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Original question:

OPA855DSGEVM: Discrepancy between the measured gain bandwidth of OPA855 Evaluation Board and the product manual

OPA855DSGEVM: Help with test measurement setup according to User's Guide

Aaron Nowack
Prodigy 10 points

Part Number: OPA855DSGEVM
Other Parts Discussed in Thread: OPA858DSGEVM,

Tool/software:

Hi there,

I recently purchased a OPA855DSGEVM and OPA858DSGEVM to evaluate the amplifier. For both boards, I have the following setup

Vcc = +2.5V

GND = 0.0V

Vee = -2.5V

In- = GND

In+ = 100 mVpp Sin Wave, Frequency = 1MHz. Generated by a signal generator with 50 ohm output

Out = Oscilloscope with 50 ohm input termination

I have the In+ connected to a T connection with one leg connected to the oscilloscope with 1M input termination. I see the input and output signals on my oscilloscope and they are both a sin wave, at 1 MHz, and both the input and output sin waves have equal amplitude of 100 mVpp

As I understand, the circuits are both configured for a Gain of +7 V/V. But I'm not seeing any gain at all. I've been re checking my test setup, the voltages, etc, and haven't found anything wrong.

If there are suggestions, or I am misunderstanding what the out of the box configuration for these EVM are, I would highly appreciate any help or clues.

Thank you

- Aaron

  • +1
    TI__Mastermind 22330 points

    Hi Aaron,

    The device is definitely configured in a gain of 7V/V out of the package. Assuming everything is terminated as expected, one reason this might be looking like it's not gained up is due to the attenuation that results from the matching circuitry at the output of the device (169ohm and 71.5ohm). These resistors were added to match to the 50-ohm termination in the scope while also providing a load of 200ohms. The device was characterized with a load of 200ohms but can obviously be changed. Nonetheless, the attenuation you will see from this setup assuming the scope is 50-ohm terminated is ~0.148V/V, when taking into account the 7V/V gain it is around 1V/V which is why it might look like it is not being gained up. This is my first guess as to why you are seeing these results. If you account for this attenuation due to the matching network, you should be seeing the result that is directly at the output of the device.

    Best Regards,

    Ignacio

  • 0 in reply to Ignacio Vazquez Lam1
    Prodigy 10 points

    Ah, thanks! Yes, using an oscilloscope probe at the output of the IC gives the expected results. Thanks!

  • 0 in reply to Aaron Nowack
    TI__Mastermind 22330 points

    Hi Aaron,

    No problem, glad we resolved the issue.

    Best Regards,

    Ignacio