Author Topic: Microcurrent gold - Reprise  (Read 4066 times)

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Offline MoriambarTopic starter

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Microcurrent gold - Reprise
« on: May 16, 2021, 01:08:16 pm »
TIFU (well yesterday but it's the same) shorting VCC to gnd through my uCurrent Gold albeit for a couple of seconds.
Although I set a limit of 20mA on the psu, it seems that I have damaged the thing.
Since I have no skill in diagnosing and repairing such things (and have a limited supply of tools), I thought this could be a good "project" but I need your guidance.
Here's a recap:
Symptoms
After doing the wrong thing, the microcurrent (it was in the milliamp range) ALWAYS shows up negative values upon powerup with the PSU disconnected.
Nothing appears blown or damaged upon visual inspection. The LED still turns on when switching on.
I tried a couple of measurements and they seem kinda off, but I don't know how to determine whether the microcurrent is really cooked or if it's fine.
First of all it gives very low readings without taking the delta for the negative starting offset. Even after doing the delta It is off wrt my EEVBlog BM235 and Keithley DM6500 of 2-3mA over a range from 5 to 20mA. Both the other meters show the same value more or less.
The same applies for the microcurrent range where the uCurrent shows values of ~100uA less than the other multimeters (around 350 uA vs 470uA). The measurements have been done using a npn current sink roughly programmed.
Anyhow the microcurrent values are always wobbly and never stable.
They were not this way before.
Questions
Can this be my paranoia and the discrepancies justified by burden voltage of the meters? I probed with either meter and the micro current separately.
What can I do to see whether there's something wrong?
Can you help/guide me step by step plz?
Tools I have
  • DM6500
  • BM235
  • Scope->SDS1104X-e (iirc, it's the 100MHz 4 Channel Siglent)
  • Linear siglent PSU (I don't recall the model unfortunately)
  • DER EE LCR meter
  • Hakko FX888D soldering iron with some tips
I think other stuff don't matter, but notably i do not have a hot air gun/station.

Cheers
« Last Edit: June 03, 2021, 06:59:31 pm by Moriambar »
 

Offline LA7SJA

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Re: Microcurrent gold
« Reply #1 on: May 16, 2021, 05:04:03 pm »
The schematic: https://www.eevblog.com/files/uCurrentRev5schematic.pdf

If you short the input pin, what do you measure on the output in the different  ranges?
I do remember a problem with one batch of the ground-splitting op-amp oscillating with some multimeter.
Can you measure the AC component (use the scope, Luke) on the negativ output with reference to the negative pole on the battery when using one of the "BAD" reading multimeter?
You should read this tread: https://www.eevblog.com/forum/testgear/questions-regarding-ucurrents-rail-splitter-subcircuit/msg2756182/#msg2756182 and this https://www.eevblog.com/forum/blog/eevblog-1057-current-murphy/?all

Johan-Fredrik
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Offline MoriambarTopic starter

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Re: Microcurrent gold
« Reply #2 on: May 16, 2021, 07:00:34 pm »
The schematic: https://www.eevblog.com/files/uCurrentRev5schematic.pdf

If you short the input pin, what do you measure on the output in the different  ranges?
I do remember a problem with one batch of the ground-splitting op-amp oscillating with some multimeter.
Can you measure the AC component (use the scope, Luke) on the negativ output with reference to the negative pole on the battery when using one of the "BAD" reading multimeter?
You should read this tread: https://www.eevblog.com/forum/testgear/questions-regarding-ucurrents-rail-splitter-subcircuit/msg2756182/#msg2756182 and this https://www.eevblog.com/forum/blog/eevblog-1057-current-murphy/?all

Johan-Fredrik

Hello and thanks for your reply.
I'm very aware of the issue but I wasn't effected either because I got it way before or way after (couldn't remember) the incident.
Anyhow:
Quote
If you short the input pin, what do you measure on the output in the different  ranges?
  • mA=>-0.21
  • uA=>-0.7
  • nA=>-4

and also
Quote
Can you measure the AC component (use the scope, Luke) on the negativ output with reference to the negative pole on the battery when using one of the "BAD" reading multimeter?
I get a high frequency noise basically, ~100mVpp

Thanks
 

Offline LA7SJA

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Re: Microcurrent gold
« Reply #3 on: May 16, 2021, 08:12:51 pm »
Hi "Moriambar"

Is this noise present if you remove the multimeter?
Do you have a picture of the scope image with and without the multimeter?
Any way you could try to remove the rail splitting op-amp and use a split powersupply (2 x 2V or two 1.5V battery)?
I think it is the LMV321 thats broken.

