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If I connect a digital multimeter directly to a DC power supply, the voltage shown on DMM is higher on the power supply.

For example, if the source is 3.29 V then DMM shows 3.31 V. What is the reason for this?

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    \$\begingroup\$ How do you know the source is 3.29V? What are you testing it with intially? Sounds like a simple tolerance issue to me; +/-20mV is not unusual, that's less than 1% error. \$\endgroup\$ – Oli Glaser Jan 30 '13 at 19:36
  • \$\begingroup\$ These numbers are shown on both device's screen. I'm using the Agilent 34970A DMM and E3631A power supply. \$\endgroup\$ – Dzung Nguyen Jan 30 '13 at 20:41
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Two things to consider:

An unregulated power supply will typically supply a higher voltage than is stated for loads which are less than its capacity. For example, if your unregulated power supply is 3.3V at 500mA, connecting a volt meter to it may result in much higher voltage readings because the meter is an extremely small load.

The values you give in your question, however, are more typical of normal precision variances in measurement gear. The accuracy of the power supply, and your meter, are probably not better than 1 to 5%. If you need greater accuracy, be prepared to allocate substantial funds to acquire a high-precision meter.

Edit:

According to your comments, you are using a high-precision meter and power supply, the Agilent 34970A Data Acquisition/Logger Switch and E3631A Power Supply.

According to the specifications for these units:

  • The E3631A Power Supply has a load and line regulation of < 0.01% + 2mV. (Datasheet)

  • The E34970A Data Acquisition/Logger Switch has an accuracy in the DC 10 V range expressed as: $$\pm(\% \text{of reading} + \% \text{of range})$$ The value given for 24-hour 23°C is 0.0015 + 0.0004. (Meaning the unit has been calibrated in the last 24 hours and is operating at 23°C \$\pm\$1°.) (Datasheet)

If I am reading the datasheets correctly, these units are very precise. Getting a value of +20mV seems like a calibration problem.

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  • \$\begingroup\$ I'm using high quality device in a university lab, they are Agilent 34970A DMM and E3631A power supply. This things happen almost to any lab station. \$\endgroup\$ – Dzung Nguyen Jan 30 '13 at 22:54
  • \$\begingroup\$ From my experience, equipment in university labs tends to not be calibrated too well. This becomes especially common with instruments not dedicated for use by a specific group on a specific system with dedicated maintenance processes and people in place. You should consider attempting to calibrate them using something that's known to be calibrated properly if you really require that level of precison. \$\endgroup\$ – Chintalagiri Shashank Jan 31 '13 at 0:24

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