Low Current Measurement A: Data Specification

There was a time when I devoted  myself entirely to the IEEE state-of-arts. I was fascinated by their new architectures, new algorithms, and new building blocks.

Later, when I sit whole days doing circuit layout. And then again, I sit whole days doing chip measurements. After those lonely days, I came to realize that research is NOT ALL about ideas. I can finally touch the pains and tears through the glory of every published idea.

Though layout and measurement can’t get many credits in a paper, they do count for much if you want the idea to be finally published. Since I’m doing ultra-low-power circuit design, to minimize errors during low-current measurements becomes an important part of my research. I put some time on learning the details.

1. Which data specification of digital multimeter (DMM) is worth a first glance?

Resolution, which refers to the smallest portion of the current in the measurement that can be observed. It is determined by the ADC in the measurement device. One common way to characterize resolution is to use digits, e.g. a 6-1/2-digit DMM. As a general rule, 1/2 digit implies the most significant digit can have the values 0, 1, or 2.

Some data show the equivalence between ADC resolution and the digits of DMM:

• 18-bit ADC – 262,144 counts – approx. 5-1/2-digits
• 22-bit ADC – 4,194,304 counts – approx. 6-1/2-digits
• 25-bit ADC – 33,554,304 counts – approx. 7-1/2-digits

Knowing the digits of DMM and the measurement range, we can calculate the current resolution in units of amps. An example:

• 6-1/2-digits(1,000,000) on 100µA range = 100µA/1e6 = 100pA

2. Then which data specification is worth a second glance?

Accuracy, which is normally expressed in two parts, namely as a proportion of the value being measured, and a proportion of the scale that the measurement is on, e.g. |% reading + % range|. An example:

• DMM 100µA range: % 0.02 + % 0.002 (either positive or negative),
• For a 50nA current input: Uncertainty = 0.02%*50nA + 0.002%*100µA=2.01nA; Reading = 47.99nA to 52.01nA.

According to the above example, it seems that the error related to the range may be the most significant factor when measuring values at the low end of the range.

Note that accuracy specs for high-quality measurement devices can be given for 24 hours, 90 days, one year, two years, or even five years from the time of last calibration. Moreover, accuracy specs are normally guaranteed within a specific temperature range.

3. One important specification might be neglected

Settling time, or response time, which ensures that there are sufficient delays for the equipment to be settled before  the measurement data is recorded. It should be paid more attention to especially when PC-controlled measurement setup is used.

An example of Current Measurement Specification of a 6-1/2-digits DMM (Courtesy:Hameg)

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