Johan-Fredrik
« Last Edit: May 16, 2021, 08:14:42 pm by LA7SJA »
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Offline MoriambarTopic starter

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Re: Microcurrent gold
« Reply #4 on: May 17, 2021, 07:50:51 am »
Hi "Moriambar"

Is this noise present if you remove the multimeter?
Do you have a picture of the scope image with and without the multimeter?
Any way you could try to remove the rail splitting op-amp and use a split powersupply (2 x 2V or two 1.5V battery)?
I think it is the LMV321 thats broken.

Johan-Fredrik
Hello, so here are the two waveforms:
with multimeter attached
1219943-0

without multimeter attached


I used the BM235 btw.

I can try to remove the splitting opamp later tomorrow I think, and I can attach it to a split power supply.

Thanks for now.
Just to learn: what makes you think it's the LMV321 that's gone bad?

Cheers
 

Offline LA7SJA

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Re: Microcurrent gold
« Reply #5 on: May 17, 2021, 08:32:02 am »

Just to learn: what makes you think it's the LMV321 that's gone bad?


The LMV321 is responsible for making a voltage division ground reference with a resistive voltage division of the battery voltage and the operational amplifier (configured as a 1: 1 buffer) to stabilize the artificial ground point.
Pin 1 on the LMV321 (U2) is VBat/2 and the op-amp is configured as X1 buffer (U2 pin 4 connected directly to U2 pin 3).
The LMV321 is the only active part in the ground circuit (the only component that can oscillate)

Johan-Fredrik
"If at first you don't succeed, skydiving is probably not for you"
 
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Offline MoriambarTopic starter

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Re: Microcurrent gold
« Reply #6 on: May 17, 2021, 08:27:51 pm »

Any way you could try to remove the rail splitting op-amp and use a split powersupply (2 x 2V or two 1.5V battery)?
I think it is the LMV321 thats broken.

Desoldered. Noise has not any ringing shape anymore and it does not change with or without the meter attached. Scopes measures about 20mVpp of noise.
The shorted readings are off though, displaying around -110 to -130 mV on the multimeter, with the selector on short. It is kind of consistent regardless of the range although it seems a bit bigger in absolute value on the mA.

So, should I buy a new LMV321?

Thanks for your help
 

Offline LA7SJA

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Re: Microcurrent gold
« Reply #7 on: May 17, 2021, 09:10:18 pm »
Hi!
Is the output a clean dc (check with a scope)?
What is your powersource, batteries or splitt power?
How is the power feed to the cirquit?
Is the ofset only present on pin 1 of U4 or on both U1 and U4?
Don't buy the LMV321 for now, you may need more parts.
What do you measure from GND black output pin (J4) to pin 2 and 5 on one of the op-amps?

Johan-Fredrik
"If at first you don't succeed, skydiving is probably not for you"
 
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Offline MoriambarTopic starter

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Re: Microcurrent gold
« Reply #8 on: May 17, 2021, 09:26:39 pm »
Hi!
Is the output a clean dc (check with a scope)?
What is your powersource, batteries or splitt power?
How is the power feed to the cirquit?


Johan-Fredrik

So, I lied… the noise is more in the 200mVpp ballpark now. I installed the new firmware on the scope today and basically all probes were set to 1x.
I attached the waveform with dc coupling across the output terminals.
The circuit is powered via my SPD3303X set in serial mod. The most negative terminal is set to the minus of the battery, the most positive to the plus and one of the other two (midpoint) are directly connected to the output - post.

Quote
Is the ofset only present on pin 1 of U4 or on both U1 and U4?
Don't buy the LMV321 for now, you may need more parts.
What do you measure from GND black output pin (J4) to pin 2 and 5 on one of the op-amps?
I will have to check this tomorrow
thanks for now
 

Offline LA7SJA

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Re: Microcurrent gold
« Reply #9 on: May 17, 2021, 09:41:13 pm »
Hi Moriambar

You did remove the coin cell? didn't you?
How was things connected and what voltages was aplied when the acident happend?

