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AD624 noise

Question asked by harvest@nanjing on Jul 11, 2017
Latest reply on Jul 12, 2017 by harvest@nanjing
1)I'm working on the low noise measurement of AD624. I use it as a differential amplifier to read the current through a sampling resistor Rs. When Rs=10 ohms, I get about output noise voltage spectral density of about  4 uv/Hz^0.5(Gain=1000) using Spectrum analyzer, which is the same with the calculated noise.When Rs=100 ohms, the measured noise(Gain=1000) is also basically the same with the calculated one(4.2 uv/Hz^0.5). However, when Rs=1000 ohms, I got about   6 uv/Hz^0.5(Gain=100) of output noise voltage spectral density, while the calculated noise should be 570 nv/Hz^0.5,
which is nearly one order difference.I'm very confused with it and would thank you very much at your earliest convenience.
2)How many is the current noise exactly?In the AD624 datasheet, I can just estimate from the current noise figure.