Designing high gain, low noise, low input amplifier?

I would suggest an input-amplifier like SSM2019. It can provide the desired gain and includes a differential input. For such a wide range of input signals you may need a option to switch input-gain.

Although the DSP can provide "digital" gain it may not be the best choice because with that gain you "boost" the THD+N of your AD-Converter. If your DSP-gain is 30dB your AD-THD+N degrades for 30dB to roughly 60dB which is not so fine ...

Hope my thoghts help a bit.

Regards Bernd

Sean,

The ADAU1761 itself has a programmable gain amplifier (PGA) in front of its ADCs. This PGA can be set for up to about +35 dB gain, plus an optional +20 dB post-gain boost. Using this would avoid the problem Bernd described in using digital gain in the SigmaDSP core.

I found this quite useful http://www.analog.com/static/imported-files/application_notes/AN_940.pdf. It gives a good outline of the fundamentals you need to consider.

HTH

Gordon

Thank you Gordon....that is a great primer. The analog world is pretty straight forward as that document shows. I am concerned about how organizing the gain stages might affect my performance within the DSP itself. JeradL's post seems to address my concerns about that (thanks JeradL).

The PGAs don't add any noise? If the PGA is operated at maximum gain, the dynamic range of the inputs is still 99 dB?

If the PGA is set to minimum (not mute) gain, does the maximum analog input level change?

Hi J3627,

Any gain circuit will add noise - hopefully the circuit is designed well so that the noise does not scale linearly with gain; adding 40 dB of gain should add significantly less than 40 dB of noise. How much less depends directly on the quality of the amplifier. If the ADAU1761 PGA is programmed with 40 dB of gain, a -40 dBV signal will be elevated to 0 dBV; any noise in the source will be brought up by 40 dB as well so the DNR of the source must be considered in the noise figure of the entire system. Please note that the noise figure listed in the datasheet was measured with the PGA gain set to minimum; we have not listed noise figures for other gain settings, and so you might have gotten the impression that the noise will not rise with increases in gain.

The short answer here is: yes, increasing the PGA gain to maximum will raise the noise floor of the PGA and of the system.

The gain staging of a system should be optimized so that each amplifier along a given signal path is running at a level just below maximum, so that peak levels will not cause clipping or excess distortion in that stage, with the consideration of the maximum input level of the following stage. This should yield the least noise in the overall system, unless there are some weak links with regards to noise.

If the PGA gain of the ADAU1761 is set to minimum, the Full-Scale input voltage is given by the formula AVDD/3.3; 1 V RMS differential will drive the ADC to 0 dBFS.

Best regards,

Coleman

Well, the only thing I found in the datasheet was "For optimal noise performance, the ALC uses the analog PGA to adjust the gain instead of using a digital method. This ensures that the ADC noise is not amplified at low signal levels." So assuming the signal source itself is noiseless, the PGA set to high gain will raise the noise level above that of the ADCs?

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