Bill wrote:
At what frequency do you have 1 nv per square root Hz with opamp and what opamp ?
With most op-amps suitable for a 10 MHz distribution amp, the input voltage noise is specified at 100 kHz or greater. For example, the OPA653 that Bruce tested is specified for 6.1 nV/rtHz (typical) at >100kHz and the graph shows that it has a 1/f noise corner in that vicinity, with a noise density greater than 100 nV/rtHz at 10 Hz.
There are op-amps that are likely suitable for 10 MHz distribution that have lower input voltage noise than the OPA653 (again, generally specified at 100 kHz or 1 MHz): AD811 (1.9); AD8007 (2.7); AD8010 (2); ADA4899 (1); EL5166 (1.7); EL5236/7 (1.5); OPA695 (1.8); THS3001 (1.6); and THS3112 (2.2). Many of these are current-feedback amps, but if you keep the resistance at the inverting input low (<200 to <75 ohms, depending on the particular amp) the inverting input current noise will be lower than the noninverting input voltage noise. If the amp has sufficient output current to drive a back-terminated 50 ohm load with authority (the OPA653 is a bit marginal in this respect, IMO), there should be no problem driving such a feedback network plus the load.
I know of one op-amp that comes close to 1 nV/rtHz at 10 Hz and being capable of useful operation as a 10 MHz distribution amplifier -- the ADA4898 (1.2 nV/rtHz at 10 Hz, 4.3 nV/rtHz at 1 Hz). These are wonderful parts, but the large signal frequency response with a 100 ohm load is less than desired for a 10 MHz distribution amplifier.
Best regards, Charles _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
