Keithley - Low Current Measurements

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Basic Current Measurements


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 current originally flowing through the circuit. To do so, the circuit is broken between points A and B, and the meter is connected as shown in Figure 1b. In the ideal case, the meter would have absolutely no effect on the circuit. For practical measurements, however, several error sources may be present. These error sources can result in substantial uncertainty in the measurement, as we will now discuss.


Any ammeter can be modeled to consist of the three separate circuit elements shown in Figure 1b: a shunt resistance (RSH) caused by the input cable connected to the meter; a generator of unwanted current (IC), which represents mainly currents generated by interconnections; and an internal resistance (RM), which includes series cable resistance. Note that RM is in series with an ideal ammeter (MI), having no resistance or current source of its own.


When the ammeter is connected in the circuit to be measured, the current indicated on the meter is equal to the current that would flow through the circuit without the ammeter inserted in the circuit, less errors caused by elements in the circuit model. These errors consist of current flowing through the model shunt resistance, currents generated by the interconnections, errors induced by the voltage burden (or drop) across the entire ammeter model, and the uncertainty of the meter itself.


With measurements of currents in the normal range (typically >1mA), errors caused by ammeter voltage burden, shunt currents, and noise current are often small enough to be ignored. 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 generally consist of a voltmeter circuit that measures the voltage drop across a shunt resistor inserted in the series with the circuit being measured. (See the discussion on shunt ammeters that follows.) The reading provided by the voltmeter is thus directly proportional to the current flow.


Unfortunately, the voltage burden (input voltage drop) produced by such meters usually ranges from 200mV to about 2V. This voltage drop is sufficient to cause errors with current measurements below the normal range. To avoid such large voltage drops, picoammeters and electrometers use a high gain amplifier with 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 minimum shunt...

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