This makes no sense!

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I'm loosing my mind. I've built a simple ph probe amp board that uses a LMC6001 to slightly amp and buffer the ph probe signal and then send it to a LMC6041 operating as a differential amp with bias to center the +/-0.65V signal centered at 0V to a +/-2.25V signal centered at 2.5V.

Both sections where bread boarded and tested before the PCM was built and both section worked exactly as needed.

This is all background for he simple fact that I have a couple of resistors that are NOT behaving! After having a problem with the final stage not working correctly (output way higher than it should be), I pulled out the LMC6041 and breadboarded the second stage once more to test it again. When that worked exactly as it should have, I decided to pull the opamp out of the bread board so I could ohm out a few points on the breadboard and compare them with the same points on the PCB.

And that's when it got weird. Here is the section of the schematic and the PCB image I am fighting with along with three test points marked on both:

schematic pic.png 7.2 Kb

Section with resistors.png 34.7 Kb

At this point in the testing, I am only testing from A to B, from B to C and from A to C. Everything other than those two resistors (60.4k and 187k) have been removed (specifically, opamp, 100 ohm output resistor and trim pot), so I literally have a simple string of two resistors to test! And this is the weirdness...

From test point A to TP B, I measure the expected 60.4k ohms.
From TP B to TP C, I measure the expected 187k ohms.
But from TP A to TP C (through both resistors in series!), I measure the exact same 187k ohms as before!

I have removed both resistors to verify that there are no shorts between any of the three PCB traces in question (none found visually or with bench meter). I have verified the ohms of each resistor and even replaced the (cheap and I buy them 200 at a time...) and STILL the measurement from TP A to TP C is 187k instead of the expected 247k ohms. And yes, the breadboard DOES measure in the 250k ohms as expected!

I've been staring at this for several hours over the last couple of days. Does anyone else see anything that stands out to them? I'm almost at the point where I am going to make a new PCB and start over. This just doesn't make any sense to me at all.

Help!

 

Clint

Last Edited: Wed. Oct 6, 2010 - 05:33 AM
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Oh, and does anyone know how to link images correctly??? I'm doing it wrong of course, but the links are there.

 

Clint

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Is the pcb double sided?
Is the pcb powered when you are making the measurements?
You could try swapping the resistors and measuring again.
Is your multimeter working ok? Have you tried another meter?
No short circut under the Resistor R6 or R6 is not shorting?

good luck

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Some invisible open-circuits (partial-opens) will repair themselves when you apply an ever-so-slight amount of pressure near them, like touching probes to surfaces. You may want to tin the traces in question (with solder, no electrolysis or chemical bath garbage).

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I had an open solder on one of the RS232 lines of a SMD Mega88 once...
I'd check it with multimeter, pushing it down and making contact...

I wasted far too much time trying to figure out why the software wasn't working... lol

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Quote:
and does anyone know how to link images correctly???

Your circuit, and math, look fine. Did you make the PCB yourself, or was it made professionally? Agree with the above, micro cracks and micro shorts can be hard to identify.

To include an image, (I use JPGs, I've never tried including a PNG), write your message and hit the Preview button instead of the Submit button. You will then see an additional box with a Browse button to locate the desired file on your PC. Double click the desired file, then click the Add Attachment, (I forget, I think it says that, it will be apparent to you at the time), button and it will be uploaded and included.

JC

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Are you sure the 60.4K is not 60.4 OHM by accident, you can easily misread the auto-scale, and in series with the 187K it will make hardly any difference in reading...

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Xantor, I could kiss you!!!! You where right on the money! Somehow, I managed to order up 60.4 ohm resistors instead of 60.4 kohm resistors! Needless to say, as I was going over it, kept mentally adding the "k" when ever I looked at it. And just as you surmised, I was ASSUMING the range was kohm instead of the actual ohm scale.

Thank you VERY much! This was driving me nuts. Of course, now I'm a bit mad at myself for ordering the wrong resistors in the first place AND for overlooking the simplest, most obvious solution (at least, obvious once pointed out!).

Everyone, thanks for the suggestions and the help with attaching images as well. Hopefully, the NEXT time I need help, I will be able present a better looking post.

 

Clint

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I went and populated the board with the hardware I previously removed and swapped out the 60.4 ohm resistor to a closer 63.8K ohm resistor. It's a bit too fat to properly center the amp with the trim pot, but it DOES prove that everything is working as expected with the amp.

Thanks again!

 

Clint

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Clint,

Your original .png schematic shows the resistor as 60.4 ohms. Maybe that is how the whole "comedy of errors" started.

Cheers,

Ross

Ross McKenzie ValuSoft Melbourne Australia

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I've been trying to get started in a project to make a pH meter for my aquarium for the longest time.

I know so little about op amps so I'm curious why the LMC6001? From the info I've found on it its really expensive.

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valusoft wrote:
Clint,

Your original .png schematic shows the resistor as 60.4 ohms. Maybe that is how the whole "comedy of errors" started.

Cheers,

Ross

Ross! LOL! Yup! I made an Excel sheet to calculate the values, and when I stuffed them into schematic, it appears I dropped a "k"!

For some reason, that makes me feel much better!

 

Clint

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emmannuel wrote:
I've been trying to get started in a project to make a pH meter for my aquarium for the longest time.

I know so little about op amps so I'm curious why the LMC6001? From the info I've found on it its really expensive.

Emmannuel,

Yes, it is expensive. And yes, it was picked for specific characteristics. Mainly, it has incredibly low input currents of 25 fA. That is "f" which stands for 1E-15 amps! Not quite a perfect opamp (input current would be 0), but REAL good.

The whole reason this is important is because of the incredibly high resistance of a pH probe. It can get up into the 1E9 ohm range. With this very high resistance, the input current of the opamp can pull down the voltage output from the probe enough to really throw off the reading if you are shooting to maximize the accuracy of the probe.

If you are only wanting something accurate to the xx.x point, you probably don't need to go quite so far in getting an opamp with such low input current. I tend to want to push things, so went to the extreme...

There are a lot of other good opamps that have low input current that don't cost so much.

Oh, one other thing: if you BUY them, they are INSANE. On the other hand, National will sample them for a shipping and handling fee of $25. And you can get 5 samples for the same shipping and handling fee. I could live with $5 opamps...

 

Clint

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Emmanuel,

Google electrometer

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Thanks for your reply Clint.

I had gone through and read the datasheet and it does seem like its a very impressive opamp.

Will it give you xx.xx accuracy or higher?

Thanks ig,

I looked up electrometers but I wasn't sure about why it would be helpful. Would an electrometer circuit be similar to what I am looking for? I've only seen those used for measuring charge.

- Emmanuel

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in order to measure charge the circuit must have very low input current.

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Ah I see thanks ig.

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emmannuel wrote:
Thanks for your reply Clint.

I had gone through and read the datasheet and it does seem like its a very impressive opamp.

Will it give you xx.xx accuracy or higher?

The opamp should be capable of xx.xxx accuracy. But a LOT will depend on the quality of the rest of the circuit and how well everything is calibrated. xx.xx was more than good enough for me, so I haven't spent a whole time digging into all the little details that are required to pull out that last order of magnitude of performance.

 

Clint