[Engineering Essentials]
What Was That Noise?
Learning the basics of noise in amplifiers against the backdrop of some new ideas on how to cope with it offers fresh perspectives on a plan of attack.
You need to take care with the test setup (e.g., proper termination at the point at which the signal is applied), and the measurement procedures themselves will be different for different pins. Since the noninverting input pin and the supply pins are tested with the amp set for unity gain, the input-referred output shift will be the same as the measured output shift.
On the other hand, some voltage gain is necessary when coupling the test signal to the inverting input and the output. This requires accounting for that gain in the calculations of EMIRR. Figure 4 shows typical results.
BUZZ AND HUM There’s the kind of noise we’ve been talking about, the noise that makes it impossible to achieve all of the precision that’s theoretically attainable with an analog-to-digital converter (ADC). Then there’s noisy noise—the hum in the speaker, accompanied by related pops and clicks, that drive you crazy during quiet musical passages.
When I spoke with Audio Precision founder Bruce Hofer and mentioned common- mode rejection ratio (CMRR), he called my attention to the Audio Engineering Society’s (AES) pragmatic and outspoken expert on audio buzz (and, by extension, CMRR), Jensen Transformers’ president, Bill Whitlock. Hofer then provided me with a copy of Whitlock’s paper from the June 1995 issue of the Journal of the AES, “Balanced Lines in Audio Systems: Fact, Fiction, and Transformers.”
Hofer said he tends to agree with Whitlock. In fact, Hofer recently modified some of Audio Precision’s manuals to reflect what Whitlock says. Hofer also supports efforts to modify the IEC specification for audio testing to better reflect an understanding of what Whitlock and others have been saying.
Since the subject is common mode, Hofer is talking about differential signals and differential amplifiers. The idea, as it was explained to me many years ago when I was an undergraduate summer intern at a local TV station, was that by using shielded twisted pair, any stray fields that got through the shield affected both wires in the pair equally. Meanwhile, the amplifiers responded only to differences in potential between the wires.
It was also standard operating procedure at the TV station to connect the cable grounds at only one end to avoid ground loops. Those approaches were pretty successful. I worked in Master Control on the 83rd floor of the Empire State Building, and there were fields from our transmitter and a number of others in close proximity, yet none of the fields floating around got coupled into the audio.
Empirically, those techniques work. But Hofer said that in terms of measurement, it’s misguided to do what we’ve always done: looking at CMRR performance (and circuit design) from the standpoint of system response to in-phase balanced signals on the wires in the twisted pair. You need to focus on the differences in impedance at the driver and receiver ends of the cable, because they cause imbalances in the signals on the cables. If you’re really going to measure real-world rejection of “commonmode” signals, your test practices must take those impedance-match imbalances and their effects into account.
BY THE BLOCK When it comes to grounding only the end of the shield on twisted pair, Audio Precision’s Audio Measurement Handbook is less prescriptive than my old TV-station mentors, some of whom had been working there long enough to have done the sound effects for The Shadow. In the section under “Balanced Devices,” it observes that most commercial cable assemblies will have the shield connected to chassis ground at both ends.
This is optimum from a standpoint of rejection of high frequency and RF interference. Theoretically, power mains-related hum problems due to ground loops will be minimized if the cable shield doesn’t connect to both the device under test (DUT) and the test equipment.
But it opines that “with balanced devices and test equipment, ground loops should not be a problem.” Still, it acknowledges that “in case of severe problems, breaking all cable shield connects between the DUT and test instrument and then making a separate large-conductor ground connection between the chassis of the DUT and the audio test set may be optimum.”
On the other hand, the handbook is all for breaking the shield in unbalanced connections, saying, “The cable from DUT output to audio analyzer should have its shield connected to chassis ground only at the audio analyzer input end.”
A very common equipment design error, referred to as the "pin 1 problem", causes it to output hum or buzz when a shield (delivering ground current) is connected to it. It is all too common since XLR connectors are mounted on PCBs rather than metal chassis. A paper on this is in the same June 1995 AES Journal as my paper. I'm working on a new paper that finally explains the origin of ground voltage differences among AC outlets. Readers may also be interested in a new IC that, for the first time, truly imitates the excellent CMRR behavior of a good audio transformer (see http://www.thatcorp.com/1200-series_High_CMRR_Balanced_Line_Receiver_ICs.html).
Bill Whitlock -June 22, 2009
As an audio technician I appreciate this article.These are good points to bring up for future designs.I discovered the wonderful world of noise trying to mate a floating ground power supply device with a referenced ground,(actually used 0 ohm resistors to the chassis),supply.It came down to lifting the ground on the inter- connect on just the input side and lifting the ground of the referenced supply,on the cable end,( very dangerous,some musicians like to drink on stage and this was the mixer for the stage sound),oh and did I mention that the input side was transformer isolated.This is common in my industry as they like to think we don't mix and match different brands of gear.Digital technology has helped,but getting it right in the first place can help with compatibility down the line.The audio world is extremely subjective and we will continue to mix and match brands,( with their different design philosophies),with out much thought of noise and compatibility.
Graham Pearson -June 22, 2009
Where is Figure 4?
Anonymous -June 15, 2009
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