Does analog gain affect the SNR?

[Yordan_Kovachev]

Hi all,
This is a very basic question. Suppose we have the following case: we have a photo detector (CMOS sensor or PIN photodiode) after which we have an analog amplifier, then ADC and then a signal processor which can apply digital gain.

My question is if we don't have any a filter or any noise reduction circuit between the photo detector and the amplifier, does the analog gain affect the SNR?
Looking at the model circuits of an optical detector (attached) I have the feeling that the amplifier is increasing the whole received signal Z(t) = S(t) + n(t) (S(t) - useful signal, n(t) - noise) and is not affecting the SNR. Could you please share your oppinion?

Thanks in advance

 

[FvM]

I have the feeling that the amplifier is increasing the whole received signal Z(t) = S(t) + n(t) (S(t) - useful signal, n(t) - noise) and is not affecting the SNR.

Only in the world of ideal components. Amplifier noise adds to existing input noise, also ADC quantization noise and non-linearity.

 

[Yordan_Kovachev]

thanks for the answer, I am aware that amplifier and ADC add noise, but my question was does the amplifier increases SNR.
I am asking because, i have recently found a statement (in a test) that analog gain increases SNR, and I am not quite sure this is absolutely correct.

 

[FvM]

Analog gain scales ADC quantisation noise relative to input signal. In this regard, it can increase (improve) the SNR as long as the amplified signal does not exceed the ADC voltage range. No idea if the said statement means something like this.

 

[dick_freebird]

That all depends where in the lineup, various noise sources
are and their relative weights.

If you have a quiet amplifier and are driving a noisy meter
at less than full scale, then more gain makes more amplitude
and better use of the meter's range / resolution. The "noise"
might also be quantization error at the measurement end.

But if the dominant term is before the amplifier then you just
amplify the incoming noise and add the noise figure of the
amp, to it.