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'[EE]: ICD2 introducing noise onto +5V line'
2006\06\22@054150 by Philip Pemberton

face picon face
Hi,
  I'm trying to sort out a problem with my homebrew battery charger, which
seems to only occur when it's hooked up to the ICD2, and even then it only
happens when the ICD2 is hooked up to my desktop machine.

  When the ICD2 is connected, the +5V power bus picks up a 0.5V amplitude
repeating pulse, with a period of about 100us. Unplugging the ICD2 from the
PC, or plugging it into a laptop instead of the desktop system completely
removes the noise.

  Besides "use the laptop", can anyone suggest a way of getting rid of this
noise? It's causing major problems with the opamps in the V/I sense circuit --
the noise is going from the Vcc supply to the opamp output. Needless to say
this makes a bit of a mess of the current readings and usually causes the
SMPSU to lose regulation.

  I managed to capture the noise on my scope - see
<http://www.philpem.me.uk/temp/noise.jpg>.

  Adding capacitors to the supply lines doesn't seem to do anything - I've
tried everything I had from 100nF to 470nF, 1uF to 100uF, and a 1000uF low-ESR
I had lying around. The noise is still there...

  Short of giving up on debugging this thing with my desktop machine and
switching to the laptop (which would involve me reinstalling MPLAB), can
anyone suggest anything else I could try that might get rid of (or at least
severely attenuate) the noise?

Thanks.
--
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2006\06\22@055238 by Michael Rigby-Jones

picon face


{Quote hidden}

Sounds like you have relatively poor supply rejection?  Is one of the inputs referenced to the Vcc rail?


>   Short of giving up on debugging this thing with my desktop
>machine and
>switching to the laptop (which would involve me reinstalling
>MPLAB), can
>anyone suggest anything else I could try that might get rid of
>(or at least
>severely attenuate) the noise?

Have you tried using a serial connection to the ICD instead of USB?  You will need to plug a 9v supply into the ICD though.

Regards

Mike

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2006\06\22@062006 by Wouter van Ooijen

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>    Short of giving up on debugging this thing with my desktop
> machine and
> switching to the laptop (which would involve me reinstalling
> MPLAB), can
> anyone suggest anything else I could try that might get rid
> of (or at least
> severely attenuate) the noise?

I would guess this noise originates from the PCs +5V supply, which is
probably the direct source of the USB +5V. Have you tried

- using the ICD2 without powering the target from the ICD2
- a powered USB hub between the PC and the ICD2

Wouter van Ooijen

-- -------------------------------------------
Van Ooijen Technische Informatica: http://www.voti.nl
consultancy, development, PICmicro products
docent Hogeschool van Utrecht: http://www.voti.nl/hvu


2006\06\22@063146 by Philip Pemberton

face picon face
Michael Rigby-Jones wrote:
>>   Besides "use the laptop", can anyone suggest a way of
>> getting rid of this
>> noise? It's causing major problems with the opamps in the V/I
>> sense circuit --
>> the noise is going from the Vcc supply to the opamp output.
>
> Sounds like you have relatively poor supply rejection?  Is one of the inputs referenced to the Vcc rail?

No, I've got a voltage divider halving the battery voltage and feeding it to
the positive input of an opamp wired as a voltage follower. The current sense
circuit is a low-side sensor; the top end of the resistor goes to the positive
input, and the sense amplifier is wired as a 30x non-inverting amplifier.

The noise is appearing on both outputs, but it's worst on the current sense
output. It's not present at all on the I(sense) opamp's positive input.

> Have you tried using a serial connection to the ICD instead of USB?  You will need to plug a 9v supply into the ICD though.

I'll try that once I can find a spare 9V power supply with the right type of
connector on it...

Thanks.
--
Phil.                         | Kitsune: Acorn RiscPC SA202 64M+6G ViewFinder
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2006\06\22@063334 by Philip Pemberton

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Wouter van Ooijen wrote:
> I would guess this noise originates from the PCs +5V supply, which is
> probably the direct source of the USB +5V. Have you tried
>
> - using the ICD2 without powering the target from the ICD2

I probably should have mentioned that the target has its own power supply.
It's coming from a Farnell Instruments L30B bench PSU. Not the best power
supply in the world, but it gets the job done.

