[time-nuts] OP-Amps for 10MHz distribution...?

Charles P. Steinmetz charles_steinmetz at lavabit.com
Wed Feb 29 02:14:07 UTC 2012

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,


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