Electronic Design

Bob's Mailbox

HI BOB,
I read your response to Arthur Williams in the April 23 column (Bob’s Mailbox). The answer as to whether or not to remove the ground plane underneath inductors is: it depends. If the inductors are cans or toroids, it does not matter as the fields are contained inside the inductor. If the inductors are air wound or chip inductors, it might be best to try and see if it improves. At 28 MHz, strip line and printed inductors won’t be used, but remove the ground plane for these. The main concern with inductors is the groundplane killing the Q through coupling. (I think you have the right insights. Thank you. /rap)

If you don’t know how to use ground planes, they can cause more problems than they solve. Ground planes should be segmented into digital and analog planes at the very least. If you know what you are doing, it is not hard to make digital and RF and analog live on the same board without shields. (If you are not sure what you are doing, those darned shields may make good Band Aids. I know several good engineers who have learned that segmenting the ground planes into digital and analog ground planes can make a big mess if you have any signals crossing those demarcations. /rap)

I enjoy reading your columns and regard them as continuing education.

RADCLIFFE CUTSHAW

HELLO, RADCLIFFE,
Well, I think you could see that this is an area where I am not an expert, so this time I am learning from you! RAP

HI BOB,
I’ve used the LM393 dual comparator in many, many applications. Usually, the open-collector output would be used as a logic signal, with no special requirements, other than a pullup resistor. However, I have a new application where the open-collector output is wire or’ed into a switching node. Therefore, the capacitance of the opencollector output is now critical. I could not find this spec’d anywhere. (If you look at the typical output curves in the datasheet, some of the waveforms may indicate if the C is large or small. /rap)

Do you know what I could expect for capacitance (to Vee or other internal nodes) for this open-collector output? (You are probably the first guy to ask the question in 10 years. Nobody at NSC will remember the answer. I think it will be more than 5 pF. Maybe 20? Probably less than 50 pF. Surely less than 100. To save time, just measure it yourself! Use a low-capacitance probe. And be a good sport and let me know the answer. /rap)

Also, the switching node is driven from other circuitry and will be switching from 0 to 1.2 V at approximately 200 kHz. Do you think this switching waveform on the open-collector output node will “disturb” the comparator at all (through internal coupling within the comparator)? Obviously, when the open-collector output pulls low, it defeats the (externally driven) switching function.

BOB BUONO

HELLO, BOB,
The LM393 circuit will not care, and its performance will be unaffected. However, your layout of the printed-circuit board between the output pin and the positive input and negative input pin will possibly have some effect. So, it might be a good idea to keep those printed-circuit foils separated. This is not an LM393 problem. This is a comparator problem, and a layout problem, and it applies to every comparator application. Every one in the world.

You might want to add a tiny bit of positive feedback from the comparator output to the positive input. Perhaps 2 mV? Standard application for comparators. RAP

HELLO MR. PEASE,
Can you direct me to some published results or describe the output phase of the standard LM317 regulator as a function of frequency? I am particularly interested in the frequency region where the regulator ZOUT becomes inductive.

(There is no simple answer to that. That’s because the phase changes to be inductive, and the inductance is a function of the load on the output. One of my techs did a study on this, and it has been published in my Troubleshooting Analog Circuits book, back in the appendices. I’ll try to find this and scan it soon. If I goof, remind me. This effect can also be minimized by putting zeners or big caps at the adjust pin to ground. Do you want to know everything these is to know about the phase, or do you just want the output error to be as flat and low as possible? The latter is easier. Have fun. /rap)

I am not an engineer. This is just a hobby for me. My main gig is as a neuroscientist. If you ever have any neuroscience-related questions, please feel free to ask. I would like the opportunity to return the favor if you do decide to respond.

TIM JARSKY

HELLO, TIM,
No problem. Best regards. RAP

Comments invited! [email protected] —or: R.A. Pease, 682 Miramar Avenue San Francisco, CA 94112-1232

Bob Pease obtained a BSEE from MIT in 1961 and was a Staff Scientist at National Semiconductor Corp., Santa Clara, Calif.

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