Signal gain and noise gain are both important to amplifier circuit design. Signal gain is obviously important because you want to have accurate control of your signal amplitude. Noise gain is also important, even though it does not directly impact signal amplitude, because it impacts amplifier stability and loop gain, both of which have an impact on signal quality. Thus, it’s important to be able to calculate both noise gain and signal gain for a particular circuit. Once you have these numbers, you can use the datasheet guidelines to optimize your circuit.
Let’s quickly review signal gain with fully differential amplifiers, both inputs are inverting inputs; there is no noninverting input, so your signal gain is When you build the board, you physically solder RF onto the board, so you know the value of RF – or do you? Depending on the particular amplifier and board design, you may need to consider extra resistance. TI’s LMH5401 fully differential amplifier has an extra 25Ω on chip between the amplifier output and the feedback connection pins on the amplifier package. Because this amplifier is only 3mm by 3mm, and because the feedback connection pins are directly adjacent to the amplifier input pins, any measurable board resistance is not likely. You should, however, consider any board traces longer than 1cm possible sources of extra resistance. So if you’re using the LMH5401, you’ll need to add onboard feedback resistance to the on-chip resistance.