decoupling capacitor

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i m using 4029 chips as an 32 bit up/down counter
but when there is electronic noise on the line
the counter state becomes randomly set

i have used 100 nF decoupling capacitors but
it didnot help

should i use 1 uF electrolytic ?
is there fast electrolytic capacitors on the market ?

thank you

Re: decoupling capacitor
oN 21-Nov-03, anotherbrick said:

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In general, on CMOS boards, I would put some bulk capacity near the
power entry for the card, and bypass it with something small for
high-frequencies. Not knowing what is in your design, it's hard to be
specific, but a 4.7uF tantalum bypassed with a 10nF cap would be a good
start.

Because caps are cheaper than debugging, I also favor distributing
small caps liberally around the board. A single 10nF per 2-3 CMOS chips
(for old and slow CMOS) would suffice. For HCMOS, I'd revert to TTL
practice, and put a 10nF per chip.

You don't say whether this is an etched board, or a prototype. Remember
that all the caps in the world won't make up for inadequate grounding.

You didn't say, either, whether you are using a regulated supply, or
what sort of noise environment is involved. More info would make
possible better suggestions.

--
Bill
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Re: decoupling capacitor
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Make sure that all unused inputs are properly tied to ground or VCC, don't
let any of them float.

Rob



Re: decoupling capacitor
oN 21-Nov-03, Rob Turk said:

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With CMOS, it is best not to tie inputs to VCC, but to tie them to VCC
through a 10K resistor. Since the input impedance is huge, a single
pullup "bus" will suffice for most circuits, or multiple resistors may
be used, whichever is more convenient.

--
Bill
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Re: decoupling capacitor
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4029's are quite slow. By noise, do you mean on the clock line, or on the
power lines ?
The clock/control lines  may need schmitt buffering, with RC filtering
before the schmitt if the signals
are really noisy. ( eg linear optical sensors )


"William Meyer"  wrote
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Why ?
I think there was a legacy reason from the very first TTL devices (before
LS),
to not tie direct to Vcc (Vih could break down, and other chips only pulled
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so 5V was a problem) - but that requirement has long gone.
 Still shows up on some circuits, 30 years later....

-jg



Re: decoupling capacitor
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CMOS inputs are not TTL inputs, nor power lines.  Under certain
conditions they can form SCRs, and get triggered by transient
spikes.  As a result the chip self destructs, shorting the power
busses.  The resistor imposes a current limit, and the chip lives
through the episode.

--
Chuck F ( snipped-for-privacy@yahoo.com) ( snipped-for-privacy@worldnet.att.net)
   Available for consulting/temporary embedded and systems.
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Re: decoupling capacitor

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Not quite. Topic was unused inputs, not external-world connections.
Unused IPs on any CMOS device can be quite safely tied to the chip
Vcc or Gnd.

Externally connected pins are a different story, and all CMOS IP structures
do have a latch-up trigger current level, normally some hundreds of mA.
On some devices, to get that injection level, you are past the voltage
rating.
It IS a good idea to have an external series R, and even decoupling C
+Schmitt
buffer on external lines, and many companies make transient diode/arrays for
protection.

-jg



Re: decoupling capacitor
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you are right ,

i m using 4029 built 32 bit counter for counting position from an
incremental encoder

however , when a contactor in electric board is powered on or off
or when a motor starts turning
the counter state becomes random - unpredictable

i think the noise on the power lines is the reason
my power supply for counter logic is linear with a transformer and 7805
i use 1 stage line filter but it doesnot help

i used .1 uF capacitor at every 4029 chip but it didnot help

is there any help ?

secondly ;
what if i change 4029 to 74HC191 ?would it help ?
is 74HC logic better for noisy environments ?

Re: decoupling capacitor
On 22 Nov 2003 08:41:43 -0800, the renowned snipped-for-privacy@yahoo.com

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It probably has to do with the power supply, external connections and
housing, PCB layout, and such like. Perhaps easy for many of us to fix
if we had it in front of us...


