• State
  • Replies 3 replies
  • Subscribers 47 subscribers
  • Views 43 views
  • Users 0 members are here
Support feedback
Options
Options
Related

OPA858DSGEVM: OPA858DSGEVM

Annu Vijayan
Prodigy 10 points

Part Number: OPA858DSGEVM

we are evaluating the OPA858DSGEVM in its default non-inverting gain of 7, but the output gain is lower than expected. We tested the board with both 3.3 V and 5 V single-supply operation, applied a 10 MHz sine wave with 240 mVpp amplitude (verified at the IN+ pin), and measured only 1.24 Vpp at the output. Could you please clarify the reason for this? We also came across discussions mentioning that R6 may affect the measured gain and Shorted R6 with 0ohm resistor.
  • 0
    TI__Intellectual 2390 points

    Hello,

    What are you using the measure your output signal? Shorting R6 will mean that there isn't any isolation resistance to the SMA or any measuring point, and this device is very sensitive to output capacitance loading. 

    Is R7 still populated?

    Thanks,

    Evan

  • 0
    Prodigy 10 points

    Hi,

    Measuring using oscilloscope, R7 populted

  • 0 in reply to Annu Vijayan
    TI__Intellectual 2390 points

    Hi Annu,

    Thanks for the clarification.

    Is your input signal centered around zero? If you are operating with a single-supply where Vs- = ground, the input common-mode of the device is at/near 0V but can't go below 0V. 

    On top of that the output swing can only go to about 1V above the negative supply, or about 1V minimum in your case. Depending on what DC value your input signal is on the non-inverting input, that will shift up the output and such with the gain accordingly. Is your output getting clipped at all?

    A good way to verify both of these is to either add a DC offset to your input signal or bring your supplies on the EVM back to split supplies to keep everything around 0V. 

    Also just so you know if R6 is populated with the default 169ohms, then the resistive attenuator with the R7 71.5ohms and 50ohm measurement equipment will attenuate the measured signal on the SMA by about 7V/V down (6.9V/V really). So with the default settings of the EVM you should just be measuring your input signal amplitude since it will be gained by 7V/V then attenuated by 6.9V/V back down with the resistor network on the output.

    If you are changing R6/R7 and depending on your input impedance of your scope the output signal will be attenuated appropriately.

    Thanks,

    Evan