AN-112: Common Mode Rejection Ratio (CMRR) in Audio Transformers
Common Mode Rejection Ratio (CMRR) is a critical specification for audio interfaces, but measurement methods vary significantly between transformer-based and direct (solid-state) inputs. This application note clarifies the differences in testing standards and explains why audio transformers often deliver superior real-world ground isolation even when their published CMRR numbers appear lower than IC-based designs.
Introduction to CMRR in Audio Transformers
Understanding some of the advantages of audio transformers as compared to direct input into active electronic devices is often misunderstood. The measurement methods for Common Mode Rejection Ratio (CMRR) often differ. CineMag performs these tests in accordance with IEEE-389-19 as may be amended by IEEE. Companies that employ devices which do not use audio transformers (“Direct Input Companies”) often ignore this test protocol thereby yielding different results which may be misleading.
How CMRR Is Measured Differently for Transformers vs. ICs
Companies that manufacture solid state amplifiers typically rate CMRR by adding in the gain to the CMRR at unity gain. Audio transformer tests which comply with IEEE-389-19 do not factor in this voltage gain factor. Analog Devices MT-042 provides a useful guide of how manufacturers of integrated circuits report CMRR data. Figure 1 below provides the equation often used to express CMRR for integrated circuits. While measurements with this circuit topology are very sensitive to resistor matching, the mathematics are fundamental to measurement calculations.
Formula 1: (1 + R2/R1) establishes the gain of the circuit. Solving for CMRR:
Hence, the gain of the circuit is added into the published CMRR. Assuming unity gain and CMRR of 80dB with gain set to +20dB, CMRR is reported at -100dB. Formula 1 gives the result as a voltage ratio. It can be converted into decibels by multiplying the result by (-20log * CMRR). Circuit gain is added into the result with this methodology.
Transformer CMRR is measured in a different manner when performed according to the IEEE transformer standard. The step-up voltage gain is ignored.
Formula 2 (IEEE 389-19): CMRR = -20 log (e2/e1) [dB]
Using IEEE 389-19, the voltage gain (or loss if step-down) of the transformer in Figure 2 is not considered in the formula. Assume for illustration that the transformer voltage gain is +20dB. If the measured CMRR at a selected frequency were 80dB, the reported CMRR would ignore the +20dB voltage gain of the transformer. The reported CMRR does not factor in the voltage gain. If Formula 1 were to be used for the transformer measurement, CMRR would be 100dB.
Ground Isolation Benefits of Audio Transformers
There is the ancillary problem of isolating the different signal grounds between systems that are powered by different power supplies. Power supply transformers (and switching power supplies) cannot 100% isolate each system from the mains power and the systems’ various signal grounds, as well as their interconnections through audio cables. This means that each stage that is not audio transformer isolated will interact, albeit at a low level. Audio transformers isolate these side paths which can inject signal through the power supplies, and and the cross-grounding of two -or more- systems.
(Caveat: There is a point where too many audio transformers in the signal path cause unacceptable phase shift and, sometimes, frequency response issues.)
References:
1) IEEE-389-19 2) Analog Devices MT-042 3) Common Mode Rejection Ratio in Differential Amplifiers Ramon Pallas-Areny & John G. Webster IEEE Transactions on Instrumentation & Measurement, Vol. 40, No 4, August 19914487 Ish Drive, Simi Valley, CA 93063
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