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Why do indicated values of unwired channels fluctuate??
With scanning recorders, the same waveforms as those of channels to which signals are being input also appear on channels that are not connected, even though the channels are isolated. Sometimes users wonder whether this indicates a malfunction, but actually this is due to a characteristic of scanning recorders in which multiple inputs are switched by a single A/D-converter prior to logging. You can prevent this phenomenon either by turning OFF the unused channels, or by shorting the +/- terminals.
To understand more about why this phenomenon occurs, let's look at an example with DXAdvanced.
On the DXAdvanced one-second sampling models, measurement is performed while the channels are switched ten at a time by semiconductor relays. For the thermocouple or low voltage input ranges, the impedance of the DXAdvanced input circuits is tens of mega-ohms, and a small capacitor is used as an input filter. Thus, when the relay switches to a channel with open input, that input remains at high impedance, and the indicated values can fluctuate due to the effects of the capacitor charge from the pervious channel. Looking at the simple diagram of the input circuits below,
- As an example, assume that a voltage is applied to CH1 (and SW1 and SW2 turn ON).
The voltage charges the capacitor C1 (hereafter called "C1").
- Next, CH2 is measured (SW3 and S4 are ON).
In this case, the CH1 charge in C1 has no means of escape since an input wire is not connected (resulting in high impedance), and this can have an effect on the A/D.
As such, this phenomenon can occur in cases like this because the input is open. However, in this type of measurement, if a sensor or other device is connected to the input, the previous CH has no effect.
For example, if a sensor (ex.: TC type K) is connected to CH2, the loop created by the sensor allows the capacitor to discharge, but in the case of DXAdvanced, the A/D waits a sufficient amount of time until the input signal stabilizes. Therefore there is no effect in terms of waveform display because the A/D facilitates measurement after the charge in the capacitor has been offset.
- For Input Ranges of 2 V, 6 V, 20 V, and 50 V
For the 2 V, 6 V, 20 V, and 50 V ranges, due to the various configurations below, 0 V results in almost all cases (even when the input is open) because the charge from R2 into C1 has an outlet (see diagram below).
- For the RTD Input Range
When the input range is RTD, the b terminal is common (excluding 3-wire insulated type RTDs). For example, Therefore, with RTD input assigned temporarily on all channels, effects are experienced if input is applied only to CH1 and the other channels are open.
- Whether or not effects are experienced differs from case to case. Open circuits are very unstable, and many causal factors are involved. In high sensitivity ranges, open circuits can even be affected by cell phones or floating potential. These conditions are not limited to DXAdvanced, and can occur on any measuring instrument.