制御用計算機のアナログ入力精度とコモン・モード電圧の関係

抄録

One of the causes of decreasing the analog input accuracy of the process control computer is the common mode voltage induced commonly on a pair of signal lines. The common mode voltage is converted to the normal mode voltage (appearing as the differential voltage between a pair of signal lines) by the influence of the unbalance resistance existing in the pair of signal lines and the leakage impedance between the signal lines and the ground when there are open scanner switches in parallel with the input signal lines.
The normal mode voltage is superposed on the signal voltage and causes an error.
The normal mode voltage, if converted at the signal source side from the input scanner, can be eliminated by a low pass filter, if converted at the amplifier side from the input scanner, cannot effectively be eliminated by the low pass filter.
In this paper, the relation between the common mode voltage and the input accuracy is discussed, in view of the following three points.
1) Referring to the equations for the conversion of the common mode voltage into the normal mode voltage, the double mixed shielding method is introduced, which is effective for decreasing the common mode voltage with the electro-magnetic shield by the method of two points' earth shielding.
The double mixed shielding method is also effective for making large the common mode rejection ratio with only electrostatic shield by the method of one point earth shielding.
2) The block switches' effect for the common mode rejection ratio and the optimum number of block switches is discussed. (The block switch is inserted in series with the specified number of scanner switches.)
3) The design method for the amplifier's settling time is discussed, in view of the relation between the scanning speed and the settling time, and between the induced noise rejection by clock pulse and the settling time.