Differences between analog and digital gain

Question

Hello, 
 I'm thinking through the analog front end for a data acquisition system interfacing to a differential sensor outputting a very small voltage. The peak amplitude of the sensor is on the order of 100-200uV. My question is more of a systems/architecture question regarding the difference between analog and digital gain. I see that some people use a digital gain by multiplying the data samples by some value after the sensor signal is sampled. Another approach is designing in an amp with a gain to bring the signal magnitude up so the ADC has a wider swing signal to sample. 
 I'm wondering which approach is better or what trades are associated with each as the digital gain seems easier done in firmware (just multiplies) rather than adding more amps and circuitry in the analog front end signal chain. I'm a little hesitant to go the digital route though because there is more dynamic range with the analog gain which seems beneficial but I don't have a great understanding of why and what some of the trade offs are. 
 
 Could someone comment/shed some light on the two different approaches here from an architecture perspective?

Emman.A · Accepted Answer

Hi Robbie, 
 
 Measuring small-signal typically needs an amplifier to gain up the signal of interest which actually helps to get better noise performance (making ADC noise negligible) and optimize the Signal to Noise ratio (SNR) of the system. This kind of optimization may not be possible if you are just implementing digital gain through a firmware/code multiplier. There is also guidance on how much gain is needed to balance out the performance in terms of SNR and THD as mentioned here . 
 Also, depending on your ADC but most of the ADCs are low input impedance and it is not advisable to connect these inputs directly to the sensors as it will introduce errors (bias current coupled to high source impedance) that might be much higher than your input signal. Using Inamp as the front end can avoid this problem due to its high Z input characteristics. Other functions and advantages that you can get from putting amplifiers on the front end are overvoltage protection, change impedance, common-mode level shift to match ADCs input range, filtering, etc. 
 I hope this helps. 
 Best regards, 
 Emman)

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Differences between analog and digital gain