[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.
It’s highly unusual to encounter only one source of intrinsic noise. If those sources are uncorrelated, they can be combined as the square root of the sum of the squares:
Thus, the total effect from two noise sources with the same energy is a 3-dB increase in total noise energy. More importantly, any noise voltage more than three or five times greater than any of the others will dominate, and the others may be neglected.
EXTRINSIC NOISE Today, it’s not so strange for a nearby cell phone to interfere with a process-control system. It does so by introducing RF interference into the signal condition amplifiers between a sensor and the analog-to-digital converter (ADC) that digitizes the signal from that sensor.
In fact, that’s the real-world example in National Semiconductor application note AN-1698 (A Specification for EMI Hardened Operational Amplifiers, www.national.com/an/AN/AN-1698.pdf). The company proposes a standard method for a new kind of “rejection-ratio” amplifier spec, “EMIRR,” or electromagnetic-interference rejection ratio.
The appnote shows a signal chain and a scope measurement that demonstrates the effects on the same circuit built using generic op amps and the company’s “EMIhardened” op amps (Fig. 2). The RF signal at the op-amp input is –20 dBVP at 900 MHz, and the op-amp voltage gain is 101.
With the standard op amp, when the phone is called, the input-referred offset voltage shifts about 0.32 mV, so the output voltage is shifted by 32 mV. With the hardened amp, the output shift is about 1 mV. For a 10-bit ADC with a 5-V input range, the difference in the ability to resolve a change in sensor voltage is 7 bits of ambiguity versus 0.2 bits.
The point of NSC’s appnote is to define how to measure the EMI hardness of an amplifier in a reasonable and standard way. But first, it’s necessary to understand the path the RF from the cell phone takes to get into the amplifier. That is, is the interference radiated or conducted? The cell phone is radiating, but even at 900 MHz, there’s not much inside the IC package that has the capture area to pick up much RF energy. The elements in the external circuit pick up the radio waves and conduct them into the package.
When determining actual numbers for EMIRR, treating the interference as conducted requires more steps in the test procedure. It’s necessary to apply signals separately to input, output, and power pins. But it also simplifies the test setup, as there’s no need for a screen room and antennas.
Once the interfering signal gets inside the package, pass-band noise problems arise when it encounters nonlinear circuit elements. “The highest nonlinearity is obtained for signals with a frequency that falls outside the band of the op-amp circuit, i.e., for frequencies at which the overall feedback is virtually zero,” according to the appnote.
“This nonlinearity results in the detection of the so-called out-of-band signals. The obtained effect is that the amplitude modulation of the out-of-band signal is downconverted into the baseband. This baseband can easily overlap with the band of the op-amp circuit,” the appnote continues. “The practical effect is that the amplifier offset voltage varies in step with the keying of the digital signal on whatever stage in the transmitter chain is being modulated.”
The engineers at NSC have defined the EMI rejection ratio as represented by Equation 14, where VRF_PEAK is the amplitude of the applied unmodulated RF signal (V) and ?VOS is the resulting input-referred offset voltage shift (V). For reasons too complicated even for the appnote, there’s a quadratic relation between the resulting offset voltage shift and the RF signal level (Fig 3).
The appnote recommends a standard test condition of 100 mVP (–20 dBVP), but notes that it might be necessary to use larger signals for measurements on amps with very good EMIRR. For those cases, it provides an algorithm for normalizing measurements under different RF signal levels.
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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