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large analogue input noise with laptop and 6024

Acquiring an analogue signal leads to large noise of +-100mV !

I'm using a NI 6024 card with shielded cable and a selfbuild connector box on a Dell D800 Laptop. The analogue inputs are used as differential inputs and connected to external amplifiers. In the box additionally the frequency output (GPCTR0_OUT and DGND) is connected to a small electronic circuit with an octal buffer (74HCT250) and 5V voltage regulator (An external 12V power supply is additionally used).
The noise is not reduced by connecting V_IN- directly or via 10k resistor to AIGIND.
There is no noise, if a battery is connected to the analogue input.

May I e.g. connect the DGND to the AIGND ?
I would be happy to receive any suggestions for solving
this problem.

Thanks a lot

Ralf
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Hi Ralf,
When using laptops and external circuitry there's always a whole lot of noise generated by the conflicting coupling of the laptop's power source and your external source. You might find that using the external power supply but powering the laptop from the battery also helps. I'd suggest that first you make sure that your laptop and circuitry are powered from the same electric line, then you can connect your external digital ground to the AIGround via a 10K resistor, to protect the DAQ board. This will help, but I believe the best way will be to use a low pass filter between your amplifier and your Analog Input, that way you'll be able to get rid of most of the high frequency noise.
In general, if you have an external digital circuit powered by an e
xternal power source and you're using a laptop I recommend you use opto-couplers, powered by the 5V of the DAQ card to get rid of any possibility of noise.
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Hi Quantumbyte,
thank you very much for your help.
The laptop and load signal amplifiers are powered from the same electric line. In the meantime I could do some more tests and figured out, that:
1. Powering the laptop from battery influences the noise (+-50mV) only very slightly(+-40mV).
2. If the external digital circuit is disconnected (power off) the noise is not reduced.
3. The connection of digital ground and analogue ground does not reduce the noise.
As you suggested I try using an analogue low pass filter.
Ralf
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