How big difference of noise between the realistic circuit and pspice simulation?

[goodnight521]

I am trying to build JFET preamplifier which is connected to a opm amplifier behind.After pspice simulation the equivalent input noise can achieve 20nv/sqrt Hz @5Hz which meets the need.However when I finish building the circuit with realistic componets the noise measured is far worse than it should be.I use DAQ to aquire output signal and measure it by labview.The equivalent input noise level is approaching 1uv/sqrt Hz at low frequency ranged from 1-100Hz.(btw I use 2sk170 input pair)

The tricky thing is that I build a simple common source amplifier with 2sk170 whose gain is 10 times.The equivalent input noise from pspice simulation and realistic measurement is 0.8nv/sqrt Hz and 100nv/sqrt Hz.I am so confused there will be such huge difference between them.

So my question is that how to build realistic circuit to reduce noise to achieve ideal result.Or do I measure the noise in a wrong way?What noise is realistic component supposed to be like 2sk170?

 

[Juergen Knoop]

Did you measure the base noise, with shorted inputs, of your DAQ?
In your other thread http://www.diyaudio.com/forums/showthread.php?s=&threadid=125023
there was no mention of that.
Measuring 8(=0,8x10)nV/rt(Hz) directly is quite a challenge.
Regards

 

[goodnight521]

Yes,I have measured the noise of DAQ which is quite small compared to the overall output noise.So how to do to make the measurement noise of circuit as small as possible to achieve ideal one?

 

[AndrewT]

what filtering are you using to measure the output noise?

 

[Pjotr]

And what is your PSU? Don’t forget that the PSU can introduce quite an amount of noise.

😉