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'[EE:] Grounding problems on breadboard'
2004\07\21@140103 by Mohit Mahajan

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Summary: How to avoid grounding problems on a breadboard while
prototyping microcontroller circuits which use ADC?

Hello,

I'm developing a pH meter using a PIC 16F870 (10bit ADC). The pH probe
is a coaxial cable with the outer wire (shield?) which is to be
grounded. I'm using a LF356 (+5V 0 -5V) and a LM324 (+5V 0V) to amplify
the pH probe signal which is then fed to the PIC (PORTA,0; AN0).

The PIC is on a PCB development board with crystal, LCD et al on it. The
LF356 and LM324 are on a breadboard, with a short wire from the output
of LM324 to the PIC.

As mentioned in a lot of subject matter that I read through to avoid
grounding problems, the grounds of the LF356, LM324 and pH probe shield
are connected together; while the PIC ground is separate from this.
These two separate grounds (wires actually) go the another PCB board on
which I've soldered a full-wave rectifier using 7805 and 7905 and a
center-tapped Xmer.

While running the PIC, the ADC reading varies almost cyclically about
150 ADC units (for eg. pH readings are 7.10 .. 7.30 .. 7.65 .. 7.93 ..
8.34 .. 8.67 .. 8.50 .. 8.14 .. 7.87 .. 7.56 .. 7.34 .. 7.05 and
repeat). No software filteration is used, although a 470pf capacitor is
soldered between AN0 and the PIC ground. The 150 ADC range is too much
to bear.

If I touch the outer shield of the pH probe, the reading is pretty
stable.

So to avoid grounding problems while prototyping such a system, what
should I do about the grounds (and power???) ?

Thanks in advance,
Mohit.

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2004\07\21@142512 by Robert Monsen

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----- Original Message -----
From: "Mohit Mahajan" <spam_OUTbiozenTakeThisOuTspamSANCHARNET.IN>
To: <.....PICLISTKILLspamspam@spam@MITVMA.MIT.EDU>
Sent: Wednesday, July 21, 2004 10:47 AM
Subject: [EE:] Grounding problems on breadboard


> Summary: How to avoid grounding problems on a breadboard while
> prototyping microcontroller circuits which use ADC?
>

Your measurements are taken with respect to ground. I'm guessing that the
PIC ADC is being used, which means they are being interpreted wrt ground on
the PIC/display board... Note that ground leads have resistance, so
variations in power requirements (LCDs, PIC, etc) will cause the grounds to
fluctuate. I'm guessing this is your problem.

If you really need to separate the grounds, I'd suggest using some kind of
VCO/VFO pair, coupled capacitively. Microchip makes VCO/VFO chips for this
very purpose (TC9400, for example). Encode the voltage to a frequency, and
decode it back to a voltage wrt ground on the pic board.

One other way would be to use a twisted pair of wires, and put a reference
voltage on one of them. Then, on the other side, use an opamp to reference
the signal to the ground on the PIC board. This is slightly dangerous, since
you need to make sure your signal or reference doesn't deviate outside the
ground-vcc rails on the pic board.

A final solution would be to use another pic (maybe a little 8pin pic) on
the collection board, and transmit the results digitally to the display unit
using a single wire interface.

Regards
 Bob Monsen

----- Original Message -----
From: "Mohit Mahajan" <biozenspamKILLspamSANCHARNET.IN>
To: <.....PICLISTKILLspamspam.....MITVMA.MIT.EDU>
Sent: Wednesday, July 21, 2004 10:47 AM
Subject: [EE:] Grounding problems on breadboard


{Quote hidden}

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2004\07\21@150055 by Eisermann, Phil [Ridg/CO]

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pic microcontroller discussion list wrote:
> ----- Original Message -----
> From: "Mohit Mahajan" <EraseMEbiozenspam_OUTspamTakeThisOuTSANCHARNET.IN>
> To: <PICLISTspamspam_OUTMITVMA.MIT.EDU>
> Sent: Wednesday, July 21, 2004 10:47 AM
> Subject: [EE:] Grounding problems on breadboard
>
>
>> Summary: How to avoid grounding problems on a breadboard while
>> prototyping microcontroller circuits which use ADC?
>>
>
> Your measurements are taken with respect to ground. I'm guessing that
> the PIC ADC is being used, which means they are being interpreted wrt
> ground on the PIC/display board... Note that ground leads have
> resistance, so variations in power requirements (LCDs, PIC, etc) will
> cause the grounds to fluctuate. I'm guessing this is your problem.

that may or may not be the problem. Grounding is certainly one of the
first things to suspect. The OP said he was having grounding problems,
but with no details of what he ckecked. 150 bits tells me there is a
fundamental layout problem somewhere.

> If you really need to separate the grounds, I'd suggest using some
> kind of VCO/VFO pair, coupled capacitively.

