Noise Figures

Typically you find a noise figure in op amp part specs that are actually referenced to a frequency, like 10kHz or 100kHz. For noise figures to be real low the inputs of an amplifier must have a fairly high quiescent current. The response of an amplifier does not follow a nice clean formula. There is however, a 1/f region and if you are ball parking this noise you can draw a straight line across the higher frequencies. See diagram below.






Here is a very useful equation used to quantify the system noise with any 2 op amps or an op amp driving an ADC part.

What's interesting about is that this equation is that it is used to calculate the system risetime and or bandwith.

Since the noise of an amplifier is reference to the input, the noise could extend up to maybe 10X or 100x the unity gain bandwidth of the amplifier, however, you are going to use the amplifier in your circuit and the noise would be cut off by the open loop gain curve.

Noise estimation for ADC parts.

SNR (N=number of bits) = 6.02 N + 1.76dB

Typically, ADCs are specified as RMS SNR. If you are getting 66.2dB (p-p) SNR then your theoretical rms SNR is about 69dB. An 11-bit ADC ideally should give you 68dB (rms).

Special thanks to Bonnie Baker at Microchip for help with this information.