Scope Probe Enhancing Performance of Circuit

[[email protected]]

I have a circuit that acts stangely enough when I connect a ground
connection of a scope probe to it.

The circuit has a TI DSP 280x Series Processor, an Isolated Switching
Power Supply All the AD inputs and GPIO are isolated to the AC Line. In
my evaluation of the performance of the circuit I found that the
systems is able to perform accordingly when the scope probe ground is
connected to the circuit. I don't even need to have the probe connected
to the circuit, just the ground. The probe must be connected to the
scope in order for this to happen. If the this connection is not made
the device sometimes delays to react to the input accordingly. I'm
using a portable scope (THS720 Tektronix with isolated ground
connections). The behavior also happens with benchtop scopes that are
not isolated.

I have all my unsuded GPIO set as inputs and with internal pullups
enabled to avoid any parasitic effects.

This is a multi-layered board with Separated Power Planes (3.3V and
1.8V) and a single ground plane.

Any suggestions on where to start?
Thanks for all your help on this in advance.

gt5513e

 

[The Phantom]

I have a circuit that acts stangely enough when I connect a ground
connection of a scope probe to it.

The circuit has a TI DSP 280x Series Processor, an Isolated Switching
Power Supply All the AD inputs and GPIO are isolated to the AC Line. In
my evaluation of the performance of the circuit I found that the
systems is able to perform accordingly when the scope probe ground is
connected to the circuit.

I had something like this happen once. We had to ship scope probes with
the product.

 

[Jim Thompson]

I had something like this happen once. We had to ship scope probes with
the product.

[snip]

I had a circuit that would only work when my middle finger was
touching a particular point.

So I bridged my finger and added that much capacitance ;-)

...Jim Thompson

 

[James Beck]

I had something like this happen once. We had to ship scope probes with
the product.

[snip]

I had a circuit that would only work when my middle finger was
touching a particular point.

So I bridged my finger and added that much capacitance ;-)

...Jim Thompson

How many pF is your middle finger?

 

[James T. White]

I had something like this happen once. We had to ship scope probes
with the product.

gt5513e's problem sounds even more expensive. He will have to ship a
Tektronix THS720 portable scope with each unit;-)

 

[Jim Thompson]

On 10 Feb 2006 06:55:31 -0800, [email protected] wrote:

I have a circuit that acts stangely enough when I connect a ground
connection of a scope probe to it.

The circuit has a TI DSP 280x Series Processor, an Isolated Switching
Power Supply All the AD inputs and GPIO are isolated to the AC Line. In
my evaluation of the performance of the circuit I found that the
systems is able to perform accordingly when the scope probe ground is
connected to the circuit.

I had something like this happen once. We had to ship scope probes with
the product.

[snip]

I had a circuit that would only work when my middle finger was
touching a particular point.

So I bridged my finger and added that much capacitance ;-)

...Jim Thompson

How many pF is your middle finger?

Don't remember now... 40+ years ago, but IIRC just a few pF.

...Jim Thompson

 

[John Fields]

I had a circuit that would only work when my middle finger was
touching a particular point.

So I bridged my finger and added that much capacitance ;-)

 

[[email protected]]

I did consider this option, but only as a quick fix for the first
million boards. Any changes will be for rev 2

 

[Tim Wescott]

I have a circuit that acts stangely enough when I connect a ground
connection of a scope probe to it.

The circuit has a TI DSP 280x Series Processor, an Isolated Switching
Power Supply All the AD inputs and GPIO are isolated to the AC Line. In
my evaluation of the performance of the circuit I found that the
systems is able to perform accordingly when the scope probe ground is
connected to the circuit. I don't even need to have the probe connected
to the circuit, just the ground. The probe must be connected to the
scope in order for this to happen. If the this connection is not made
the device sometimes delays to react to the input accordingly. I'm
using a portable scope (THS720 Tektronix with isolated ground
connections). The behavior also happens with benchtop scopes that are
not isolated.

I have all my unsuded GPIO set as inputs and with internal pullups
enabled to avoid any parasitic effects.

This is a multi-layered board with Separated Power Planes (3.3V and
1.8V) and a single ground plane.

Any suggestions on where to start?
Thanks for all your help on this in advance.

gt5513e

If you don't need a scope attached to the probe perhaps you should just
ship each unit with a probe attached .

 

[[email protected]]

I do need the scope! I can fit the probe in the design budget but the
whole scope, that's a different story!

 

[Keith Williams]

I threatened to do this once. The circuit (an ECL logic circuit,
at that) only worked with a scope probe attached.

I had a circuit that would only work when my middle finger was
touching a particular point.
Yep.

