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
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