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

[Bruce Griffiths]

Charles P. Steinmetz wrote:
> 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
>
Wideband current feedback opamps tend to have a non inverting input 
current noise floor in the 10-20pA/rtHz region.
Unless the output to inverting input feedback resistor value is less 
than around 250 ohms or so the equivalent input noise for a gain of 2 is 
greater than that of an OPA653.
Unfortunately the value of this resistor cannot be arbitrarily reduced 
without destabilising the amplifier.
The AD811, AD8007, AD8010, THS3001, THS3112 current feedback amplifiers 
have a higher equivalent input noise floor than the OPA653 for a voltage 
gain of 2.
For sufficiently high gain the equivalent input noise floor of these 
current feedback opamps will be lower than that of the OPA653.
However the output phase noise floor (limited by the maximum input 
signal amplitude and the amplifier equivalent input noise) with such 
high gain amplifiers will tend to be greater than that of the OPA653 
with a gain of 2.

The LMH6702 current feedback amp is stable with a 237 ohm feedback 
resistor and has an inverting input noise current floor of 18pA/rtHz.
The equivalent input noise for a voltage gain of 2 is -158dBm/Hz.
The corresponding lower limit to the phase noise floor is about -174dBc/Hz
A maximum output of around +13dBm in a 50 ohm load is achievable with 
distortion below -70dBc @ 10MHz.

Bruce