So I bridged my finger and added that much capacitance ;-)

Tried that (adding the scope capacitance); no joy. Turned out that
the wire to the pin was broken. The scope probe added enough
weight that it made contact. I chased that one for a while!

 

[Keith Williams]

No, that's unaccounted for "parasitics". ;-)

Sometimes you just gotta drain the swamp.

 

[Michael A. Terrell]

What more do you expect? After all, it is J.T.

--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida

 

[John Fields]

No, that's unaccounted for "parasitics". ;-)

Sometimes you just gotta drain the swamp.

 

[David Harmon]

On Fri, 10 Feb 2006 09:08:05 -0700 in sci.electronics.design, Jim

I had a circuit that would only work when my middle finger was
touching a particular point.

So I bridged my finger and added that much capacitance ;-)

Alternately, a drop of super glue between the part and your finger
would have done.

 

[amdx]

On 10 Feb 2006 06:55:31 -0800, [email protected] wrote:

I have a circuit that acts stangely enough when I connect a ground
connection of a scope probe to it.

The circuit has a TI DSP 280x Series Processor, an Isolated Switching
Power Supply All the AD inputs and GPIO are isolated to the AC Line. In
my evaluation of the performance of the circuit I found that the
systems is able to perform accordingly when the scope probe ground is
connected to the circuit.

I had something like this happen once. We had to ship scope probes with
the product.

[snip]

I had a circuit that would only work when my middle finger was
touching a particular point.

So I bridged my finger and added that much capacitance ;-)

...Jim Thompson

How many pF is your middle finger?

Don't remember now... 40+ years ago, but IIRC just a few pF.

...Jim Thompson

I once got a job at a consumer electronics repair shop because
of scope probe capacitance!
I walked in to put in an application, showed my resume, and he said he
had just hired someone. He saw I had some Sony experience and said
he had a projection TV that no one had been able to fix, he ask if I'd be
interested in looking at it. The projection TV had no raster, no output
from the tubes. I sat it up and started probing around and after
about ten minutes he looked over and said "hey you got it going". I had
hung the probe on part of the high voltage osc. circuit and it fired up.
I was on the backside and hadn't noticed light was coming out of the tubes.
Turns out someone previously had replaced a cap. with one only 1/10
of the correct value. The scope probe added enough capacitance to
keep it running. So he started thowing repair work my way.
Mike

 

[Jim Thompson]

On Fri, 10 Feb 2006 09:08:05 -0700 in sci.electronics.design, Jim


Alternately, a drop of super glue between the part and your finger
would have done.

Reminds of a true story... over in Sun City, our local retirement
community, a gal suspected hubby of infidelity.

While he slept she super-glued his "privates" to his leg. When he
awoke, in need of a pit stop, there was sheer panic... made the
newspapers because he called 911 ;-)

...Jim Thompson

 

[Fred Bartoli]

I did consider this option, but only as a quick fix for the first
million boards. Any changes will be for rev 2

I'll fix it for free, then you give me half the scope price for each of the
first million boards.

 

[Ancient_Hacker]

It happens.

A while ago I hung an old Heathkit signal tracer onto a test point in
a fancy Fluke signal generator. The test point was in a PLL lock
circuit.

Turns out the PLL would lock only with the signal tracer probe on the
test point. Very weird.

After a little investigation it turned out the signal tracer had a
leaky input capacitor. Substituting a resistance decade box in place
of the probe showed the circuit needed between 11 and 12 megohms on
that point to achieve lock.

The capacitor leakage: 11.6 Megohms. How cool is that?


But bvack to your problem-- first I'd put the scope PROBE on the
circuit board ground, and leave the scope ground lead dangling.

Apparently there's a fair amount of hash on that, if just hanging a
piece of metal onto it makes a difference.

 

[Allan Herriman]

It happens.

A while ago I hung an old Heathkit signal tracer onto a test point in
a fancy Fluke signal generator. The test point was in a PLL lock
circuit.

Turns out the PLL would lock only with the signal tracer probe on the
test point. Very weird.

After a little investigation it turned out the signal tracer had a
leaky input capacitor. Substituting a resistance decade box in place
of the probe showed the circuit needed between 11 and 12 megohms on
that point to achieve lock.

The capacitor leakage: 11.6 Megohms. How cool is that?


But bvack to your problem-- first I'd put the scope PROBE on the
circuit board ground, and leave the scope ground lead dangling.

Apparently there's a fair amount of hash on that, if just hanging a
piece of metal onto it makes a difference.

I've seen PLL designs that would work better with the application of a
finger. From memory, one would reduce its phase noise by about 10dB
when touched. It turned out that the leakage current would cause a
steady state phase shift that moved the phase detector away from its
dead zone.

I hate the 4046.

Allan