Johan-Fredrik
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Offline fzabkar

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Re: Microcurrent gold
« Reply #10 on: May 18, 2021, 05:49:00 am »
FWIW, the MAX4239AUT amps are configured with a gain of 10. The datasheet specifies a Minimum Stable Closed-Loop
Gain of 10V/V. This means that the amp is operating right at the limit of its spec.

https://datasheets.maximintegrated.com/en/ds/MAX4238-MAX4239.pdf

Quote
Unlike conventional operational amplifiers, the MAX4238/MAX4239 have a maximum gain specification. To maintain stability, set the gain of the MAX4238 between AV = 1000V/V to 1V/V, and set the gain of the MAX4239 between AV = 6700V/V and 10V/V.

I suspect that this poses no problem, but I thought I'd just make the observation.

It might be interesting to replace one of the amps with a MAX4238.
« Last Edit: May 18, 2021, 05:50:55 am by fzabkar »
 

Offline MoriambarTopic starter

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Re: Microcurrent gold
« Reply #11 on: May 18, 2021, 06:37:43 am »
Hi Moriambar

You did remove the coin cell? didn't you?
How was things connected and what voltages was aplied when the acident happend?

Johan-Fredrik

Yes I removed the battery, but thanks for asking.
Well when the accident happened the input+ terminal was connected to VCC and input - directly to GND.
I don't remember how much voltage was supposed to be there tbh (I was doing too many things at once) but definitely between 5 and 9V, but the PSU should've been CC limited to 20 or 40mA of current (again: too many things at the same time and I got distracted).

Later today/tomorrow (depending on your timezone too) I'll do the measurements you asked.

Thanks!
 

Offline MoriambarTopic starter

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Re: Microcurrent gold
« Reply #12 on: May 18, 2021, 06:42:17 am »
FWIW, the MAX4239AUT amps are configured with a gain of 10. The datasheet specifies a Minimum Stable Closed-Loop
Gain of 10V/V. This means that the amp is operating right at the limit of its spec.

https://datasheets.maximintegrated.com/en/ds/MAX4238-MAX4239.pdf

Quote
Unlike conventional operational amplifiers, the MAX4238/MAX4239 have a maximum gain specification. To maintain stability, set the gain of the MAX4238 between AV = 1000V/V to 1V/V, and set the gain of the MAX4239 between AV = 6700V/V and 10V/V.

I suspect that this poses no problem, but I thought I'd just make the observation.

It might be interesting to replace one of the amps with a MAX4238.
Hello and thanks for your interest.

Unfortunately I have no available MAX4238.
 

Online Kleinstein

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Re: Microcurrent gold
« Reply #13 on: May 18, 2021, 08:33:19 am »
The max4239 should be OK with a gain of 10. It is chosen because of higher bandwidth. For just use with a DMM and DC the max4238 would be OK too, but for the use with a scope the higher BW is very nice.

Oscillation of the virtual ground is a known issue with some OPs used in the circuit - there are many OPs similar to the LMV321 and some are more prone to ocillation than others. So one batch of the µCurrent got OPs prone to oscillation.

The linked schematics (https://www.eevblog.com/files/uCurrentRev5schematic.pdf) is already for the updated circuit that fixed the oscillation problem of the LMV321 by adding R10.  The older version may not have R10 there. Ideally one would add an extra cap from the batery neg side to the virtual ground - but only with R10 in place.

At the other end R8 at the output is inside the loop and making things harder for the signal output and one may get oscillation there. Shorting out R8 (for the  may actually help.
 

Offline MoriambarTopic starter

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Re: Microcurrent gold
« Reply #14 on: May 18, 2021, 11:05:05 am »
Hello!
Here are the measurements with the device on short and split psu input

Is the ofset only present on pin 1 of U4 or on both U1 and U4?
U1 has an offset of around +2mV, while U4 displays -30mV
Quote
What do you measure from GND black output pin (J4) to pin 2 and 5 on one of the op-amps?
-1.997V and +1.995V respectively.

Thanks
M.
 

