Eight Tips for making better use of digital multim

Make better use of the eight tips of digital multimeter measurement

tip 1 to avoid the measurement error caused by connection, test line and digital multimeter connection

the simplest way to eliminate the error caused by connection is to conduct zero adjustment measurement. For DC voltage or resistance measurement, select the appropriate measurement range, then connect the probes together and wait for a measurement - this is the case closest to zero input - and then press the null button. The reading obtained below will deduct the result of zero adjustment measurement. Zeroing measurement is very suitable for the structure. Of course, we are also very clear about the combination of DC and resistance measurement functions. But this technology is not suitable for AC measurement. The AC converter cannot work well in the lower part of the range; The analog converter of Agilent 34401A digital multimeter does not specify the technical index below 10% full scale. Agilent 34410a and 34411a Digital Multimeter use digital technology, which can measure up to 1% full degree, but it cannot be used to measure short circuit

connection

if you connect with different metals, it will form a thermocouple junction. The thermocouple junction generates a voltage that varies with temperature. Although this voltage is very low, if you are measuring a small voltage or your system has many connections, you need to take this problem seriously. It can be considered that this thermocouple junction is at the DUT, relay (multiplexer) and your digital multimeter. This offset can be minimized by using copper copper junctions

when measuring resistance, you can use bias compensation to measure any bias voltage and deduct this error. Figure 1 shows two measurements in the offset compensation measurement. The first measurement has a current source, and the second measurement has no current source. Subtract the first reading from the second reading, and then divide by the known current source current value to obtain the actual resistance value. Since two readings are taken in the measurement, the reading speed will be reduced, but the measurement accuracy will be improved. Bias compensation can be used for both two-wire and four wire resistance measurements

figure 1

offset compensation using two measurements. The first measurement is standard ohm measurement; The second is to measure the offset generated by thermoelectric EMF. The voltmeter reading is the difference between these two measurements divided by a known current source

connecting line

four wire ohm method is the most accurate method to measure small resistance. This method can automatically deduct the test line resistance and contact resistance. See Figure 2 for four wire resistance measurement connection. Using a known current source and measuring the voltage generated by the resistor, the unknown resistance value can be calculated. A set of additional test lines is used to carry the current to the unknown resistor, and the voltage generated on it can be measured through the voltage sensing line. No current flows through the voltage sensing line, so it will not produce a voltage drop

no current flows through the voltage sensitive line. The digital multimeter divides the measured voltage value by the known current to obtain the unknown resistance value

internal digital multimeter offset

automatic zeroing is used to eliminate the internal error source of the digital multimeter. When automatic zeroing is enabled, the digital multimeter internally disconnects the input signal after each measurement and obtains a zero reading. Then subtract the zero reading from the next measurement. This avoids the influence of the bias voltage in the input circuit of the digital multimeter on the measurement accuracy. Automatic zeroing is always enabled in four wire measurement, but you can disable the automatic zeroing function in order to improve the measurement speed. When automatic zeroing is disabled, the digital multimeter takes a zero reading and then deducts it from all subsequent measurements. Each time you change the function, range or integration time, a new zero reading will be taken

prompt 2 measuring large resistance

stability time effect

capacitance in parallel with resistor will produce stability time error after initial connection and range change. Modern digital multimeter inserts a trigger delay, which gives the time to stabilize the measurement. The length of the trigger delay depends on the selected function and range. When the combined capacitance of cables and devices is less than hundreds of PF, these delays are sufficient for resistance measurement, but if there is a parallel capacitance on the resistor, or you measure a resistance higher than 100 K Ω, the default delay may not be enough. Due to the influence of RC time constant, it may take a long time to stabilize. Some precision resistors and multifunctional calibrators use capacitors in parallel (1000 PF to 100 μ F) Together with the high-value resistor, it filters out the noise current injected by the internal circuit. Due to the dielectric absorption (infiltration) effect in cables and other devices, it is possible to increase the RC time constant and require longer stability. When we finally reach the field that we have been theoretically sure that the specific fiber structure can lead to the improvement of mechanical properties. In this case, you may need to increase the trigger delay before testing

offset compensation in the presence of capacitance

if there is a shunt capacitor on the resistor, it may be necessary to turn off offset compensation. When offset compensation takes a second reading without a current source, it will measure any voltage offset. However, if the device has a long stability time, it will cause biased measurement with errors. The digital multimeter will use the same trigger delay for bias measurement to try to avoid the problem of stabilization time. Increasing the trigger delay is another solution to make the device completely stable

connection in high resistance measurement

when you measure large resistance, insulation resistance and surface pollution will cause considerable errors. Various precautions need to be taken to keep the high resistance system "clean". The test line and fixture are very sensitive to leakage caused by moisture absorption of insulating materials and "dirty" surface facial mask layer. Compared with PTFE Teflon insulator (109 Ω), nylon and PVC are relatively poor insulators (1013 g Ω). If you measure 1 m Ω resistance under wet conditions, the contribution of nylon or PVC insulator leakage to the error can easily reach 0.1%

prompt 3 AC measurement with DC bias

many signals contain AC and DC components. For example, asymmetric square waves contain these two components. Many audio signals also contain DC offset generated by DC bias current, which is used to drive output transistors. In some cases, dc+ac voltage needs to be measured, while in other cases, only AC component may be required. For this audio example, the amplifier gain is to compare the input AC voltage with the output AC voltage by conducting new experiments and repeated experiments with a larger load

most of these advantages make it more and more widely used in various industrial fields to study the wear problem of machines. Several modern Multimeter use a DC isolating capacitor in front of AC RMS converter. It isolates the DC voltage and allows the multimeter to measure only the AC value