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Unformatted text preview: ECE 209: Circuits and Electronics Laboratory Notes for Lab 2 (Meters, Measurements, and Errors) 1. The digital voltmeter (DVM) • Because voltage is defined at one point with respect to another point, then voltmeter must be placed at two points in the circuit (i.e., in “parallel” with the device under test). • To reduce impact on circuit, voltmeter has very high internal resistance (i.e., an open circuit ). • Because voltmeter resistance is very high, if placed in series with a device, it stops all current. 2. The ammeter • Because current is defined at a point, then ammeter is inserted in series to do measurement. • To reduce impact on circuit, ammeter has very low internal resistance (i.e., a short circuit ). • Because ammeter resistance is very low, if it is placed in parallel with a device, current skips the device and usually fries the ammeter and possibly other parts of the circuit. 3. The digital multimeter (DMM) or digital volt-ohm meter (DVOM) • Combines multiple measurement tools into one unit. – Internally, DMM is a slow and high-precision analog-to-digital converter measuring voltage. – Versatile: voltage, current, capacitance, inductance, resistance, temperature, frequency, etc. * Convert to ammeter by measuring voltage from measured current × known resistance. * Convert to ohmmeter by measuring voltage from known current × measured resistance. * Measure temperature with a thermocouple . * Measure capacitance by finding rise time of a known step across a known resistance. * With an on-board DSP , lots of other useful measurements or conversions are possible. • High-precision models (e.g., 6.5+ digits) can be PCI cards or larger bench-top units (e.g., function generator size) with computer interfaces. These models have advanced/extensible feature sets. • RMS measurements are often AC coupled (i.e., DC offset/average component is stripped out). – So RMS measurements may be incorrect when waveform has nonzero offset. – Remember that RMS 2 = RMS 2 AC + DC 2 ....
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This note was uploaded on 08/05/2011 for the course ECE 209 taught by Professor Staff during the Fall '08 term at Ohio State.
- Fall '08