Twisted pair signal wiring: This acts by successive twisted loops canceling each other’s pick-up of magnetically induced voltages (reduction of mutual inductance M).
When you have taken all the above measures you could still be left with some unwanted voltage, effectively in series with the signal. This is called Series Mode interference and is usually ac. Occasionally it could come in dc form. E.g. leakage currents in electrolytic tanks or a bare thermocouple clamped to a conductor carrying direct current. In the dc form, prevention by good isolation is the only remedy since you cannot filter out dc from a dc signal.
A low-pass filter will reduce ac interference. Such filters can be combinations of capacitors, resistors and inductors in the signal lines or they can be digital filters built into the controller’s electronics. Their job is to allow passage of the dc process signal and attenuate (reduce) the interfering ac. Such filters increase the response time of the controller or indicator to changes in signal level. Since temperatures don’t change all that fast, this is not usually a problem. If you are scanning multiple inputs into a time-shared instrument containing a filter you may have to put up with a slow scanning speed in order to wait out the settling time of the filter as it acquires each dc signal. Alternatively you could use a filter on each input.
Modern controllers and indicators use another technique to deal with the remaining ac superimposed on the dc signal. They digitize the average dc signal over a period that contains an equal number of canceling positive and negative half cycles of the ac interference. This means that the interfering frequency has to be known by the digitizing process. Usually 50 or 60Hz is anticipated.
We now come to series mode and common mode definitions and isolation techniques.
Common Mode voltage means that both terminals of the signal are elevated in voltage relative to ground. For example a thermocouple could be welded directly to the live (i.e. not neutral) end of an ac heater. The ac voltage at this point relative to ground is called the Common Mode voltage.
Instrument input circuits are usually designed to cope with sensors that are connected to sources that could be up to some 250V above ground. This elevation can be either an unavoidable process requirement or by accidental contact with a live electric heater.
In the presence of common mode there is always some resistive or capacitive leakage-current to ground from each signal wire and the leakages are not necessarily equal. These currents pass through the two thermocouple conductors – also unequal in resistance. The difference in IR volt-drops in these conductors can produce a series mode interference component which you can deal with as above.
Instrument manufacturers design to minimize the effect of interference and specify a Series Mode Rejection Ratio, defined as:
The value of the series mode interference voltage/the error in indication that it causes – both expressed in volts.
You will see rejection ratios typically of 103. In this example 1V of series mode interference would cause a 1mV measurement error.
Likewise the Common Mode Rejection Ratio is:
The value of the common mode voltage/ the error in indication that it causes – both expressed in volts.
For common mode you will see numbers in the order of 108. In this example 100V common mode causes 1μV (one microvolt) of error.
Instead of publishing these simple numerical ratios it has long been the custom in manufacturer’s specifications to convert them to decibels. 103 converts to 60dB and 108 converts to 160dB. To convert this weird unit back again so you can work with it, divide the dB number by 20 and use it as the exponent of 10.
