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?

--
Jean-Pierre Coulon

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On 9/25/24 15:16, 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?

How do you think we can know without you showing us exactly what you
did?

Jeroen Belleman

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Jean-Pierre Coulon <[email protected]> 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?

Are you looking at output with a scope- could be oscillating?

--
piglet

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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?

The NE521 is quite fast - it can get up to 55MHz - and it's not
difficult to make it self-oscillate if the voltage difference between
the input isn't high enough to swamp capacitative feed-through or ground
loops.

I've had to crawl through circuits which were in production when a batch
of faster comparators - not NE521 parts - showed up a marginal layout.
And tightening up the layout didn't always keep on working.

--
Bill Sloman, Sydney

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Jean-Pierre Coulon <[email protected]> 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?

If the strobe pins are high, the output will be undefined.


--
~ Liz Tuddenham ~
(Remove the ".invalid"s and add ".co.uk" to reply)
www.poppyrecords.co.uk

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On 9/25/24 19:19, Liz Tuddenham wrote:

Jean-Pierre Coulon <[email protected]> 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?

If the strobe pins are high, the output will be undefined.

That's not so. The NE521 has no latch of flipflop. What they call
strobes are actually inputs to NAND gates. If any of the strobes
are low, the associated output(s) will be forced high.

The output is a traditional Schottky TTL gate. If the OP sees
something in between a definite high or low, it's probably
oscillating. That could be because one or both of the comparator
inputs are driven from a too-high impedance, or it could be
feedback due to poor layout, or it could be poor or missing
power supply decoupling, or something else yet.

In summary, let's wait until he shows us what he did.

Jeroen Belleman

P.S. Half of the equivalent schematic in the datasheet is
nonsense.

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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. I oscillates with a smaller hysteresis.


--
Jean-Pierre Coulon

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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".

--
Bill Sloman, Sydney

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Jeroen Belleman <[email protected]> wrote:

On 9/25/24 19:19, Liz Tuddenham wrote:

Jean-Pierre Coulon <[email protected]> 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?

If the strobe pins are high, the output will be undefined.

That's not so. The NE521 has no latch of flipflop. What they call
strobes are actually inputs to NAND gates. If any of the strobes
are low, the associated output(s) will be forced high.

The output is a traditional Schottky TTL gate. If the OP sees
something in between a definite high or low, it's probably
oscillating. That could be because one or both of the comparator
inputs are driven from a too-high impedance, or it could be
feedback due to poor layout, or it could be poor or missing
power supply decoupling, or something else yet.

In summary, let's wait until he shows us what he did.

Jeroen Belleman

P.S. Half of the equivalent schematic in the datasheet is
nonsense.

Perhaps I should have prefaced my reply: "The data sheet says" if the
strobe pins are high...


--
~ Liz Tuddenham ~
(Remove the ".invalid"s and add ".co.uk" to reply)
www.poppyrecords.co.uk

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"offset" has a particular meaning for its designers. Tha datasheet says:
output undefined if abs(Va_Vb) < offset. Forget what you know about the
offset of an opamp! One could expect better from a bipolar differential pair :-)


--
Jean-Pierre Coulon

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Jean-Pierre Coulon <[email protected]> wrote:

"offset" has a particular meaning for its designers. Tha datasheet says: output undefined if abs(Va_Vb) < offset. Forget what you know about the offset of an opamp! One could expect better from a bipolar differential pair :-)

Well, you could improve the layout, or add an amplifier in front. How fast
do you need it to be?

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC / Hobbs ElectroOptics Optics, Electro-optics, Photonics, Analog Electronics

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On 26/09/2024 9:04 pm, Jean-Pierre Coulon wrote:

"offset" has a particular meaning for its designers. Tha datasheet says: output undefined if abs(Va_Vb) < offset. Forget what you know about the offset of an opamp! One could expect better from a bipolar differential
pair :-)

What's wrong with that?

The offset for any particular pair of nominally identical transistors
can be more tightly defined than that, though it can be expected to
drift with temperature, but this is a data sheet talking about all the
possible parts that the manufacturer might sell under that part number.

If the two inputs are closer in voltage than the data sheet value for
the maximum offset, they can't say anything reliable about the state of
the output.

It can be seen as a cop-out, but it's hard to see how they could be more specific.

--
Bill Sloman, Sydney

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On 9/26/24 13:04, Jean-Pierre Coulon wrote:

"offset" has a particular meaning for its designers. Tha datasheet says: output undefined if abs(Va_Vb) < offset. Forget what you know about the offset of an opamp! One could expect better from a bipolar differential
pair :-)

It's Va-Vb, the difference. Not Va_Vb.

They mean that they can't tell if the output will be either
high or low; Not that it is hovering somewhere in between.
This is pretty much what one should expect from a comparator
offset. Of course, for *really* small inputs, the output
will start wiggling randomly. You're not supposed to go there.

I agree Vos=6mV is pretty poor for a bipolar differential
comparator, but hey, this is an old design. It can easily
compare voltages less than the offset, but you'd have to
get serious about correcting for it. It gets slower, too.

Jeroen Belleman

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