• Is anybody using NE521 comparators?

    From Jean-Pierre Coulon@21:1/5 to All on Wed Sep 25 15:16:20 2024
    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

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)
  • From Jeroen Belleman@21:1/5 to Jean-Pierre Coulon on Wed Sep 25 16:58:21 2024
    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

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)
  • From piglet@21:1/5 to Jean-Pierre Coulon on Wed Sep 25 14:58:25 2024
    Jean-Pierre Coulon <coulon@cacas.pam.oca.eu> 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

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)
  • From Bill Sloman@21:1/5 to Jean-Pierre Coulon on Thu Sep 26 01:17:14 2024
    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

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)
  • From Liz Tuddenham@21:1/5 to Jean-Pierre Coulon on Wed Sep 25 18:19:10 2024
    Jean-Pierre Coulon <coulon@cacas.pam.oca.eu> 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

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)
  • From Jeroen Belleman@21:1/5 to Liz Tuddenham on Wed Sep 25 23:50:32 2024
    On 9/25/24 19:19, Liz Tuddenham wrote:
    Jean-Pierre Coulon <coulon@cacas.pam.oca.eu> 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.

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)
  • From Jean-Pierre Coulon@21:1/5 to Bill Sloman on Thu Sep 26 08:14:48 2024
    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

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)
  • From Bill Sloman@21:1/5 to Jean-Pierre Coulon on Thu Sep 26 17:10:17 2024
    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

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)
  • From Liz Tuddenham@21:1/5 to Jeroen Belleman on Thu Sep 26 09:36:36 2024
    Jeroen Belleman <jeroen@nospam.please> wrote:

    On 9/25/24 19:19, Liz Tuddenham wrote:
    Jean-Pierre Coulon <coulon@cacas.pam.oca.eu> 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

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)
  • From Jean-Pierre Coulon@21:1/5 to All on Thu Sep 26 13:04:46 2024
    "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

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)
  • From Phil Hobbs@21:1/5 to Jean-Pierre Coulon on Thu Sep 26 11:19:38 2024
    Jean-Pierre Coulon <coulon@cacas.pam.oca.eu> 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

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)
  • From Bill Sloman@21:1/5 to Jean-Pierre Coulon on Thu Sep 26 21:24:32 2024
    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

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)
  • From Jeroen Belleman@21:1/5 to Jean-Pierre Coulon on Thu Sep 26 15:07:50 2024
    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

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)