EMRFD Message Archive 9791

Message Date From Subject
9791 2014-02-25 20:44:10 bob_ledoux Low Noise Amps for Tayloe Integrating Detector
In 12/2009, (message 3851),  Dan suggested the following list for low noise amps following the quad detector.  Are there any recent additions to the list? 

As far as ultra low noise op-amps, I have usually used the LT1115 (0.9
nV/sqrt Hz), but for a couple of dollars more you can get the LT1028 or
LT1128. and move down to 0.85 nV/SqrtHz.

In my better DC receiver designs, I almost think that ultra low
distortion might almost be a better goal (IP2/IP3 performance). The
ultra low distortion parts include:

LT1115 (0.9 nV/sqrt Hz) - THD+N (0.0002%) Use as a reference

AD797 (0.9 nV/sqrt Hz) - THD+N (0.0001%) Also use as a reference

LM4562 (2.7 nV/Sqrt/Hz) - THD+N (0.00003%)

LME49713 (1.9 nV/SqrtHz) - THD+N (0.00008%)

OPA1611 (1.1 nV/SqrtHz) - THD+N (0.000015%)

OPA211 (1.1 nV/SqrtHz) - THD+N (0.000016%)
9792 2014-02-27 12:03:12 arfghans Re: Low Noise Amps for Tayloe Integrating Detector
What everyone forgets to do is NOISE MATCH the opamp to the source impedance. This applies to both inputs of the opamp, and a complete analysis of the circuit surrounding the opamp required if you really want an optimum result. Minimum input noise voltage (ein) is only half the problem; you must consider input noise current (iin) as well. As a simple rule of thumb, the best noise figure will occur when Rsource = ein/iin.  You can start with the typical values on the opamp datasheets, which may be stated at 1 kHz, and that's a good place to work for a DC receiver. If you want to get fancy, it has to be integrated over the full bandwidth of interest. TI had a nice spreadsheet tool for opamp noise calculations, but I haven't searched for it in quite a while.

What happens in opamp input circuit design is that they pick a bias current for the transistors based on conflicting requirements including noise voltage and current, bandwidth, gain, linearity, and so forth. A BJT running high collector current will exhibit very low noise voltage but very high noise current... Perfect for noise matching, say, a 100-ohm source. But with a high-z source, that same opamp will produce a large effective noise voltage:  (iin * Rsource) will be much larger than ein. That's why we like JFET opamps for high-z sources; their current noise is usually very small. I don't know what the source impedances are for the Tayloe circuit, but I bet it's somewhere in the middle, maybe several k ohms. Definitely worth some analysis. THEN you can pick the best opamp with regards to noise.

Gary, WB9JPS
9793 2014-02-27 12:58:10 Tayloe, Dan (NSN ... Re: Low Noise Amps for Tayloe Integrating Detector

The source impedance depends on the system impedance as seen through the front in band pass filters.  The detector looks at baseband like an R/C low pass filter at baseband where the series R is the system impedance and C is the detector cap.  If there are four detector caps, the impedance looks like 4x the system impedance (200 ohm for a 50 ohm system) or 100 ohm for a two capacitor detector  configuration.  It is nice from a receiver dynamic range perspective to have a detector that has a user configurable band pass response to roll off signals either side of the desired signal.


The 50 ohm input into the band pass filter can be transformed into a higher source impedance into the detector to better match the Voltage/Current noise characteristic of a (potentially) less expensive pre-amp.  The tradeoff is that when using 5v bus switches, there is a limited dynamic range through the switch.  Thus raising the impedance for a better noise match will increase the signal voltage, reducing the dynamic range.  That can give rise to a noise vs. dynamic range tradeoff.  Nothing new there!


-          Dan