Vernier Tech Info Library TIL #1026

Question

The readings from my Current and/or Voltage Probe are erratic, but if I measure the voltage with a multimeter instead, I get a solid reading. What is wrong with my sensor?

Answer

Multimeters are designed to average the data and update their display only once a second or so. In many power supplies, there is a 120Hz ripple in the signal. Sometimes the noise can be measured on the multimeter by using the AC mV setting, but a multimeter usually masks the noise by averaging several readings before displaying a value. In our sensors, we give you an exact reading at one instance, which can appear to make the reading change erratically.

When collecting data from a noisy signal like this, the noise typically does not look like a sine wave on the screen because LabPro's sampling rate is set much lower than the frequency of the noise in the signal. However, faster sampling rates can alias the signal and produce a nice sinusoid at a much lower frequency than the sampling rate. The best way to reduce this problem in your data is to increase averaging or to sample at higher frequencies.

To verify that your Voltage Probe is working correctly, the best test is to measure the voltage across a battery. Because the battery is a simple DC voltage, there are no problems from signal noise.

Another simple test is to keep your circuit the same and set the software sampling rate to 1000 points/second for 0.1 seconds. If the ripple is the problem, you will see a drastic variation in the voltage with a period of 60 or 120 Hz (in North America) [or 50 or 100 Hz, outside North America].

If this is the problem, you can correct it by trying the following:
1. Get a different power supply, use batteries instead of a power supply, or add a capacitor to the output of your power supply to steady its output.
2. To minimize this problem in Logger Pro, turn up the Oversampling in dialog box where you control the data collection rate. In Logger Pro 2x, set the oversampling to as large a number as you can. On Logger Pro 3.1 and newer, just turn on Oversampling. You can also set the sample rate to a number not a factor of 60 [or 50 outside North America] to improve things.

Other problems stem from grounding loops. This happens when the power supply ground reference does not match the ground reference of the interface (which is the same as the ground reference for the computer). One way to tie these together is to attach the ground wire of the voltage probes that came with LabPro to the earth ground connection of the power supply. Do not connect the ground wire to a negative or positive supply node.

Created by: jhopkins on December 14 2001
Last updated by: jhopkins on November 05 2009