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How to select the right ammeter. (Test & Measurement).

As modern electronic components become smaller and use less power, associated R&D of new material and components require measuring very low currents. Nanoamp ([10.sup.-9]A) measurements and lower are becoming increasingly more common. Measurement of such low-level currents requires careful ammeter selection and use. However, the correct choice of instrument depends not only on the magnitude of the current, but also on characteristics of the signal source.

Imagine a circuit with a current ([I.sub.s]) running through it. If the current is directed through an ammeter, it is reasonable to expect that the ammeter reading ([I.sub.M]) will be essentially equal to the original [I.sub.s]. However, when the current is very low or the source generating the current acts very differently from an ideal current source, measuring the current adequately can be a challenge. When a current measurement is non-trivial, choosing the right ammeter can make a significant difference in your results.

Ammeter architectures

The most common source of error in any current measurement lies in the fact that the ammeter has a non-zero input resistance. The voltage developed across the meter results in less voltage across the device under test (DUT); if the reduction is substantial, it will result in significantly lower current flowing than before the meter was inserted. In other words, the ammeter is not reading the current the user intended to measure.

There are two main types of ammeter architectures, shunt ammeters and feedback ammeters.

Shunt ammeters are found in almost all digital multimeters (DMMs). These meters measure current by intentionally inserting a resistance ([R.sub.shunt]) in the current path, and thus developing a voltage across the shunt resistor at the input terminals. This voltage is proportional to the current being measured.

The main problem with shunt ammeters is their inherently high input resistance. The high voltage to be developed across the shunt resistance ([R.sub.shunt]), typically ranges from 100mV to 1V. We call this voltage the voltage burden ([V.sub.B]).

Quantitatively, the error caused by the shunt ammeter is stated as the ammeter's voltage burden divided by [R.sub.s], the output resistance of the DUT.

Feedback ammeters are closer to "ideal" and should be used for current measurements where a source of low voltage is generating the current to be measured. This current would be altered significantly by a DMM's high voltage burden. Instead of developing a voltage across the terminals of the ammeter, a feedback ammeter develops a voltage across the feedback path of a high gain operational amplifier. This voltage is also proportional to the current to be measured, but it does not appear at the input of the instrument. As a result, sensitive feedback ammeters such as electrometers and picoammeters have voltage burdens typically limited to 200[micro]V. A schematic illustration of a feedback ammeter is shown in the figure.

The effective input resistance of a feedback ammeter is:

[V.sub.B]/[I.sub.M] = [R.sub.F]/A

where [R.sub.F] is the ammeter's feedback resistor and A is the op-amp gain. Given that A is on the order of [10.sub.6], the input resistance and voltage burden are very low. Again, for any ammeter, the error is [V.sub.B]/[R.sub.S]. However, in the case of the feedback ammeter, this becomes:

[] = [V.sub.B]/[R.sub.S] = ([I.sub.M]*[R.sub.F]/[R.sub.S])/A

[I.sub.M] [approximately equal to] [I.sub.S] [1- [R.sub.F]/[R.sub.S]*A]]

Note that the error has been effectively reduced by a factor of A (typically in the range of 1,000,000) compared to the shunt ammeter.

The feedback ammeter's low voltage burden is also the reason why it is the chosen instrument for measuring low currents. In order for a DMM to measure a very low current, it would need a very large shunt resistance to generate enough voltage to be measured precisely. This high shunt resistance would create an even larger voltage burden, significantly reducing the current to be measured. The feedback ammeter gets around this problem by allowing a large feedback resistor to be used without causing a significant voltage burden problem; hence the low current can be measured without the ammeter causing it to change.

Web Resources for Ammeters:

Ammeter: Device used to measure electric current

Ted Thorbjornsen Business Manager at Keithley Instruments Inc.
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Author:Thorbjornsen, Ted
Publication:R & D
Geographic Code:1USA
Date:Jul 1, 2003
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