PCB News - Several special cases of measurement results

1. The influence of capacitance

When a capacitor is connected between the tested pins, it should be in an open circuit relationship, but the measurement path charges the capacitor when the switch is closed, and the two measurement points are like a path. At this time, the measurement result read from the voltage comparator is path. For this kind of false path phenomenon caused by capacitance, the following two methods can be used to solve: appropriately increase the measurement current to shorten the charging time, so that the charging process ends before reading the measurement results; add the inspection of true and false paths to the measurement software The program segment.

2 Influence of inductance

If an inductor is connected between the tested pins, it should be in an open-circuit relationship, but since the static resistance of the inductor is very small, the result measured with a multimeter is always a path. Contrary to the case of capacitance measurement, at the moment when the analog switch is closed, there is an induced electromotive force due to the inductance. In this way, the inductance can be correctly judged by using the characteristics of the fast acquisition speed of the detection circuit. But this is in contradiction with the measurement requirement of capacitance.

3 The influence of analog switch jitter

In the actual measurement, it is found that the analog switch has a stable process from the open state to the closed state, which is manifested as the fluctuation of the voltage VA, which makes the first few measurement results inconsistent. For this reason, it is necessary to judge the results of the path several times and wait for the measurement results to be consistent. Confirm later.

4 Confirmation and recording of measurement results

Considering the above various situations, in order to adapt to different tested objects, the software program block diagram shown in Figure 3 is used to confirm and record the measurement results.

In order to eliminate the influence of capacitive components and analog switch jitter, it is necessary to extend the measurement time, and to eliminate the influence of inductive components, it is necessary to use the induced electromotive force to determine in a short time. For this purpose, two counters are set up in the program: the number of passes counter and The number of open circuit counters. Setting the number of passes N is to eliminate the false path effect caused by the charging of the capacitor at the moment the analog switch is closed, that is, when the results of N passes are read cumulatively, the charging of the capacitor is generally completed, and the measured points are confirmed as paths. Setting the number of breaks n is to eliminate the interference caused by the jitter of the analog switch. Generally, when the results of n breaks are continuously measured, it indicates that the jitter of the analog switch has ended before it can be confirmed as a break. But because the induced electromotive force of the inductance is the largest at the moment the analog switch is closed, and then drops rapidly, if the first and second measurement results are both open, it is confirmed as open. Due to the contradictions among several situations, the value of the counter and delay is determined according to the actual situation on the basis of weighing the three situations.

Of course, when using the above program to judge, if there is a small resistance, small inductance or large capacitance between the tested pins, it may be misjudged as a path. This kind of problem can be easily checked by the software in the measurement results. If both ends of a two-pin component are in the same network, the above judgment error may exist, which should be eliminated after confirmation.

The above is the introduction of several special cases in the measurement results of PCB design. Ipcb is also provided to PCB manufacturers and PCB manufacturing technology.