Low Current Measurements

Number 100

Application Note Series
Basic Current Measurements

Low Current Measurements
tions, errors induced by the voltage burden (or drop) across the entire ammeter model, and theuncertainty of the meter itself. With measurements of currents in the normal range (typically >1mA), errors caused by ammeter voltage burden, shunt cur­ rents, and noise current are often small enough to beignored. In these cases, the displayed current reading is simply equal to the actual current plus or minus inherent meter uncertainty, (UM). Meters designed to measure these normal currents generallycon­ sist of a voltmeter circuit that measures the voltage drop across a shunt resistor inserted in the series with the circuit being meas­ ured. (See the discussion on shunt ammeters that follows.) Thereading provided by the voltmeter is thus directly proportional to the current flow. Unfortunately, the voltage burden (input voltage drop) pro­ duced by such meters usually ranges from 200mV toabout 2V. This voltage drop is sufficient to cause errors with current mea­ surements below the normal range. To avoid such large voltage drops, picoammeters and electrometers use a high gain amplifierwith negative feedback for the input stage. As a result, the voltage burden is greatly reduced—on the order of 200µV or less. This low voltage burden reduces both measurement errors and the minimumshunt cable resistance that must be maintained to pro­ vide a given meter accuracy. Consequently, no special measures need be taken to obtain unusually high cable resistance. Typical picoammeters orelectrometers that employ feedback ammeters generally provide sensitivities to 1fA (10 –15A) or less and typical accuracy of 0.1% to 3%.

In a typical circuit (see Figure 1a), a source causes a current(I) to flow through the circuit. The goal of any electrical current measurement is to insert an ammeter in series with the circuit so that the current measured on the ammeter is identical to the cur­...