Re: Is anybody using NE521 comparators?

On 26/09/2024 4:14 pm, Jean-Pierre Coulon wrote:

On Thu, 26 Sep 2024, Bill Sloman wrote:
 

On 25/09/2024 11:16 pm, Jean-Pierre Coulon wrote:

If the difference between both inputs is not a least 50 mV the output remain at about 1.5 or 2V instead of beeing close to V+ or ground. What am I doing wrong?

>
Getting it to oscillate faster than your oscilloscope can follow?

 Yes this was an oscillation. A bit fast for my scope but visible.
 I worked around with a 10 mV hysteresis but it is unpleasant because I want to detect when an about 20 mV high signal goes to zero. It oscillates with a smaller hysteresis.

If you've got control over the circuit layout, you can look for the likely feedback paths and try to reduce them - if the printed circuit layout was sub-contracted, this can sometimes be quite easy.
I remember an occasion from early in my career - at EMI Central Research - where we got given a layout by a subsidiary company and could see - by inspection - that a particular amplifier chip was going to oscillate.
We could rotate the chip by 180 degrees and separate the input track from the output track, which made the power supply tracking a bit messier, but turned out to solve the problem.
The subsidiary had already made and loaded 210 of the boards, and refused to junk them, choosing instead to take out the chip (which did indeed oscillate) and bodge in a two transistor amplifier, which must have cost a fortune.
Crankier problems can call for extra ground track between sensitive tracks to provide a measure of capacitative shielding.
On one occasion I ended up soldering a length of heavy copper wire onto a ground trace, which lowered it resistance enough to kill off a nasty ground loop.
The production modification was less messy - we split the track so that the offending high current went down one track while the  other track served as OV voltage reference - "star earthing".
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Bill Sloman, Sydney