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Measuring from different ground points and getting different voltages?
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nms
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4th June 2012
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Measuring from different ground points and getting different voltages?

I'm calibrating my analog synth and when I connect the probe to the ground point at the PSU I get a different measurement than I do using the ground located on the main board with the test point ring. I heard that some ground points further downstream carry a small amount of current that's leaked over which causes this.

When I connected one probe to the psu gnd and one probe to the other one on the board I measured 32mv which was the exact difference I was seeing when I used one ground or the other. So this tells me that ground point on the board is carring 32mv of current. I heard that if there's current on the ground line it will cause any readings made with that ground to be lower, due to the resistance caused by said current.

Have I got this right so far?

So then the question is, for all my trimmer adjustments on the main board which gnd should I use for my measurements? Go with the clean ground right at the source in the PSU? Or should I be using the test point gnd with the loop on the board (despite it carrying 32mv) where all the trimmers and components I'm testing live?

Which gnd should be my reference?
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Are you measuring a final output voltage or some internal node?

Ground is not a voltage but a concept so the either use the ground on the output jack, that the output signal should be referenced to.

Internal voltage nodes should probably be measured wrt to local ground near that node.

JR
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I'm measuring the voltage at one of the component test points where I need to adjust a trimmer. But this behavior holds true for any test point measurements on this thing.

If I use the gnd located on the psu my reading will be 32mv higher than if using the main board's test point gnd location which is located after a few caps. The more caps the gnd passes through the more current the gnd accumulates. I heard this was from slight leakage?

I'm starting to think I should use the PSU gnd when adjusting the psu voltage.. but use the main board's test point gnd when measuring and adjusting trimmers located on the main board.
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Yes, but you don't sound convinced.

Perhaps listen to what you are measuring. I suspect the 32mV you measure between the grounds is nasty hum, if you use the PS ground to make audio board measures the tests points probably contain the sum of the audio signal and the hum due to the ground difference.

JR
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Have you tried (with power off) measuring the resistance between the test point and the power supply? Also, have you tried reducing the difference by making sure the ground connection to the main board is clean, tight and otherwise as low resistance as possible?
Best,
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Hi
Unless I have missed it, you haven't said whether you are measuring AC or DC.
I would tend to go with John's suggestion and measure your test points relative to a ground that is nearby (on the same circuit board). It also depends what voltage levels you are trying to measure, 32mV is not much if the test point is looking at 5 or 10 Volts.
If it is really crucial, the manual would suggest where you are measuring relative to although you may discern this by looking at the schematics.
Does your testmeter measure 'zero' if you touch both probes to the SAME point? This may seem a 'silly' excercise but it can be instructive. You can 'fool' digital multimeters.
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Hey guys thanks for the posts.

I'm measuring DC voltages. I turned it off and measured for current between gnd points and it disappeared giving a zero reading. The current is definitely there by design. The connections are good but the gnd trace runs alongside the 5v rail and after each filter cap the curren on the gnd is larger. I just recapped the whole synth and there was current at that gnd location before & after. My PSU gnd measures not even 1mv of current.

First step in calibration is setting the PSU output to 15v +/-10mv. I'd think using the gnd reference given by the PSU gnd tab would be good to use for that measurement. Otherwise I'd be setting it to output 15.030 in order to get a 15.000 reading at the main board GND TP.

Next step is adjusting the bias trimmer in the middle of the board to 7.700v but I'd think for all board measurements I should use the local GND TP and not pay attention to the 32mv current on that gnd. I think?
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Most analog synths have separate digital ground and analog ground. This concept isolates switching noise from the digital side (esp from CPUs) from leaking into the audio side. There will be some difference between these two grounds as you proprogate away from the power supply.

When in doubt - use the local ground not power supply ground.

And 32mV difference does not necessarily equate to 32mA of current, it is a bit more involved than that.
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Hi
It sounds as if your 'ground' is a bit 'resistive, or at least more than one might hope.
No matter, do as you say and measure the supply 15 Volts reference to it's local ground and the the others against the ground on the board near each test point.
The 7.700 Volts will need to be measured against local ground on that board as it is most likely that whatever it is feeding is taking it's 'ground' reference from there and not from the supply.
It would be great if your ground traces had zero resistance so would all be the same irrespective of any currents flowing, but this is of course impossible unless you are at -273 Centigrade in which case your problems will be far worse!
You could 'beef up' the grounds but it may actually introduce other problems so unless you want a serious project it is better left alone.
The filter caps will not be drawing current themselves (unless you have them backwards in which case they will explode soon) so it is simply the current that is being taken by the other parts that give this 'drop'. You should see a similar effect on the other power rails as you measure away from the supply board. If the resistance of the supply track is similar to the ground track, you would see similar voltage loss measuring across it, meaning that at an extreme you may see 2 X 32mV (64mV) drop.
You may now appreciate the rudiments of balanced audio and why chassis and analogue grounds are not necessarily the same thing.
Matt S
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Superb. Thanks for taking the time guys. Couldn't have asked for a more informative explanation. It'd be nice not to have any current on that gnd line but 32mv isn't bad I guess. The important thing is just knowing which gnd to use for reference during measurements.
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why are you talking about currents? Only knowing the actual resistance of the ground path (dvm says 0, right) could tell us the current flowing from a 32mv voltage source.
Or am i misunderstanding something here?
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oh, btw, it might be a good idea to check where the device to be tuned is taking it´s reference from. As Matt pointed out, it is propably nearby. But something rings in my head with synth shematics where vcos had their own connections and supply, sperated from other circuitry. So it might be you psu point after all. Just an option to consider and propably not the case.
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oops, haha.. you're right that was a misuse of "current". I meant to reference the presence of voltage on that line.

