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OPA858DSGEVM: Unexpected Output Question

John Albrecht
Intellectual 1110 points
Part Number: OPA858DSGEVM
Other Parts Discussed in Thread: OPA858

Tool/software:

Hi TI Experts, 

I am using the OPA858 in an RX application, where it is receiving signal from an antenna before attenuating it and sending it to a high speed ADC. 

When using the OPA858EVM i expected an output attenuation of gain 7, as mentioned in the DS but that is not at all what i observed. I ended up troubelshooting and removing R6 and R7, which fixed the output and gave me the 7 gain with my 600MHz signal. 

What are R6 and R7 for in this design, and why would it compromise the gain of the amplifier? I'm assuming that R6 is a series output resistor to limit current, and R7 is a shunt to GND for 50Ohm impedance matching, but they acted more like a resistor divider making the output attenuation lower than expected. 

Thanks for your help with this!

Best Regards,
John Albrecht 

  • 0
    TI__Intellectual 2480 points

    Hi John,

    The amplifier gain is set by R2 and R4, so G= 1 + R2/R4 (= 7 in this case).

    The R6 and R7 are for different purpose. R6 acts as an isolation resistor and helps in keeping the opamp stable if a capacitive load is connected at the J5.

    R7 in parallel with R6 makes the overall impedance looking in from the J5 connector equal to 50ohms. So if a 50ohm load is connected at J5, the impedance will be matched (50ohms on both source and the load side) and hence will reduce signal reflection. But in this case overall signal voltage will get attenuated by 6dB.

    Regards

    Anant