Hello;
On a laptop, this type of noise is most commonly attributed to the laptop's own power supply. If you watch the DC power supply provided to the laptop through the in-line converter, you can see that, differentially, the DC voltage is steady. However, if you look at either end with respect to a solid reference, you can see a voltage pattern at 60 Hz (wall power frequency) with a very high peak-to-peak voltage. This causes an unstable AIGND, and therefore will produce an unstable, noisy signal.
There are a few different ways to overcome this phenomenon:
- Unplug the laptop from wall power. This will allow the laptop to run off of its DC power supply, and therefore completely take away the possibility of the unstable AIGND. If running on the laptop's battery power is not an option for you, then follow one of the other suggestions below.
- Reference your measurement to AIGND. When in differential mode, this can be done by connecting a wire from ACH(-) directly to AIGND. Doing so will allow the signal to ride on the varying reference and will, in-turn, stabilize the differential measurement. Making this connection sets up a Referenced Single Ended Mode analog input operation. However, this setup will not reject common mode voltage.
- If rejecting common mode voltage is important in your measurements, and you want true differential mode, there is another solution. Instead of connecting a plain wire directly from ACH(-) to AIGND, you can connect well-matched bias resistors from each input channel to AIGND. There is a balance, though, between the size of the resistor and the amount of common mode rejection and noise. Larger resistors provide a greater ability to reject common mode voltage. The larger the resistance, however, the greater the separation from AIGND, and thus more noise is present. A smaller resistor will produce less noise, but at the same time provide less common mode voltage rejection. You will need to experiment with different resistance values to find the perfect balance between noise and common mode rejection for your application.
Hope this helps.
Filipe A.
Applications Engineer
National Instruments