Anyway there are 2 gnd points mentioned here. One on PSU and one upstream on the main board. If you put one probe on each gnd you get a measurement of 32mv. The gnd on the board is carrying 32mv while the one at PSU is a true zero voltage gnd point. When measuring voltages around the synth your readings will be 32mv lower if using the gnd on the board vs the one at the PSU. The 32mv that has accumulated on the board's gnd point causes resistance when using that gnd point for measurements.
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They are all true grounds, or else they would label them something else. It is simple physics that grounds will experience voltage differences from even modest distances.

It is worthwhile figuring out the intent of the design and set up procedure.

JR
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Here's what I had read:

"Currents in grounding wire

Ground wires should not carry current except during faults. If the ground wire carries any current there will be a potential difference between different grounding points because the current flowing in wire causes voltage drop due to wire resistance. This is why a common wire which works as neutral and grounding wire is very bad thing.

When there is separate wiring for grounding you can't still completely avoid the current flowing in grounding wires! There will always be some capacitive leakage current form the live wire to the ground wire. This capacitive leakage current is caused by the fact that the wiring, transformers and interference filters all have some capacitance between the ground and live wire. The amount of current is limited to be quite low (limited to be between 0.6 mA to 10 mA depending on equipment type) so it does not cause dangers and big problems. Because of this leakage current there is always some current flowing in the ground wire and the ground potentials of different electrical power outlets are never equal. "
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Quote:
Originally Posted by nms View Post
Here's what I had read:

"Currents in grounding wire

Ground wires should not carry current except during faults. If the ground wire carries any current there will be a potential difference between different grounding points because the current flowing in wire causes voltage drop due to wire resistance. This is why a common wire which works as neutral and grounding wire is very bad thing.

When there is separate wiring for grounding you can't still completely avoid the current flowing in grounding wires! There will always be some capacitive leakage current form the live wire to the ground wire. This capacitive leakage current is caused by the fact that the wiring, transformers and interference filters all have some capacitance between the ground and live wire. The amount of current is limited to be quite low (limited to be between 0.6 mA to 10 mA depending on equipment type) so it does not cause dangers and big problems. Because of this leakage current there is always some current flowing in the ground wire and the ground potentials of different electrical power outlets are never equal. "
HUH? Safety ground wires in outlets?

For the record there can also be current/voltage induced into ground runs (loops) due to magnetic fields, but i don't want to make this more confusing.

There can be multiple different local Ground voltages, all are correct in the proper context.

JR
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Quote:
Originally Posted by nms View Post
Here's what I had read:

"Currents in grounding wire

Ground wires should not carry current except during faults. If the ground wire carries any current there will be a potential difference between different grounding points because the current flowing in wire causes voltage drop due to wire resistance. This is why a common wire which works as neutral and grounding wire is very bad thing.

When there is separate wiring for grounding you can't still completely avoid the current flowing in grounding wires! There will always be some capacitive leakage current form the live wire to the ground wire. This capacitive leakage current is caused by the fact that the wiring, transformers and interference filters all have some capacitance between the ground and live wire. The amount of current is limited to be quite low (limited to be between 0.6 mA to 10 mA depending on equipment type) so it does not cause dangers and big problems. Because of this leakage current there is always some current flowing in the ground wire and the ground potentials of different electrical power outlets are never equal. "
That relates to RESIDENTIAL AND COMMERCIAL WIRING (read: homes and businesses) and is totally unrelated to audio device wiring such as analog synths. "Ground wires should not carry current except during faults" A ground buss in an analog synth carries return current BY DESIGN.

Power distribution in analog synths and residential/commercial dwellings are two VERY different things.
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Quote:
Originally Posted by JohnRoberts View Post
There can be multiple different local Ground voltages, all are correct in the proper context.
JR
Yeah that definitely makes sense. I'm going to set the PSU voltage using the gnd on the PSU then use the board gnd witht he TP ring on it for all board measurements. Thanks gents.
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