INA228: does not reading correctly and i don't have a consistent error

Nestor Ortinez

Part Number: INA228

Tool/software:

Hello

I am working with a PCB design using an INA228, and it was working fine until we decide to calibrate them, and discover the error on the measurements that we are having is bigger than we expected, at this moment i am trying to configure it using a shunt resistor of 16mOhm, and trying to read the same load with different voltage, resulting on different currents, and as for compassion I am using a DMM Keithley6 1/2 recent calibrated,

this is the design that I am using

at 9 V this is my graphic current in mA

at 13.5V this is my graphic current in mA

and at 16V this is the behavior current in mA

I already tried to change the value on the ShuntCal to 4025d, 4050d, and 4000d, this for the extra values from the shun 16.1, 16.2 or 16.0

change the number of measurements from 1 to 256,

change the time to take the measurement to 4120 us

the voltage is almost close I only have 0.1V of difference vs DMM and this is calculated by (VUS_voltage) * 195.3125/1000000

the LSB current that I declared is 0.000019073486, I got that from 10/2^19

i tried to change the ADC_range to 1, and then according to the data sheet, to establish the SHUNT_CAL, I should multiply by 4 the value, but when I did that, the values were to different, so I leave it by one,

the only thing that I saw is that as i go to higher voltage, the error diminish

i don't know what else to do, since I don't have an standard offset to add, since each measurement the error is very different

5 months ago

0 Guang Zhou 5 months ago

TI__Mastermind 35030 points

Hi,

I think it might be a good idea to compare shunt voltage instead of current at this time.

For that can you please measure at the input pins of INA228 the real input differential voltage, and compare to Vshunt reported by INA228?

Once we achieve agreement of the two, we can move on to resolving current difference if it is still there.

Regards

Guang

0 Nestor Ortinez 5 months ago in reply to Guang Zhou

Prodigy 20 points

Hello Guang,

sorry for the late answer I was performing the modifications on the code to read the shunt voltage, since i hadn't implemented yet, so, I check the shunt voltage give it by the INA228 vs the voltage across the resistance using the DMM,

the values are very similar, and almost without difference, the differences that are show I think they are for the change on the power supply or by hand. . .

here is the data, I don't know if there is a better way to share this information.

Shunt Volatege RAW	DMM Shunt Voltage	Difference	Power Supply
0.013343	0.010595649	0.002747	8.86
0.013345	0.007682402	0.005663	8.85
0.013346	-0.360042016	0.373388	8.86
0.013347	1.79579548	-1.782448	8.85
0.013346	0.013395458	-0.000049	8.86
0.013346	0.013425175	-0.000079	8.86
0.013348	0.013396614	-0.000049	8.85
0.013349	0.013403367	-0.000054	8.85
0.013351	0.013448066	-0.000097	8.85
0.013353	0.013453783	-0.000101	8.85
0.013353	0.013441121	-0.000088	8.85
0.013353	0.013454769	-0.000102	8.83
0.010581	0.013475598	-0.002895	13.47
0.008832	0.005852424	0.002980	13.47
0.008833	0.70859399	-0.699761	13.47
0.008833	0.008918767	-0.000086	13.47
0.008833	0.008923868	-0.000091	13.47
0.008833	0.008932843	-0.000100	13.47
0.008833	0.0089258	-0.000093	13.47
0.008833	0.008928197	-0.000095	13.47
0.008833	0.008935582	-0.000103	13.47
0.008833	0.008944086	-0.000111	13.48
0.008833	0.008931331	-0.000098	13.47
0.008833	0.00892266	-0.000090	13.48
0.008833	0.008947978	-0.000115	13.47
0.00884	0.008920495	-0.000080	13.46
0.00884	0.008922852	-0.000083	16
0.007544	-0.08208083	0.089625	16
0.007544	-0.08878507	0.096329	16.01
0.007544	0.00761904	-0.000075	16
0.007543	0.007624569	-0.000082	16.01
0.007543	0.007629033	-0.000086	16.01
0.007543	0.007624516	-0.000082	16.01
0.007543	0.007621739	-0.000079	16.01
0.007543	0.007620835	-0.000078	16.01
0.007543	0.007621655	-0.000079	16.01
0.007543	0.007621848	-0.000079	16.01
0.007543	0.007626821	-0.000084	16.01
0.007543	0.007627902	-0.000085	16.01

0 Guang Zhou 5 months ago in reply to Nestor Ortinez

TI__Mastermind 35030 points

Hi Nestor,

It is good that shunt voltage is almost identical between DMM and INA228, although some differences are quite large, such as the 4^th line.

