DMM 4072 capacitance reading and possible inaccuracy

[Gazlar]

Ok so im part of a team that has built an ATE and now we are going through a verifcation process. We are veryfiying Resistors and Capacitor readings for repeatability and for accuracy.

To do this we have a PCB populated with capacitors and resistors but done this in such a way that none of the components will interfer with eachother during taking readings.

Our RnD department then used a HamEg 8118 (WebSite link) to read the components and used those readings as a reference point to compare to the ATE readings.

The resistors ranged from 47ohms to 2.2Meg and our testing showed that, worst case, the readings were 1.5% off from the HamEg device which is pretty good. Most of the time th readings were betlow 0.5%

The problem appears when we get to the capacitors.

The first bunch of capacitors were 47nf (100nf Range) and tester read them ok, with the worst result being 0.29% out. Once we move the range up and measure bigger capcitors we start to see like an offset appear.

470nf (1uf Range) were 5% out

4.7uf (10uf Range) were 15% out

10uf (100uf Range) were 20% out

So as an experiment I did the following.

Took a blank PCB and put one capacitor on it at 470nf.
Diconnected the DMM from the ATE and plugged in the leads from a fluke multimeter. This removed any bias etc that the test fixtures, pins could cause as I will be connected straight to the PCB.

I then measured the component and got a value of 446.8nf

I then repeated using the HamEg Device and got a reading of 497.46nf 

and just for fun I used a fluke capacitor meter and got a readnig of 484nf

I reapeated the above test a few times for repeatability and the results were the same so the testing was stabile.

I have tried the Open/Short Compensation and that did pretty much nothing to the reading.
I thought about DC Bias but was informed that it wasn't necessary for this type of capacitor.

So my question is why is there an issue reading capacitors with my DMM-4072?
We have another DMM in the facotry which I tried the test on and it was the same so its not a faulty DMM. Also I used the NI DMM soft panel for the readings.

[Henrik_Volkers]

The first thing that comes to my mind: Chip (SMD) capacitors are voltage dependend. So depending on the method and bias and test amplitude  the capacity will change!

(and 40% change is nothing unusual)

Get some mica capacitors for the low range and some foil capacitors for the higher ranges (they are quite bulky for values >1µF 😉   (and temper them after soldering ) to compare the capacity ... if you want  close values. But since your actual used parts seems to have that dependency: Use test methods for your capacitors (well for any part on your PCB) that match the setpoint conditions.

Take a generater , place a resistor in series to measure the current, apply a AC plus DC bias voltage , vary the bias ...    

This effect should be noted in the datasheet of the capacitor, at least the manufactor should be able to  tell you, if you ask for it 😉

[Gazlar]

Thanks for the reply Henrik!

I'm not great with electronics and have very little knowledge, if any , so please forgive me if the following questions sound dumb.

The first thing that comes to my mind: Chip (SMD) capacitors are voltage dependend. So depending on the method and bias and test amplitude  the capacity will change!

(and 40% change is nothing unusual)

So are you saying that if a capacitor depends on a voltage greater than what the dmm supplies then you have to expect different capacitiy readings?

Get some mica capacitors for the low range and some foil capacitors for the higher ranges (they are quite bulky for values >1µF    (and temper them after soldering ) to compare the capacity ... if you want  close values. But since your actual used parts seems to have that dependency: Use test methods for your capacitors (well for any part on your PCB) that match the setpoint conditions.

So are you saying that using the DMM to read capcitence may not work due to capacitor dependancies on voltages etc so use other methods such as the one you mentioned below?

[Henrik_Volkers]

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@Gazlar wrote:

Thanks for the reply Henrik!

I'm not great with electronics and have very little knowledge, if any , so please forgive me if the following questions sound dumb.

The first thing that comes to my mind: Chip (SMD) capacitors are voltage dependend. So depending on the method and bias and test amplitude  the capacity will change!

(and 40% change is nothing unusual)

So are you saying that if a capacitor depends on a voltage greater than what the dmm supplies then you have to expect different capacitiy readings?

Simple said: The actual capacity (impedance)  of your capacitor is (or can be) a function of the applied voltage (and frequency, temperature,...) . Different LCR-meters use different methods/voltages/frequencies to measure the impedance.  

Get some mica capacitors for the low range and some foil capacitors for the higher ranges (they are quite bulky for values >1µF    (and temper them after soldering ) to compare the capacity ... if you want  close values. But since your actual used parts seems to have that dependency: Use test methods for your capacitors (well for any part on your PCB) that match the setpoint conditions.

So are you saying that using the DMM to read capcitence may not work due to capacitor dependancies on voltages etc so use other methods such as the one you mentioned below?

If a value of part is critical in a certain application, it should be measured under conditions close or equal to the conditions during actual use. (Very general 😉 )

All of your LCR-Meters show the value they actually see (maybe with different uncertaincies) and they are possibly all rigth. Just the conditions are different in a way you haven't thougth of.

Get a EE-engineer with skills in electronic (PCB) design in your team 🙂 

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