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OPA855DSGEVM: 50ohm output impedance

Part Number: OPA855DSGEVM
Other Parts Discussed in Thread: OPA855

Hello - What is the reason for the specific TIA output resistor divider (169ohm / 71.5ohm) to provide a 50ohm output impedance. This provides a 200ohm load to the TIA.

Alternatively, a 50ohm series resistor at the TIA output, would provide a 100ohm load to the TIA (including the 50ohm input impedance of the instrument).

With a +/-2.5V supply, max current would be +/-25mA. This appears reasonable based on the datasheet, is it desirable to limit the output current further?

Thanks!

  • Hello Majid,

    The OPA855 can indeed drive a 100 ohm load as you have stated. The reason a 200 ohm load is used in the EVM is to replicate the test conditions in the electrical characteristics portion of the OPA855 datasheet. You will notice that there are slight differences in some of the curves when the load resistor is changed as can be seen in figure 3 of the datasheet. 

    Best,

    Hasan Babiker