Best regards,
Spehro Pefhany
--
"it's the network..."                          "The Journey is the reward"
snipped-for-privacy@interlog.com             Info for manufacturers: http://www.trexon.com
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Re: decoupling capacitor
oN 22-Nov-03, anotherbrick said:

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1. How is the encoder powered? Is its power adequately filtered?
2. what does your supply look like? Is it half-wave, or full wave?
3. How much bulk capacity in the supply before the 7805?
4. Is there high-frequency filtering ahead of the 7805?
   Note: you might also want to consider clamping the unregulated
supply, to prevent nois spikes from going higher than unfiltered volts
+ 5, and lower than ground.
5. If you've not done so, read the 7805 app notes. Throwing a large cap
on the output of the regulator is a Bad Thing, as it will open the
feedback loop, and reduce the ability of the regulator to protect you.
IIRC, the recommended output cap for a 7805 is 1uF.
6. Make sure you have a 4.7uF or 10uF at the input to the 7805, close
to the device. Failure to provide such a cap can cause problems with
instability.
 
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If you power the shaft encoder from a remote supply, rather than from
the supply on the counter circuit, you will introduce many potential
problems. If it is necessary to use a remote supply, then consider
using optical couplers to import the signals from the encoder.
 
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I'm suspicious of grounding and shielding issues, as well as the
coupling of noise from the ground system on which the motor is powered.

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74HC is not likely to help. First is to be sure your ground is clean.
Next, make sure your unregulated power is clean. Then, make sure you
have adequate power and ground distribution downstream of the regulator.

NOTE: If this is a wire-wrapped circuit, then unless the board has a
ground plane, you are at some risk of noise. Your decoupling is only as
good as the conductors to which it is attached.

--
Bill
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Re: decoupling capacitor
Hi, now we get the full picture. Contactor coils are notorious for this
problem, put a mov across the coils.

Re: decoupling capacitor
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the

How it it connected ?
True quadrature encoder interfaces are non trivial - done properly
they count 4 per full phase, and include filtering to ensure that if
chatter does occur on a single edge, the state engine can correctly
track it.
If you connect Qa to CP and Qb to DIRN, you get one count per full phase,
but besides the effect of reduced resolution, edge chatter on  the CP will
spin the
counter.

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How random ?
Is it a advancing a few counts, or do the MSB's change ?

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Unplug, and short the incremental encoder IPs and see what the
contactor-injection effect changes to.
If you have a external reset line, unplug and short that too.
Once you have the counters stable with no wired signals,
then add them back in one at a time.

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4029 @ 5V logic is very slow, which can be a good thing for injected noise
effects.
It also has lower linear region IP currents, which is why it has not been
replaced by
HC/HCT/VHC/LVC families - if the 4000 family is OK at your frequency, it can
be
the best design solution.

-jg




Re: decoupling capacitor
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it gets a very large number
i.e. the most significant bits also change
but it is fully random which bits change

the board has ground plane
but no plane for +5V

on the same board with the counters
there is a 16 bit DAC which closes the position control loop
the incremental encoder is at the back of motor
and the DAC outputs speed command to a thyristor servo drive
which drives the motor
incremental encoder is isolated but DAC ground is connected to servo driver

there is no reset line of counter
i cannot run the board with encoder input shorted
i need encoder input to run the machine
(it is a grinding machine )

Re: decoupling capacitor
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driver

Does your logic circuits share any ground paths with your high-power
components? If you have a common section of thin wire or traces, then this
may cause spikes to propagate through ground. The common section may act as
an inductor, making this even worse. Make sure all logic and all power
circuits have a single point of ground.

Does the DAC output directly influence the speed of the servo? 16-bit is
very high resolution for speed control, especially if it's limited to 0-5V
in range. Are you sure you are observing erratic counter behaviour, and not
noise on your DAC output? Is it sufficiently buffered with a low-noise
opamp? Do you have a picture on the web somewhere of the schematics you're
using?

Have you peeked around with a scope? Have you tried if it's easy to inject a
signal into the counter with a signal generator? Take a generator with 1V
and 1kHz output and a 10K resistor in series. Probe the pins of your
counters and see if any causes spurious changes.

Last but not least, double-check each counter chip for bent pins. It
wouldn't be the first time where a chip with ground or VCC bent under the
socket seems to sort-of-work OK.

Rob



Re: decoupling capacitor

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Put a 0.1uf ceramic disk cap as close as possible to VCC and GND leads.




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