IMO, that's premature at this point. It's more likely to be incorrect
(or possibly missing altogether) grounding. Possibly defective software.
Or maybe even a problem in the display routine.

First verify what & where the problem is using basic techniques.

For example, measure the resistance between the two grounds (unpowered
of course).

measure the voltage potential between the two grounds (while powered).

Then disconnect the pH circuitry altogether. Replace it with a simple
trimpot. Does the problem go away?

disconnect the controller from the pH circuitry. Measure it's output.
Is it fluctuating the same way the ADC readings are? In other words,
perhaps your amplifier is really an oscillator...

Thats all the time I have for now, but if you do some simple &
fundamental diagnostics, i'll bet you find the problem pretty quickly.

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2004\07\22@003956 by Mohit Mahajan

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> measure the voltage potential between the
> two grounds (while powered).
Same, 0V on multimeter.

> Possibly defective software...
> even a problem in the display routine.
Using 16x1 LCD, the first 8 characters show pH and the next 8
temperature (of the form "14.00pH, 100.0^C"). While running the LCD
shows temperature properly - using a LM335, connected to PIC PORTA,1;
AN1. Since the ADC routine is shared by both channels (pH and
temperature), I believe it is alright and so is the display routine.

> Then disconnect the pH circuitry altogether.
> Replace it with a simple trimpot.
> Does the problem go away?
Yes, the problem goes away. The PIC shows a very stable reading.

> disconnect the controller from the pH circuitry.
> Measure it's output.
Using a multimeter, the output of the pH circuitry is stable.

The temperature measurement and display is alright.The problem happens
only when the pH circuit is connected to PIC. I've even tried connecting
the pH probe outer to a large metal case but the fluctuations still
persist.

The point is that I get a very stable pH reading when I touch my finger
to the pH probe outer wire. Can anybody suggest why this might be the
case?

Regards,
Mohit.




pic microcontroller discussion list wrote:
> {Original Message removed}

2004\07\22@004950 by Robert B.

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----- Original Message -----
From: "Mohit Mahajan" <@spam@biozenKILLspamspamSANCHARNET.IN>
To: <KILLspamPICLISTKILLspamspamMITVMA.MIT.EDU>
<snip>
> The point is that I get a very stable pH reading when I touch my finger
> to the pH probe outer wire. Can anybody suggest why this might be the
> case?
>
> Regards,
> Mohit.


I've been told a human finger is like a 20pF shunt to ground.  Did you try
bypassing the probe to ground with a small cap? (20pF might be a good place
to start)  It could be picking up stray jitters from the PIC's circuitry and
just needs a little damping.

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2004\07\22@140432 by Robert Monsen

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----- Original Message -----
From: "Mohit Mahajan" <RemoveMEbiozenTakeThisOuTspamSANCHARNET.IN>
To: <spamBeGonePICLISTspamBeGonespamMITVMA.MIT.EDU>
Sent: Wednesday, July 21, 2004 9:37 PM
Subject: Re: [EE:] Grounding problems on breadboard


{Quote hidden}

What is the impedance of the probe? I understand that ph probes are very
high. Since you are reading it with a bipolar opamp, that may be an issue.

http://pdfserv.maxim-ic.com/en/an/A0512.pdf

{Quote hidden}

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2004\07\22@174944 by 4HAZ

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I would think PH measurements would not need extremely fast sampling as they
would be slow to change.
So a small cap across the signal to Gnd. should clean-up any RFI et.al.

$.02 - KF4HAZ - Lonnie

----- From: "Robert Monsen" <rcmonsen@C

{Quote hidden}

> > > {Original Message removed}

2004\07\23@013935 by Mohit Mahajan

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> Did you try bypassing the probe to ground
> with a small cap? (20pF ...)
and
> So a small cap across the signal to Gnd.
> $.02 - KF4HAZ - Lonnie

Thanks a lot Robert and Lonnie. A 0.1uF cap across the signal and ground
did it. Funny, I tried 20pF, 470pF and 1uF before posting the problem,
and but not the 0.1uF, 0.01uF. Should be more careful in the future.
Sorry.

Mohit.

{Original Message removed}

2004\07\23@145706 by John N. Power

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> From:         Mohit Mahajan[SMTP:biozenEraseMEspam.....SANCHARNET.IN]
> Sent:         Thursday, July 22, 2004 1:07 AM
> To:   EraseMEPICLISTspamMITVMA.MIT.EDU
> Subject:      Re: [EE:] Grounding problems on breadboard
. . .

>> measure the voltage potential between the
>> two grounds (while powered).
> Same, 0V on multimeter.

. . .

{Quote hidden}

You may have a high frequency oscillation problem in the pH front end
measurement which doesn't show up on a multimeter. For that matter,
you may have 60 Hz pickup or any other kind of noise pickup which
you need a scope to see.

John Power

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