PGA noise voltage vs frequency (why?) - Page 1
Forum section: electronics / projects

author: SArepairman
author_rank: Frequent Contributor
timestamp: June 01, 2014, 08:31:58 pm
content: How come a PGA has the weird property of noise voltage / sqrt hertz going down when gain is increased?Like, If I wanted a gain of 100 with a op-amp/precision resistor, I would get op amp noise * 100, I am looking at a Ti datasheet for a PGA and it showsSo at a gain of 100 that thing beats the crap out of any other part, even the most expensive low noise amplifiers suck compared to it at G = 100 (lt1028 is 100nV/sqr hz for instance). I know the LT1028 is meant as a DCish device, but still, I don't think there is any part that I have ever seen that can beat that PGA at higher gains.It seems to make sense to use that PGA as a fixed gain amplifier then use a trimpot and a low noise amplifier to get the precise gain.

author: Marco
author_rank: Super Contributor
timestamp: June 01, 2014, 09:41:14 pm
content: I don't know why it goes down with amplification, but it's input noise so you still have to multiply with amplification just like you did for the LT1028.

author: SArepairman
author_rank: Frequent Contributor
timestamp: June 01, 2014, 09:58:12 pm
content: Quote from: Marco on June 01, 2014, 09:41:14 pmI thought that was output noise.it seems kind of weird to list input noise on a device with only an input and an output, but I guess thats the convention, its not like there is a point of doing the calculation yourself. I guess thats how they measure it.the RTI elsewhere in the datasheet should have clued me off, I think that means reference to inputstill wonder why it goes down though, is it just switching in smaller resistors in the front? Like G=1 is 10k and 10k, g=10 is 1000 and 10k, g=100 = 100 and 10000, so the dominant noise source is the gain set resistor? thats the best I could think of.

author: nickm
author_rank: Regular Contributor
timestamp: June 02, 2014, 01:35:55 am
content: There are two noise sources in INAs. The input buffers have their noise which is ein dominated, and the fixed gain output diff amp consists of resistor noise. The noise contribution of the buffer amps will be very low (some get close to 1nV/rtHz) multiplied by the gain that the INA is set at. The noise contribution of the output diff amp is constant irregardless of gain but will be much higher than the INAs (like 10 - 100nV/rtHz). At low gains like 1 the total noise will be dominated by the diff amp. At high gains the noise will be dominated by the input buffers. But since the gain is referred to the input it is divided by the gain of the system. So at a gain of 1000 it converges to the ein of the input buffers, whereas at a gain of 1 it is the resistor noise of the diff amp.

author: ejeffrey
author_rank: Super Contributor
timestamp: June 02, 2014, 04:42:12 am
content: Quote from: SArepairman on June 01, 2014, 09:58:12 pmIt isn't weird at all. That means that the noise is in the same units as your signal, which is probably something you care about a lot more than the arbitrary units of the amplified signal so you can easily calculate the SNR. If your oscilloscope vertical amplifier has 20 nV/sqrt(Hz), that has a direct relevance on how small a signal you can measure.