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OPA858: OPA858

Dan Clark
Prodigy 30 points
Part Number: OPA858

I have the NEW OPA858 EVM on my bench with a signal being fed to the input and I see no gain. I see a signal with unity gain.I can move the PD jumper to low and the output drops to zero.

The paperwork says it should have the gain set to seven. My power supply is plus VCC of 2.2 volts and minus VEE of 2.2 volts dc with a ground in between. The schematic shows it is set up in single ended mode with R3 shorted to ground I checked it and this is true. I have an input signal on J2 of +/- 350mv square wave 20KHZ and I am seeing an output of +/349mv.

I think there is a BooBoo on the board - I checked all the resistances and found R4 = 68 ohms, R2 = 187 ohms ?, R6 = 132 ohms and R7 = 65 ohms.

R2 on the schematic should be 453 ohms to get a gain of 6. R2 is a 403 size resistor with no writing on it.

You would think I should still get a gain of 2.

Did I get a dud? Or am I doing something wrong?

Thanks, 

Dan

  • 0
    TI__Genius 14166 points
    Hi Dan,

    I think this is simply a result of the resistor dividers at the input and the output. For long traces and high frequency circuits, we have to impedance match the signal chain links to prevent reflections. To terminate a 50 ohm transmission line connected to J2, we put a 50 ohm to ground at the amplifier's input. However, this also divides the 350Vp signal in half. This signal is then gained up by 7 at the amplifier output. In order to terminate any possible reverse reflections into the output, we also put a 71 ohm resistor to ground at the output, which in parallel with 169 ohms makes 50. However, this divides down the signal from the output by 3.36. The result is 1/2 * 7 * 1/3.4 ~ 1. The reason the measured resistances were lower is because there are parallel resistors in the circuit. To measure a resistor you must first take it off the board.

    Best regards,

    Sean
  • 0 in reply to Sean Cashin
    Prodigy 30 points

    Why doesn't someone state that in the publication. "Don't expect any total gain from this circuit, it is set to unity due to the impedance matching circuitry."?

    After reading the manual I expected a gain of 7 at the output!

    Thanks,

    Dan

  • 0 in reply to Dan Clark
    TI__Genius 14166 points
    If your signal frequency is less than 50MHz you can just remove the impedance matching to get a gain of 7. Remove R1 & R7 and change R6 to 0Ω. If your signal is above 50MHz, you need to have it but you can simply change R2 to a 3.61kΩ resistor to increase the gain to 7V/V.