**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,

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.

Hi, Emman

Thanks for your reply.

I checked again and update the data. In my experiment, the current through the sampling resistor will not change(about 50uA，I already know), so the output voltage of AD624 can verify that I set the right gain.

If I choose Rs=1000 and G=1000, the output will be 50V and the input will exceed the maximum differential input voltage in the datasheet(VDL=10V/G).

I measured the noise spectral density at 1 kHz.

- Rs(ohms) Gain Measured output voltage(V) Measured output noise(uV/Hz^0.5) Calculated output noise(uV/Hz^0.5)
- 10 100 0.052 0.53 0.4
- 10 1000 0.52 4.1 4
- 100 100 0.51 0.85 0.42
- 100 1000 5.3 7.9 4.2
- 500 100 2.53
**3.8 0.49** - 1000 100 5.25
**8 0.565**

It seems that the source resistor (500 ohms or 1000 ohms) has a great influence on the noise indeed although the current noise of AD624 is only 300~400 fA/rtHz. The same phenomena happens on the AD8429 (voltage noise:1 nv/rtHz,current noise 1.5 pA/rtHz): Rs=1000 ohms,Gain=100 ,the measured output noise is about 9 uv/rtHz while the calculated should be 0.44uv/rtHz. I can't explain the reason.

Hi Run,

The current noise of the AD624 at 1kHz is around 300 to 400 fA/rtHz.

It is interesting to see that the measured noise at Gain of 100 is higher than your previous measurements at Gain of 1000 though I understand that there is an increase in series resistor but the current noise of the AD624 is too small..

Can you make sure that the inamp is really configured at Gain of 100? The measured output noise (6uV/rtHz) that you have is actually correlated to the calculation of having 1000 ohms series resistor if the gain is set to 1000.

Can you also try to measure the noise with this condition:

1. Rs =100 and Gain =100

2. Rs=1000 and G =1000..

I just want to make sure that you have a valid and working setup.

Best regards,

Emman