Normal Mode Rejection, NMR
The operational amplifier (OP Amp) with differential inputs is an essential component in industrial signal conditioning modules, SCMs. Half a century ago designs using OP Amps were very costly and not suited for industrial environments. Today, modern silicon monolithic semiconductor circuits provide the opportunity to design low cost, high gain, low noise, and very stable operational amplifiers. These operational amplifiers are available as multiple units in one package, which ensures that parameters of each unit track with temperature.
Perhaps one of the most important design benefits of operational amplifiers is their ability to process differential inputs. This allows design engineers to create industrial signal conditioning modules, SCMs with differential input pairs. The major advantage of this is the elimination of the effects of equal voltages appearing on both inputs.
As a refresher example, suppose one wants to measure the DC line current on a 90 VDC motor with a 10 milli-ohm shunt for use in a feedback control system. For a motor current of 15 amps, the shunt voltage is 0.150 VDC=(15*0.010). The voltage at each end of this shunt is 90 and 90.15 VDC. Clearly the measurement’s important information is contained in the 0.150 VDC shunt voltage; therefore, the SCM must not be influenced by the common 90 VDC on each end of the shunt. This is where the common mode rejection, CMR, specification of a SCM saves the day. A typical CMR specification for Dataforth SCMs is in excess of 120 dB, which means the effect of the input 90VDC common voltage has a reduction factor of one million.
Normal Mode Rejection, NMR, is another very important specification for SCMs. In the above review example, the “information containing” voltage is 0.150VDC, but as we all know there is most likely considerable 60Hz or 50Hz noise on this 0.150 VDC voltage, perhaps with a magnitude greater than our signal. Again this is where the NMR specification of SCMs saves the day. The Normal Mode Rejection, NMR, specification tells us how well the SCM’s low-pass filter attenuates the 60Hz or 50Hz noise. A typical NMR specification for Dataforth SCMs is in excess of 90dB, which means the 60Hz or 50Hz noise appearing as a differential input is reduced by a factor of 56,000.
Dataforth SCMs have internal multiple pole (5 to 7 poles) filtering that provide excellent NMR. You are encouraged to examine Dataforth’s complete line of SCMs and related products at http://www.dataforth.com. For references, see Dataforth’s Application Notes; AN103 on Common Mode Rejection, AN112 on Filters, and AN113 on Phase shifts.