# Digital Multimeters (DMMs) and Precision DC Sources

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## Instrument Accuracy | DMM Vs SMU | Voltage Measurement

Hi Engineers,

I have a question on instrument accuracy. Could someone help me in understanding how to choose a instrument for the below DC Voltage Regulator voltage measurements.

Typical O\P voltage of regulator at no load = 5V.

Load Regulation (0 to 100mA) is 0V to 0.05V

I wanted to measure the o\p voltage of a regulator. I can either measure using SMU (0.02% + 200uA Accuracy at 6V operating range) or DMM( Six and Half digit). As we already know DMM gives more accurate and precise measurement. But do I need that much accuracy for the above measurements? (Yes or No: Please provide the reason)

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## Re: Instrument Accuracy | DMM Vs SMU | Voltage Measurement

SMU = SOURCE Measure Unit.  You are not measuring the input voltage of the device (ie the power you are sourcing into the device); you are measuring the output of the regulator.  Therefore, a DMM is the device you should be using.

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## Re: Instrument Accuracy | DMM Vs SMU | Voltage Measurement

We can set a SMU to Source Current mode with the below settings and measure the voltage. Can you explain in more detail on why we shouldn't use SMU to measure voltage.

O\P current = 0A.

Voltage Range and Compliance = Voltage to be measured + 1V

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## Re: Instrument Accuracy | DMM Vs SMU | Voltage Measurement

@Incognito_Guy wrote:

Can you explain in more detail on why we shouldn't use SMU to measure voltage?

An SMU is a basically a power supply with very accurate measurements on its output.  It is not designed to measure the output of other devices.  You should be using a DMM.

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## Re: Instrument Accuracy | DMM Vs SMU | Voltage Measurement

Sounds like a perfect application for an SMU to me.  Not only is it accurate enough to measure the output voltage of "5V typical," it can also provide the 100mA load to check the load regulation.  Remote sense should be used for the load regulation test to ensure you're not just measuring the resistance of the connecting leads.

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## Re: Instrument Accuracy | DMM Vs SMU | Voltage Measurement

You can go with either DMM or SMU. There are advantages and disadvantages to both approaches.

DMM

• High accuracy and resolution
• Measure only voltage or current at a time (on one DMM)
• One PXI slot per DMM

SMU

• Limited accuracy and resolution (relative to DMM)
• Measure current and voltage simultaneously
• Can act as a load by sinking current
• High density (number of channels per PXI slot)

From my experience with both silicon validation and production testing,

• DMM is used in cases where SMU is not available
• For LDO/Regulator testing very high accuracy or precision is not required compared to precision ADC/DAC and voltage references
Santhosh
Soliton Technologies