Author Topic: Non linear offset on start and end of nA range [NOISE!]  (Read 1252 times)

maxb

  • NewMember
  • *
  • Posts: 4
Non linear offset on start and end of nA range [NOISE!]
« on: October 26, 2020, 06:26:56 AM »
Hi,

I am running some test to understand de values of my just arrived current ranger.
I run my tests with a DC PSU isolated from ground earth, and a Voltmeter isolated from ground earth.
I measure a current through a 10k or a 1M and 10M resistor for my tests with SPF turned off.
Turning on SPF don't change the measurements.

It reveals that my CR seam to have a problem on nA range. There is some HUGE non linear offset from 0 1000 nA then from 2000 to 3000 nA.
Have you already seen this ?
I attach some pictures of my simple installation showing the DC supply with 1M resistor and the response on the voltmeter.

Here are my measurements :

## Resistor 10k connected
### mA range
0 V => 0.0
10 V =>1.1 mA
20 V =>2.1 mA
30 V =>3.1 mA

### µA range
0 V => -0.4 µA (expect 0 ; offset -0.4)
10 mV => 0.6 µA (expect 1 ; offset -0.4)
20 mV => 1.7 µA (expect 2 ; offset -0.3)
30 mV => 2.7 µA (expect 3 ; offset -0.3)
100 mV => 9.8 µA
200 mV =>19.9 µA
1 V => 101 µA
2 V => 202 µA
3 V => 303 µA
30 V => 3047 µA (expect 3000 ; offset +47)

### nA range
0 V => 94 nA (expect 0 ; offset +94)
10 mV => 370 nA (expect 1000 ; offset -630)
20 mV => 824 nA (expect 2000 ; offset -1176)
30 mV => 1334 nA (expect 3000 ; offset -1666)
60 mV => 1781 nA (expect out of range)


## Resistor 1M connected
### µA range
0 V => 0.0 µA
1 V => 1.0 µA
2 V => 2.0 µA
10 V => 10.0 µA
20 V => 20.0 µA
30 V => 30.1 µA

### nA range
0 V => 431 nA (expect 0 ; offset +431)
10 mV => 437 nA (expect 10 ; offset +427)
20 mV => 445 nA (expect 20 ; offset +425)
30 mV => 450 nA (expect 30 ; offset +420)
100 mV => 481 nA (expect 100 ; offset +381)
200 mV => 535 nA (expect 200 ; offset +335)
300 mV => 590 nA (expect 300 ; offset +290)
400 mV => 649 nA (expect 400 ; offset +249)
500 mV => 709 nA (expect 500 ; offset +209)
600 mV => 774 nA (expect 600 ; offset +174)
700 mV => 837 nA (expect 700 ; offset +137)
800 mV => 906 nA (expect 800 ; offset +109)
900 mV => 976 nA (expect 900 ; offset +76)
1 V => 1049 nA (expect 1000 ; offset +49)
2 V => 1957 nA (expect 2000 ; offset -43)
3 V => 2654 nA (expect 3000 ; offset -356)


## Resistor 10M connected
### µA range
0 V => 0.0 µA
1 V => 0.1 µA
2 V => 0.2 µA
10 V => 1.0 µA
20 V => 2.0 µA

### nA range
0 V => 442 nA (expect 0 ; offset +442)
10 mV => 445 nA (expect 1 ; offset +444)
20 mV => 446 nA (expect 2 ; offset +444)
50 mV => 446 nA (expect 5 ; offset +441)
100 mV => 450 nA (expect 10 ; offset +440)
200 mV => 454 nA (expect 20 ; offset +434)
500 mV => 468 nA (expect 50 ; offset +418)
1 V => 493 nA (expect 100 ; offset +393)
2 V => 543 nA (expect 200 ; offset +343)
5 V => 713 nA (expect 500 ; offset +213)
10 V => 1043 nA (expect 1000 ; offset +43)
20 V => 1918 nA (expect 2000 ; offset -82)
30 V => 2608 nA (expect 3000 ; offset -372)

Felix

  • Administrator
  • Hero Member
  • *****
  • Posts: 6866
  • Country: us
    • LowPowerLab
Re: Non linear offset on start and end of nA range
« Reply #1 on: October 26, 2020, 10:16:36 AM »
Have you measured the total resistance including the wires and breadboard?
I see standard resistors, basic jumper witres, and a breadboard which is really not a good way to do any electronics with precision.

If you have a perfect load and a perfect voltage that does not change with the load, then you can "expect"  the readings that you say you expect.

I can guarantee all CRs that leave my office have near ZERO offset.
For 0v, you should SHORT the inputs and then read the output. There should be virtually no offset, output should be 0.0x mV.
If there is an offset its possible the opamps were damaged, typically I see this with improper use - overloading/ground looping.

(I think you mean) the LPF, is mostly useful for scope and digital measurements, not so much for raw readings with a DMM.
And this was purchased from WELECTRON, correct?

maxb

  • NewMember
  • *
  • Posts: 4
Re: Non linear offset on start and end of nA range
« Reply #2 on: October 26, 2020, 10:52:46 AM »
Have you measured the total resistance including the wires and breadboard?
I see standard resistors, basic jumper witres, and a breadboard which is really not a good way to do any electronics with precision.

If you have a perfect load and a perfect voltage that does not change with the load, then you can "expect"  the readings that you say you expect.