Offline LA7SJA

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Re: Microcurrent gold
« Reply #15 on: May 18, 2021, 04:49:46 pm »
Hi M.

If you lift pin 3 U4 and bridge it to GND, what du you measure on the output?

Johan-Fredrik
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Offline MoriambarTopic starter

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Re: Microcurrent gold
« Reply #16 on: May 18, 2021, 05:45:02 pm »
Hi M.

If you lift pin 3 U4 and bridge it to GND, what du you measure on the output?

Johan-Fredrik
I think the task of lifting one pin of such a tiny ic are well beyond my soldering capabilities.
Perhaps I'll give it a try but I smell failure all the way
 

Offline LA7SJA

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Re: Microcurrent gold
« Reply #17 on: May 18, 2021, 06:45:30 pm »
Hi Moriambar don't give up!
I just tryed on a scrap pcb with an rather large soldering iron and a needle, it went great  ;D
I agree I think U1,U2 and U4 are bad and maybe R1 and or R9, check if the green LED goes out when the voltage is lower than 2.5V. Maybe you can ask Dave to qoute you the spare parts.

Johan-Fredrik
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Online Kleinstein

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Re: Microcurrent gold
« Reply #18 on: May 18, 2021, 07:32:10 pm »
A first point would be to check if the oscillation comes from the max4239 or the virtual ground. The measured frequency is near 12 kHz which is about where the AZ frequency of the max4239 is. There is a kind of beat frequency visible, indicating 2 near by frequencies.

An oscillating OP usually oscillates at 1 frequency and often more close (still below) to the upper loop bandwidth. So with the LMV321 as a follower this would be more like a little (e.g maybe a factor 2 or 3) below 1 MHz. 12 kHz would be rather slow for a LM321 with unity gain.

This kind of points to seeing the AZ frequencies of the 2 max4239 and not the virtual ground oscillating. There may still be some contribution of the virtual ground increasing the  amplitude.

A first point to check would be the µCurrent revision - it helps when you know the actual circuit and not just approximately.
It it is really the Rev 5 circuit as in the plan (with R10 as shown), adding some capacitance (e.g. 100 nF-50 µF) from the battery neg side to the virtual ground could help, as it would lower the virtual ground impedance quite a bit.

With some AC cuperimposed, not all meters may correctly measure to the mV range.
The DC voltage is still at least approximately OK - so I would expect the OPs not to be really bad.

For a test of the OPs rough working may apply a AC signal of suitable size to the input and check the signal output and intermediate (pin1 of U1) with the scope.
 

Offline MoriambarTopic starter

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Re: Microcurrent gold
« Reply #19 on: May 18, 2021, 07:53:27 pm »
Hi Moriambar don't give up!
I just tryed on a scrap pcb with an rather large soldering iron and a needle, it went great  ;D

Well you're god, Johan-Fredrik. I can BARELY solder packages suck as those of the amplifiers on the microcurrent and it takes a lot of time and effort for me to do so!
Quote
I agree I think U1,U2 and U4 are bad and maybe R1 and or R9, check if the green LED goes out when the voltage is lower than 2.5V. Maybe you can ask Dave to qoute you the spare parts.
Well the resistors are too small, 0603 packages are too difficult, I can solder them but it's something really tasking for me. Imagine the tiny packages on the microcurrent!

Btw the led goes out around 2.65V IIRC.
As for the spare parts, I guess it's way cheaper imo to get them locally than from australia since I'm in Italy… I could try to replace U1, U2 and U4, but if the resistors are involved, then it's a new microcurrent for me.

Well I don't know how to proceed now, should I order the components? The lift and ground is just a ridiculous maneuver for me lol. I tried but it's pointless lol.
 

Offline MoriambarTopic starter

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Re: Microcurrent gold
« Reply #20 on: May 18, 2021, 07:57:47 pm »
A first point would be to check if the oscillation comes from the max4239 or the virtual ground. The measured frequency is near 12 kHz which is about where the AZ frequency of the max4239 is. There is a kind of beat frequency visible, indicating 2 near by frequencies.

An oscillating OP usually oscillates at 1 frequency and often more close (still below) to the upper loop bandwidth. So with the LMV321 as a follower this would be more like a little (e.g maybe a factor 2 or 3) below 1 MHz. 12 kHz would be rather slow for a LM321 with unity gain.

