Electrometer is a highly refined digital multimeter (DMM). Electrometers can be used for virtually any measurement task that a conventional DMM can and offer the advantages of very high input resistance when used as voltmeters, and ultra-low current sensitivity with low voltage burden when used as ammeters. Electrometers are superior to DMMs by three to eight orders of magnitude in these respects. That makes them the instruments of choice for measuring voltages with high source impedance or currents with low source impedance (i.e., signals from non-ideal sources). Electrometers can also measure electric charge directly.

Why is Very High Input Impedance Necessary for Multimeters?

When measuring voltage, the input impedance of the meter must be many decades higher than the impedance of the voltage source. For example, if the meter’s input impedance is only 1GΩ (typical of DMMs), and the source of voltage has 10MΩ of impedance, then the meter will introduce a 1% error due to its relatively low input impedance. In contrast, an electrometer with 1014Ωinput impedance will cause only a 0.00001% error. It’s also important for the voltage measurement instrumentation to have a low bias current, because any current coming out of the meter input will be forced through the source, and change its voltage. Electrometers use active cancellation to reduce bias current to the single femtoamp level.

Why is Low Voltage Burden Critical in Ammeters and Multimeters?

Voltage burden is the voltage that appears across the ammeter input terminals when measuring. A DMM uses a shunt ammeter that requires voltage (typically 200mV) to be developed across a shunt resistor in order to measure current. This voltage burden will reduce the actual current flowing in the circuit, and reduce accuracy. An electrometer uses a feedback ammeter to reduce this terminal voltage by several orders of magnitude. Some electrometers go a step further, adding a compensating voltage that eliminates any residual offset voltages at the ammeter input, down to as little as 20μV.

Applications of Electrometer

  • Electrometer can be used as microvoltmeter or millivoltmeter or voltmeter to measure ultra low voltages in the range of microvolts (uV) to several hundred volts (500V).
  • Electrometer can be used as femtoammeter or picoammeter or nanoammeter or milliammeter to measure ultra low currents in the range of femtoamperes (fA) to milliamperes (mA).
  • Electrometer can be used as high precision megohmmeter to measure very high resistance upto teraohms or petaohms.
  • Electrometer can be used for electric charge measurements in the range of femtocoulombs (fC) to microcoulombs (μC).