8 tips to teach you how to use a multimeter correctly

Tip 1: Avoid measurement errors caused by connections, test leads, and DMM wiring

The simplest way to eliminate errors caused by wiring is to perform a null measurement. For DC voltage or resistance measurements, select the appropriate measurement range, connect the probes together and wait for a measurement—the closest you can get to a zero input—then press the null button. The readings below will have the null measurement subtracted. Null measurements are great for DC and resistance measurement functions. But this technique is not suitable for AC measurements. AC converters do not work well in the lower part of the range; the analog converters in the Agilent 34401A DMM are not specified below 10% of full scale. The Agilent 34410A and 34411A DMMs use digital technology and can measure all the way to 1% of full scale, but they cannot be used to measure shorts.

connect

If you connect dissimilar metals, you create a thermocouple junction. A thermocouple junction generates a voltage that varies with temperature. This voltage is low, but if you are measuring small voltages, or your system has many connections, it is a significant issue. Think of the thermocouple junctions as being at the DUT, at the relay (multiplexer), and at your DMM. Using copper-copper junctions minimizes this offset.

When making resistance measurements, you can use offset compensation to measure any offset voltage and deduct this error. Figure 1 shows two measurements taken in an offset compensated measurement, the first with a current source and the second without. Subtracting the second reading from the first and dividing it by the known current source value gives the actual resistance value. Since two readings are taken in the measurement, the reading speed is reduced, but the measurement accuracy is improved. Offset compensation can be used for both two-wire and four-wire resistance measurements.

figure 1

Offset compensation using two measurements. The first measurement is a standard ohm measurement; the second is a measurement of the offset produced by the thermal EMF. The voltmeter reading is the difference of these two measurements divided by the known current source.

Connection

The four-wire ohm method is the most accurate way to measure small resistances. This method automatically deducts the test lead resistance and contact resistance. The four-wire resistance measurement connection is shown in Figure 2. Using a known current source and measuring the voltage developed by the resistor, the unknown resistance value can be calculated. An additional set of test leads is used to carry the current to the unknown resistor, and the voltage developed across it can be measured by the voltage sensing leads. No current flows through the voltage sensing lead, so it does not produce a voltage drop.

No current flows through the voltage-sensitive wire. The DMM divides the measured voltage by the known current to obtain the unknown resistance value.

Internal DMM Bias

Autozero is used to eliminate error sources within the DMM. When autozero is enabled, the DMM disconnects the input signal internally after each measurement to obtain a zero reading. This zero reading is then subtracted from the subsequent measurement. This prevents the offset voltage in the DMM input circuit from affecting measurement accuracy. Autozero is always enabled in four-wire measurements, but you can disable it to increase measurement speed.

Use the AutoZero feature. When AutoZero is disabled, the DMM takes a zero reading once and then subtracts it from all subsequent measurements. A new zero reading is taken each time you change function, range, or integration time.

Tip 2: Measuring large resistors

Stable time effect

Capacitance in parallel with a resistor can cause settling time errors after initial connection and after range changes. Modern DMMs insert a trigger delay that gives the measurement time to settle. The length of the trigger delay depends on the function and range selected. These delays are adequate for resistance measurements when the combined capacitance of the cables and fixtures is less than a few hundred pF, but if there is capacitance in parallel with the resistor, or if you are measuring resistances greater than 100 kΩ, the default delay may not be enough. Settling may take considerable time due to the effects of the RC time constant. Some precision resistors and multifunction calibrators use capacitors (1000 pF to 100 μF) in parallel, which, along with high value resistors, filter out noise currents injected by internal circuitry. Dielectric absorption (wetting) effects in cables and other fixtures may increase the RC time constant and require longer settling times. In this case, you may need to increase the trigger delay before making the test.

Bias Compensation in the Presence of Capacitors

If there is shunt capacitance across the resistor, it may be necessary to turn the offset compensation off. When the offset compensation takes the second reading without the current source, it will measure any voltage offset. But if the device has a long settling time, this will result in an erroneous offset measurement. DMMs will use the same trigger delay as for the offset measurement to try to avoid settling time issues. Increasing the trigger delay is another solution to allow the device to fully settle.