I can guarantee all CRs that leave my office have near ZERO offset.
For 0v, you should SHORT the inputs and then read the output. There should be virtually no offset, output should be 0.0x mV.
If there is an offset its possible the opamps were damaged, typically I see this with improper use - overloading/ground looping.

(I think you mean) the LPF, is mostly useful for scope and digital measurements, not so much for raw readings with a DMM.
And this was purchased from WELECTRON, correct?

Hi Felix,

Thank you for your response.

As you can see, I did a lot of measurements. I did not measure precisely the resistor with breadboard and wires. I don't expect to have a great accuracy.
My expectation which I indicate are from Ohm's law, and I agree that a 5 to 10% precision is ok.
For mA and µA range, my measurement are ok with the 10% precision window.
But my measurements shows that I can measure correctly the nano current only from 1000 nA to 2000 nA. Whatsoever the resistor value is.
But before 1000 nA and after 2000 nA, the measurement deviate. My total resistor is not varying in those specific ranges.

If I short the 2 inputs, I got a 0 nA measurement (0.0 mV).

If the problem is my poore precision devices, How can I test properly the nA range ?

Yes, I ordered this from WELECTRON.

Felix

  • Administrator
  • Hero Member
  • *****
  • Posts: 6866
  • Country: us
    • LowPowerLab
Re: Non linear offset on start and end of nA range
« Reply #3 on: October 26, 2020, 12:36:10 PM »
Quote
If I short the 2 inputs, I got a 0 nA measurement (0.0 mV).
There you go, noise. So all those 0v with offsets are false negatives. I suspect much of the rest is also noise picked up from your environment.

nA is indeed sensitive to noise and this is noted in the guide.
I see the behavior is amplified in nA and with the larger resistors you use, which makes sense, higher resistance will be give more noisy readings especially with the thin wires and breadboard which act like antennas for all the electrical noise sources in your lab.

Use short thick wires, solid strong connections. Connect the resistors straight to your PSU, eliminate the breadboard. Maybe also try a battery instead of a PSU, see if that's different.
You could use a load measurement using your PSU also, instead of passing voltage through all those antennas.

maxb

  • NewMember
  • *
  • Posts: 4
Re: Non linear offset on start and end of nA range
« Reply #4 on: October 27, 2020, 06:18:52 AM »
Hi Felix,

By pluging directly the 10M ohm resistor on the PSU and with 2 banana wire plus an aligator clamp, I get the same result.

I am able to measure the expected 100 nA in a 10 Mohm resistor with 1V on my dc supply on the mA range : (I measure 0.10 mV)
For 2V, I am able to measure 0.20 mV, 0.30 mV for 3V It's pretty accurate.

But in the nA range, it's totally not the case. After a switch to nA range, I measure 0.37 mV for the same 100 nA expected to flow in the resistor.

So it cannot be à problem of noise. If I have 400 nA of noise in the circuit, why is it picked only by the nA range, and not by the mA range ?

Moreover, if I put a 1M ohm resistor in parralele to the 10M ohm, it add 1000 nA in the current ranger (with 1V on the PSU), and I correctly measure 1100 nA.
So it really seem there is a non linear offset voltage on my current ranger on the nA range.
For now I have to add some resistor in parallel to use my current ranger in the "linear part" of the nA range.

By the way, I don't know if it's a part of the problem, but, I use the display of the current ranger to show values, and I see it jumping all over the place.
For example, while measuring the 1100 nA in the 10M // 1M parallel resistor from 0 to 2000 nA while I measure nA currents. The multimeter show a smooth 1100 mV.

Thanks for your help.

Felix

  • Administrator
  • Hero Member
  • *****
  • Posts: 6866
  • Country: us
    • LowPowerLab
Re: Non linear offset on start and end of nA range
« Reply #5 on: October 27, 2020, 09:08:23 AM »
Code: [Select]
So it cannot be à problem of noise. If I have 400 nA of noise in the circuit, why is it picked only by the nA range, and not by the mA range ?
I mentioned before that nA is sensitive to noise. What you see in the mA range could be different because its much less sensitive to noise. You are using extremely large resistors which are prone to picking up noise.
The OLED jumping a lot also indicates there's noise (it only does averaging not RMS).
Why don't you hook up a scope to the output and see exactly?

I would take a battery and a 1M resistor. You should get around 3000nA in nA range, or 3uA in uA range. Try that on your bench or take that somewhere else and see if there's a difference. Use short wires.

maxb

  • NewMember
  • *
  • Posts: 4
Re: Non linear offset on start and end of nA range
« Reply #6 on: October 28, 2020, 11:22:54 AM »
Hi,

I checked with à battery instead of my PSU and Yeah, I can measure a 11 nA current through a 20M resistor.
The measurement are stable.

Do I am condemned to run my tests on a battery ?
For now, I am unable to filter the noise of the PSU.

Felix

  • Administrator
  • Hero Member
  • *****
  • Posts: 6866
  • Country: us
    • LowPowerLab
Re: Non linear offset on start and end of nA range [NOISE!]
« Reply #7 on: October 28, 2020, 12:24:54 PM »
You're not condemned but, if the PSU is noisy then perhaps you need a better supply for very low currents.
I assume your products will eventually not include a bench PSU but a battery.
Be aware of other traps like the high value resistors, their type, the leads you use to connect them. Everything matters, and errors and noise are amplified as you go down in range.