This kind of points to seeing the AZ frequencies of the 2 max4239 and not the virtual ground oscillating. There may still be some contribution of the virtual ground increasing the  amplitude.

A first point to check would be the µCurrent revision - it helps when you know the actual circuit and not just approximately.
Hello, and thanks for your reply. Yes it's Rev5 as advertised on the board too. Please do mind that my beginnitude prevents me to understand a single thing of the rest you said above.
Quote
It it is really the Rev 5 circuit as in the plan (with R10 as shown), adding some capacitance (e.g. 100 nF-50 µF) from the battery neg side to the virtual ground could help, as it would lower the virtual ground impedance quite a bit.

With some AC cuperimposed, not all meters may correctly measure to the mV range.
The DC voltage is still at least approximately OK - so I would expect the OPs not to be really bad.

For a test of the OPs rough working may apply a AC signal of suitable size to the input and check the signal output and intermediate (pin1 of U1) with the scope.
Ehr… so… what should I do?

Thanks
 

Offline MoriambarTopic starter

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Re: Microcurrent gold
« Reply #21 on: May 19, 2021, 04:35:16 pm »
Ok I just checked R1 and R9 in circuit using an LCR meter on DCR and on my keithley. Those are within my ability to replace. I tried for a couple of hours the pin-lifting, but I cannot do it.
Btw R1 measures 0.01 Ohms, while R9 measures 9.99 Ohms.

So should I get U1, U3 and U4 replaced or should I throw away the microcurrent gold? Or are there any other tests that do not involve pin-lifting?
 

Online Kleinstein

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Re: Microcurrent gold
« Reply #22 on: May 19, 2021, 06:33:34 pm »
The LMV321 seems to be already removed - so not really much sense in chicking it in circuit in a non destructive way. Chances are it would be replaced with a new one at the end. The LMV321 or a similar replacement is very cheap, so little sense in putting the old one back. It is more the shipping that could hurt.

The 10 Ohms resistor still measures OK - at least good enough. The othere 2 shunts at the input are relatively difficult to damage and thus very likels still OK.
One could also check R12 in circuit with the switch set to the nA position. I would not really expect it to be bad, but just in case.


One point one could test is checking the switch contacts, with an ohms meter.

Another possible point may be just to short out R8 - as in the plan it does more harm than good and may increase the ringing at the output.

One could test the amplifiers with an AC signal to the input and the scop at the output and maybe pint 1 of U1. This should tell if the amplifiers still works, at least roughly.
Usually OPs are Ok or fully broken - partially working is a rare case.

The offset readings look odd - how are they measurend. One should get 1 offset reading, at least for the mA and µA range. There is no need for an external shunt. the 0.01 Ohms resistor is essentially like a short. An external short is more like a possible source for errors (e.g. thermal EMF from the cable and connectors) - zero current is best realized with open inputs.
 
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Offline LA7SJA

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Re: Microcurrent gold
« Reply #23 on: May 19, 2021, 07:46:35 pm »
Quote
One could also check R12 in circuit with the switch set to the nA position

If you by this mean R2 (10K) you would need about 50V to destroy it (in a year). But wy not  :-+
I agree regarding ordenary op-amps but this op-amp(s) are quit a different beast. I think it has been proven that the virtual ground op-amp is oscilating earlyer in the diagnostic.

Moriambar! do not giv up. (This is what's called learning)
If you send Dave an PM and ask if he could sell you the spare part (I would be suprised if he turn you down)

Johan-Fredrik

PS Sorry I'm so slow to respond but the chemotherapy makes me so slow and lazy. :(


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Online Kleinstein

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Re: Microcurrent gold
« Reply #24 on: May 19, 2021, 08:03:31 pm »
With the given frequency the oscillations shown don'T look like the LMV321 is oscillating. This was a possible problem with the earlier version of the µCurrent, but should be fixed with version 5. The frequency and beat frequency very much looks like the maxa4239 internal AZ frequencies. The questing is a little why so strong.

R12 is the 270 ohms resistor just in front of the OP. It is unlikely that it is bad, but in theory possible. Choosing the nA range is suggsted as the 10 K resistor would not effect the testing of R12 much.
 


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