> - a powered USB hub between the PC and the ICD2

I don't have any spare powered USB hubs :(

Thanks.
--
Phil.                         | Kitsune: Acorn RiscPC SA202 64M+6G ViewFinder
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2006\06\22@101346 by Bob Axtell

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Philip Pemberton wrote:
> Wouter van Ooijen wrote:
>  
>> I would guess this noise originates from the PCs +5V supply, which is
>> probably the direct source of the USB +5V. Have you tried
>>
>> - using the ICD2 without powering the target from the ICD2
>>    
>
> I probably should have mentioned that the target has its own power supply.
> It's coming from a Farnell Instruments L30B bench PSU. Not the best power
> supply in the world, but it gets the job done.
>
>  
>> - a powered USB hub between the PC and the ICD2
>>    
>
> I don't have any spare powered USB hubs :(
>
> Thanks.
>  
I think you have the several devices that are used on different branch
circuits, each one having a different "GROUND"
level. Try moving the mains for all devices to the same branch.

Otherwise, my money is on a defective PC 5V power supply feeding the ICD2.

--Bob

2006\06\22@103643 by Peter van Hoof

face picon face
My guess would be you have a groundloop.
If you can run either the computer or the powersupply of the target without ground your probably disappears

Peter van Hoof

{Original Message removed}

2006\06\22@112849 by Philip Pemberton

face picon face
Peter van Hoof wrote:
> My guess would be you have a groundloop.
> If you can run either the computer or the powersupply of the target without ground your probably disappears

I think that might be it, actually...
The power supply floats its output - there's no connection between mains
ground and the 0V output. The scope chassis and the PC PSU, on the other hand,
do seem to ground 0V to mains ground...

I've got the PC plugged into a four-way socket strip, and the scope is plugged
into that via a 5m 2-way socket strip (serving as a double adapter). I
wouldn't have thought you'd get a ground loop if they were all running off the
same mains cable though?

Thanks.
--
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2006\06\22@135450 by Gerhard Fiedler

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Philip Pemberton wrote:

> I wouldn't have thought you'd get a ground loop if they were all running
> off the same mains cable though?

Always when you have two separate ground connections between two devices,
you have a ground loop. Of course, the bigger the loop, the bigger the
chance of it creating problems, so having a big ground loop that goes all
the way to your central mains connection box is worse than having a loop of
a few meters around your bench. But a loop is a loop... if the current can
go in circles, it has a tendency to do that :)

That's probably a case where a certain ground connection resistance is
helpful. Not advisable, nonetheless.

Gerhard

2006\06\23@014323 by Xiaofan Chen

face picon face
On 6/22/06, Philip Pemberton <@spam@philpemKILLspamspamdsl.pipex.com> wrote:
>
> > Have you tried using a serial connection to the ICD instead of USB?  You will need to plug a 9v supply into the ICD though.
>
> I'll try that once I can find a spare 9V power supply with the right type of
> connector on it...
>

I think there is an 9V supply comes with the original ICD2 but maybe you
did not buy it...

There is a related thread in Microchip Forum --> related to ICD2 causing
noise in the circuit and problem with ICD2 causing a big loss of US$1 million
due to the use of ICD2 to support on-site debugging of an Hart device.

forum.microchip.com/tm.aspx?m=125645&mpage=2
http://forum.microchip.com/tm.aspx?m=127223


Regards,
Xiaofan

2006\06\23@154141 by Philip Pemberton

face picon face
Gerhard Fiedler wrote:
> Always when you have two separate ground connections between two devices,
> you have a ground loop. Of course, the bigger the loop, the bigger the
> chance of it creating problems, so having a big ground loop that goes all
> the way to your central mains connection box is worse than having a loop of
> a few meters around your bench. But a loop is a loop... if the current can
> go in circles, it has a tendency to do that :)

Sorted!
I've just installed MPLAB 7.40 on the laptop, set up the ICD2 and the noise is
gone. I can't believe I spent nearly a fortnight trying to figure out where
this noise was getting in, only to find out that it was caused by a ground loop...

Ah well. It's sorted now, and the regulation loop is pretty stable. I'm going
to spend a few minutes doing some efficiency calculations, then see about
tweaking the hardware a bit - the current sense circuitry is dire at best, and
the PIC A/D really needs to be run from an external 2.5V reference instead of
the +5V supply.

Then all I have to do is add support for RS232 control of the SMPSU's
parameters (charge algorithm, current and voltage), and maybe add support for
"burp" charging and charge profile logging. I think it was Dave VanHorn that
mentioned that you can tell cheap NiMHs from decent ones based on charge
profile - I'd like to try that, and maybe upload some pretty graphs somewhere.

Thanks for the help, guys!

Thanks.
--
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