Just to add to what Rich has stated - the CMRR will also vary with frequency for both passive and active systems.
These variation are caused by small differences in the LCR coupling of the probes and the finite bandwidth of any amplifier. I remember using a very high gain product (A Thermal Imager - the noise figure was down around (800pV/Hz^0.5)and the end of runway radar bursts of RF were demodulated by the combined non-linearities and finite CMRR to wipe-out the IR image. I think this story might also be relevant to the debate about spurious emission in safety applications - although I believe that 'proper' safety circuits will be designed so that a single fault (such as a junction s/c or capacitor o/c)will not affect system safety. Best regards and Happy New Year Gregg -----Original Message----- From: [email protected] [mailto:[email protected]]On Behalf Of Rich Nute Sent: Monday, December 31, 2001 1:46 PM To: Cortland Richmond Cc: [email protected]; [email protected] Subject: Re: Fw: working voltage measurement Hi Cortland: > One can also use a pair of probes known to be well balanced and take the difference between the A and B channels, or > use a differential input accessory or plugin. There is then no possibility of high-voltage on the instrument chassis. "Balance" is really Common-Mode-Rejection-Ratio, CMRR. There are two schemes for difference measurements. One is measuring the high and low signals independently (through Channel A and Channel B), and then subtracting the two (A and B) signals. This requires both channels to have exactly the same gain (and phase shift) throughout the frequency range of interest. A 1% gain difference yields a 100:1 CMRR, which is quite poor -- almost useless. Another scheme is the difference amplifier, where the high and low signals are applied to the + and - inputs of the same amplifier. A good difference amplifier is capable of 100,000:1 CMRR at low frequencies. The CMRR of a difference amplifier falls off as the frequency increases. A difference amplifier may not have sufficient bandwidth to accurately measure the high frequencies in a SMPS. I have found the portable, battery-operated scope to be the most accurate and easy way of measuring waveforms within a SMPS. Best wishes for the New Year, Rich ------------------------------------------- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ To cancel your subscription, send mail to: [email protected] with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Michael Garretson: [email protected] Dave Heald [email protected] For policy questions, send mail to: Richard Nute: [email protected] Jim Bacher: [email protected] All emc-pstc postings are archived and searchable on the web at: No longer online until our new server is brought online and the old messages are imported into the new server. ------------------------------------------- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ To cancel your subscription, send mail to: [email protected] with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Michael Garretson: [email protected] Dave Heald [email protected] For policy questions, send mail to: Richard Nute: [email protected] Jim Bacher: [email protected] All emc-pstc postings are archived and searchable on the web at: No longer online until our new server is brought online and the old messages are imported into the new server.
