedance ratio balun. E.g. 1:10
    2. Use a opamp as difference amplifier

    But anyways first I will try to reduce R6 and R7 to 25 Ohms while removing R8 and see how it works. My application doesn't care really about the linearity of the amplifier, I more need to see if there is some pulse or not.

  • Hi Tobias, the amplifier will still see a low load under those conditions. That is not really a problem. The EVM was just designed to show the amplifier in the best possible light.