6/1/16: Noise Analysis in Precision Analog Designs

Question

For a single-ended sensor, does the added noise from additional differential driving components add more noise than the benefit of using a differential input ADC, as opposed to using a single-ended ADC?

ScottH · Accepted Answer

Unfortunately there's not an easy answer to this question. If you have a high impedance sensor, for example a pH probe, the sensor will likely dominate the noise, so any extra dynamic range you get from a differential ADC doesn't help, you're only gaining the noise by an extra factor of 2 and not gaining anything in terms of sensitivity or dynamic range. 
 If you have a low-impedance sensor on the other hand, followed by some low noise amplifier, such that the input noise sources are small and the noise sources after the gain stage contribute significantly, then you may be able to take twice the gain in your low noise amplifier and improve your sensitivity. 
 Here's an interesting example of a circuit where attenuation in the SE-diff stage allows higher gain in the input amplifier stage to improve the noise performance: http://www.analog.com/media/en/technical-documentation/application-notes/AN-1264.pdf)