[time-nuts] OP-Amps for 10MHz distribution...?
Charles P. Steinmetz
charles_steinmetz at lavabit.com
Wed Feb 29 02:14:07 UTC 2012
>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.
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