Connections for high resistance measurements

Insulation resistance and surface contamination can cause considerable error when you are measuring large resistances. Various precautions need to be taken to keep high resistance systems "clean". Test leads and fixtures are very sensitive to leakage caused by moisture absorption by insulation materials and "dirty" surface films. Nylon and PVC are relatively poor insulators (1013 GΩ) compared to PTFE Teflon insulators (109 Ω). If you are measuring a 1 MΩ resistor in wet conditions, leakage from the nylon or PVC insulator can easily contribute 0.1% of the error.

Tip 3: Use DC bias to make AC measurements

Many signals contain both AC and DC components. For example, an asymmetrical square wave contains both. Many audio signals also contain a DC offset caused by the DC bias current used to drive the output transistor. Some situations require measuring the DC+AC voltage, while other situations may require only the AC component. For this audio example, the amplifier gain is the comparison of the input AC voltage to the output AC voltage.

Most modern multimeters use a DC blocking capacitor in front of the AC RMS converter. This isolates the DC voltage, allowing the multimeter to measure only the AC value. More importantly, the multimeter can scale the AC signal for the best measurement. For example, when measuring the AC ripple of a power supply, the multimeter isolates the high level of DC, while amplifying the AC signal according to the range selected for the AC component.

To make the most accurate AC+DC measurements, measure the two components independently. The multimeter can achieve the best possible DC measurement by using the appropriate range and integration time to suppress the AC component. When making AC measurements, select the appropriate range based on the AC component. You can calculate the AC+DC RMS value using the following formula:

True RMSAC+DC = √ (AC2 + DC2)

Agilent's new 34410A and 34411A use DC blocking capacitors when making AC voltage measurements. AC measurements are made digitally, which results in faster settling times and the ability to handle higher crest factors, which is often encountered when measuring pulse trains. When measuring pulses, make sure the pulses do not contain frequencies above the multimeter's bandwidth. The 34410A and 34411A can measure AC signals up to 300 kHz. If there is a significant AC component at frequencies below 8 kHz, the 34410A and 34411A have a DC function with peak detection to accurately measure both DC and AC components. For higher frequency signals, you can measure the AC component alone and use the formula to calculate the AC+DC measurement result.

Tip 4: Use a digital multimeter to measure low frequency AC signals

Most modern multimeters can measure AC signals with frequencies as low as 20 Hz. However, some applications require measuring lower frequency signals. To make such measurements, you need to select the right multimeter and configure it appropriately. Consider the following examples:

The Agilent 34410A and 34411A multimeters use digital sampling technology to make true RMS measurements as low as 3 Hz. It uses digital methods to increase the settling time to 2.5 s at slow filters. To make the best measurements, you should pay attention to:

1. It is very important to set the correct AC filter. The filter is used to smooth the output of the true RMS converter. At frequencies below 20 Hz, the correct setting is LOW. In the LOW filter setting, insert a 2.5 s delay to ensure that the multimeter is stable. Use the following command to set the low filter.

VOLTage:AC:BANDwidth MIN

2. If you know the maximum level of the signal being measured, you should set the manual range to help speed up the measurement. The long settling time of each low frequency measurement will significantly slow down the auto range.

We recommend that you set the manual range.

3. The 34401A uses a DC blocking capacitor to block the AC RMS converter from measuring DC signals. This allows the multimeter to measure the AC component using the best range. When measuring sources with high output impedance, sufficient time is required to ensure that the DC blocking capacitor stabilizes. The settling time is not affected by the frequency of the AC signal, but is affected by any changes in the DC signal.

The Agilent 3458A has three methods for measuring AC RMS voltage; its simultaneous sampling mode can measure signals as low as 1 Hz. To configure the multimeter for low frequency measurements:

1. Select the synchronous sampling mode:

SETACV: SYNC

2. When you use the simultaneous sampling mode, the input signal is DC coupled for the ACV and ACDCV functions. In the ACV function, the DC component is mathematically subtracted from the reading. This is an important consideration because the combined AC and DC voltage levels can cause an overload condition even if the AC voltage itself is not overloaded.