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Analog Dialogue Current Issue

A Reader Notes
High-Frequency Signal Contamination

by Leroy D. Cordill*

I found your article on high-frequency signal contamination (“Ask The Application Engineer-14,” Analog Dialogue 27-2, 1993) interesting and would like to offer some additional comments.

EMI/EMC requirements are becoming more important to designers of industrial equipment as analog signal sensitivities are increased while more “RF generators” (higher-frequency digital circuits) are incorporated into the same equipment. Therefore, I would like to see a good application note relating to the issue of RF susceptibility produced by someone such as Analog Devices. By “good”, I feel it should cover:

a. rules of thumb about the types of circuits where you will likely have trouble
b. some explanation of the phenomenon
c. general grounding/shielding approaches for equipment
d. “fix” type approaches to minimizing the effects when items from (c) can’t be implemented
e. bench-level testing techniques.

(At least I’m not aware of any such application note in existence; maybe one exists and I haven’t found it.) Based on my own experience, I offer the following comments on the above five areas:

Regarding (a), I generally see the problem with low-level input or preamp circuits involving a voltage gain of 50 V/V or more. In my case, the signals are usually from thermocouples, RTDs, pressure sensors, etc., and the required signal bandwidth is less than 100 Hz. And I’m trying to maintain signal integrity suitable for conversion by a 10-to-14-bit A/D converter.

For (b), my “model” of the effect is that the error gets created by rectification of the rf at the base-emitter junctions at the inputs of the op amp, and essentially becomes a large input offset voltage for the op amp. This introduces errors into dc-coupled circuits that cannot be corrected for by any usual low-pass filtering of the signal.

One observation I have made regarding this susceptibility problem is that it is primarily related to bipolar-type op amps (741, 5558, OP05, OP07, OP27, AD708, OP220, etc.) If I swap to a FET-input op amp (TL082, TL032, OP80, OP42, AD845) the error will largely disappear. (Due to other considerations, this is not usually a permanent solution, but helps to identify error sources during EMC testing.)

Also involved is the RF impedance at the two input nodes of the op amp. If (in a typical inverting configuration) the feedback path has a capacitor for low-pass filtering, it aggravates the problem as one input node of the op amp sees more of the RF than the other. If this is the situation, I’m not sure a wide-bandwidth op amp would help (regarding suggestions for using an AD830). Even without an intentional discrete capacitor in the feedback loop, PC-board layout makes it difficult to count on matched impedances at the two inputs.

Regarding (c), a good RF ground to the chassis is important for the signal common; but I find the shielding/grounding aspects of the equipment design relate more to the ESD requirements than RF (continuous-wave) susceptibility problems. I also try not to rely on these (shielding/grounding) to a great extent, since I find them very uncontrollable during the life of a piece of field- customizable equipment.

For (d), my best, most consistent prescription is placing a small capacitor directly across the input pins on bipolar op amps. I have used 100-1000 pF for this purpose in various circuits; it usually significantly reduces or eliminates the problem up to the level of interference that I plan for. I have found that with this in place on the critical parts of the circuit, the requirements for extreme care in grounding and shielding of cables are greatly reduced.

Regarding (e), I agree that a small walkie-talkie is useful, but primarily as a go/no-go test on the equipment when it is all assembled, in the enclosure, etc. However, for pc board or circuit-level work, I have two problems with the walkie-talkie technique: (1) you will get many unkind remarks from the guy on the next bench over if he’s trying to breadboard a low-level circuit and is not ready for EMI testing yet; and (2) if you start attaching leads to various points in the circuit to determine where the problems are, and then apply RF in a radiated fashion, you have so many antennae, both to your circuit and to the various test gear, that you will have no idea what is happening.

I prefer to use an RF signal generator and apply the interference in a conducted fashion. This allows much better control of which items get RF applied to them. I don’t use a lot of RF power, as I usually connect the output of the generator directly to some connector or cable supplying the low-level signal of interest, or in some cases the body of a sensor. A few hundred millivolts of RF signal is generally sufficient to identify problem circuits. I manually sweep from about 10 MHz to 100 MHz. While this is not a quantitative type of test, it is a very useful qualitative technique.

Some of the RF generators I have used for this are older model units-usually acquired at garage sales for $5 to $20 each:

Advance Schools, Inc., Model IGB-102
Heathkit Model IG-102 (same as above)
Precise Model 630

I hope this may be useful, and, as I mentioned would like to see a good application note put together on this subject by someone who can add some additional information regarding performance implications of adding a capacitor on the op-amp inputs for various circuit configurations.

*RR 3 Box 8910, Bartlesville OK 74003. Leroy Cordill, a design engineer with Applied Automation, Inc., has been involved in designing process gas chromatographs for about 20 years. His areas of design have included system architecture, analog, digital, and serial communication circuits, as well as GC detectors and valves.

Thanks to Mr. Cordill for a useful contribution to the Dialogue, and for throwing down the gauntlet to our Application Engineers. They have accepted the challenge; so keep your eyes on the “Worth Reading” page in future issues. Having said that, we feel obliged to point out that the challenge is to get it together in one place; much of the material he suggests already exists in the Analog Devices literature (and elsewhere). For example, the System Applications Guide devotes pages 1-13 thru 1-55 to remote sensor application problems-including an exhaustive discussion of RFI rectification in high-accuracy circuits. Other good sources include the Applications Reference Manual, Chapter 3 and Bibliography of the Transducer Interfacing Handbook, and Part 5 and Bibliography of the Analog-Digital Conversion Handbook.