# ADE7880 calibration, gain matching issue

Hello,

I am currently working on the calibration of an ADE7880 (I am working on the EVAL-ADE7880EBZ). For this I used the application note AN-1171. Here is the procedure that I carried out for this calibration:

First of all, I used a precise source (Omicron CMC 256 plus) to generate the precise currents/voltages.

1. I generated a fixed current of 5Arms (corresponds to about 40% of my full scale), then I measured the AIRMS, BIRMS, CIRMS values (average of 500 readings).

2. Using the AN-1171 formula, I calculated BIGAIN and CIGAIN and updated these values.

3. I calculated the value of [I/lsb] by dividing 5Arms by my decimal value obtained on the ADE7880 (on phase A).

4. I then calculated the values obtained for AIRMS in Arms using this conversion value, for different voltages in the range (0.1/0.5/1/3/5Arms) to see if my accuracy approached the expected 0.1%.

5. I then arrive at very poor accuracies. On phase C for example: 5.8% for 0.1Arms, 1.4% for 0.5Arms, 0.75% for 1Arms, 0.16% for 3Arms and 0.01% for 5Arms.

6. I performed exactly the same procedure for the voltage by generating a voltage of 230Vrms and then checking the range of the signal (10/50/110/230/300Vrms) and I also obtain insufficient precision.

To be noted :

- I use current transformers (CR8410-100, Ir = 20A, r=1000) to measure the current.
- The current source (CMC 256plus) is accurate to within 0.1% over the entire range. So I think it's normal that I can't reach an accuracy of 0.1% (specified by the ADE7880) over the whole range for current and voltage because the source is not precise enough, but I'm still very far from that at the moment.

Can you tell me if my procedure is correct? How can I improve these results?

Maxime

Changing the laylout
[edited by: Orwick at 10:34 AM (GMT -4) on 28 Oct 2020]
• Maxime

What are your IRMS and VRMS hex values with 5A and 220V ?

Is the PGAGAIN of 1?

What is the burden you are using ?  Are you using 2 resisters for a differential burden?

On the ADE7880 eval board is there a single 1 Meg resistor or a couple 500K?

This looks like and offset issue which means you either have to much noise or cross talk from the voltage channel.

If you look at IRMS at .1A and vary the voltage does the error on the IRMS measurement change?

Dave

• What are your IRMS and VRMS hex values with 5A and 220V ?

With xIGAIN at 0x00, here are my values for IRMS :
- 0x18BDAC (AIRMS)
- 0x18EF8D (BIRMS)
- 0x18955A (CIRMS)

With xVGAIN at 0x00, here are my values for VRMS (220V) :
- 0x25B0AF (AVRMS)
- 0x259E21 (BVRMS)
- 0x259FDE (CVRMS)

Is the PGAGAIN of 1?

Yes it is.

What is the burden you are using ?  Are you using 2 resisters for a differential burden?

I use a 30 Ohm burden (separated into 2 burdens of 15 Ohms to have a differential burden), this allows me (in theory, a maximum full scale current of 11.93 Arms).

On the ADE7880 eval board is there a single 1 Meg resistor or a couple 500K?

There is a single resistor of 1 MOhm.

This looks like and offset issue which means you either have to much noise or cross talk from the voltage channel.

If you look at IRMS at .1A and vary the voltage does the error on the IRMS measurement change?

The measured value changes slightly if I change the voltage.

For example:
- With 0.1A and 1V, I have the value 30132d (average)
- With 0.1A and 10V, I have the value 30353d (average)

Do you have any advice on how to solve the problem? Not sure if I understand the "cross talk from the voltage channel" ?

Thank you a lot !

• For the voltage divider  typical the voltage only varies 10% in the field so this in not usually and issue but.. We have found a single 1M resistor is not the best choice for performance. The capacitance between the pads of the 1 Meg resistor will create leakage that is proportional to the voltage input and cause non linearity. Short the 1 Meg resistor and add 2 500K or 3 330K in series as shown. See if VRMS improves.

As for the current channel.

Please verify current (.1 ,.3, 1 ,3 ,5)using a hand held meter set to current  does the measured current match the supplied current?

I possibility is your ct is non linear. I noticed there is no indication of the linearity of the ct on the web site. Try a different manufacture.

Irms and Vrms offset can be used to correct the error but you should try to minimize the error first.

• I tested with 3 resistors of 3 x 330 ohms, the result is clearly better for voltage accuracy!

I get the following accuracy for phase A :
10V : 0,9%
50V : 0,13%
110V : 0.14%
200V : 0.02%
230V : 0%
250V : 0,02%
300V : 0,05%

After modifying the AVRMSOS register, I get the following details :
10V : 0%
50V : 0,12%
110V : 0.15%
200V : 0.02%
230V : 0.0%
250V : 0,02%
300V : 0,05%

I measure voltages with a multimeter with an accuracy of at least 0.1%.

So I come with the following questions:

• I have an improvement for very small voltages around 10V by modifying AVRMSOS, but not in the intermediate values (50V, 110V), is this normal?
• What is the use of ferrites (E1A/E2A/E3A etc), maybe they are the ones that cause non linearity problems (in my test with 330k resistors, I short-circuited)?
• Do you see a way to further increase this precision (in the intermediate values) to reach 0.1% everywhere?Maybe it is only due to the accuracy of the measuring device (but I found it strange that it is only in the intermediate values).

I checked the RMS currents, they match what I inject. I will order other CTs and see if I get better results. I will get back to you after this.

• Good to see this fixed the VRMS issue. The issue now is noise. In a perfect work with low noise .1% is possible. Your source might be noisy at 10V and 50V this can be due to auto ranging in the source. Try using the FFT in the eval software to see the noise from the source at your test levels.

The ferrite have very low current and should not cause to many issues. The ferrites only needed for system level EMC and only used if there is an issue. Shorting is fine.

Dave