Next is to correlate your previous DMM current readings with DMM shunt reading divided by 16mOhm. Do they agree? Further correlate both to current reported by INA228. Doing so can reveal if the shunt resistor is accurate, and if there is any math error.

Regards

Guang

0 Nestor Ortinez 5 months ago in reply to Guang Zhou

Prodigy 20 points

Hi Guang,

I was checking again at a different test voltage, in this case 9V, and I can se a bigger error, than the one on the 13.5 or 16V which is even weird, but now I compared the shunt voltage read by the DMM, INA228 and also currents and the conversions

using the following values.,

also I was checking that on the program I set the shunt value as 0.0162 ohm, so, when I change that to the spreadsheet, the values between the current obtained by the INA and the one obtained by the Ohm law, are almost the same

I am adding both tables.

	Shunt Value		0.016	Ohm
INA 228					DMM
Voltage Read INA228	Current Read INA228	Shunt Voltage	Current by Shunt mA		Current read by DMM mA	Shunt read by DMM	current by shunt mA
8.8	819.48	0.013276	829.75		850.5629	0.013415	838.4168
8.8	819.48	0.013276	829.75		850.8193	0.013411	838.1635
8.8	819.48	0.013276	829.75		850.8242	0.013431	839.4272
8.8	819.37	0.013274	829.625		850.7852	0.01342	838.7515
8.8	819.37	0.013274	829.625		851.0354	0.013413	838.2984
8.8	819.37	0.013274	829.625		851.01	0.013423	838.9198
8.8	819.74	0.01328	830		851.0544	0.013431	839.4602
8.8	819.74	0.01328	830		851.2238	0.013442	840.0973
8.8	819.74	0.01328	830		851.211	0.013454	840.8795
8.8	819.9	0.013282	830.125		851.1479	0.013422	838.8762
8.8	819.9	0.013282	830.125		851.2674	0.013424	839.0165
8.8	819.9	0.013282	830.125		851.2189	0.013427	839.1741
8.8	819.99	0.013284	830.25		851.1889	0.013406	837.8547
8.8	819.99	0.013284	830.25		851.381	0.013408	837.9911
8.8	819.99	0.013284	830.25		851.3989	0.013403	837.6643
8.8	819.85	0.013282	830.125		851.2842	0.013405	837.8257
8.8	819.85	0.013282	830.125		851.5042	0.013414	838.3881
8.8	819.85	0.013282	830.125		851.4465	0.01341	838.1482
8.8	819.85	0.013282	830.125		851.3886	0.013415	838.4082
8.8	819.79	0.013281	830.0625		851.5431	0.013408	837.9946
8.8	819.79	0.013281	830.0625		851.5157	0.013415	838.4247
8.8	819.79	0.013281	830.0625		851.4237	0.013413	838.3046
8.8	819.85	0.013282	830.125		851.539	0.013386	836.6251
8.8	819.85	0.013282	830.125		851.5828	0.013405	837.7948
8.8	819.85	0.013282	830.125		851.4691	0.013398	837.3539
8.8	819.97	0.013283	830.1875		851.6177	0.013403	837.6613
8.8	819.97	0.013283	830.1875		851.6197	0.013414	838.3647
8.8	819.97	0.013283	830.1875		851.4496	0.013431	839.434
8.8	820.19	0.013287	830.4375		851.6349	0.013418	838.6478
8.8	820.19	0.013287	830.4375		851.6473	0.013407	837.9136
8.8	820.19	0.013287	830.4375		851.5227	0.013416	838.5046
8.8	820.27	0.013288	830.5		851.7194	0.013409	838.0904
8.8	820.27	0.013288	830.5		851.7229	0.013404	837.7743
8.8	820.27	0.013288	830.5		851.4994	0.013406	837.8689
8.8	820.34	0.01329	830.625		851.7288	0.01342	838.7508
8.8	820.34	0.01329	830.625		851.7766	0.013423	838.9389
8.8	820.34	0.01329	830.625		851.5989	0.013419	838.6622
8.8	820.34	0.01329	830.625		851.8037	0.01344	839.982
8.79	820.46	0.013292	830.75		851.8261	0.013428	839.229
8.79	820.46	0.013292	830.75		851.642	0.013422	838.8956
8.79	820.46	0.013292	830.75		851.8131	0.013424	838.9826
8.79	820.46	0.013292	830.75		851.8047	0.013425	839.0404
8.79	820.46	0.013292	830.75		851.6625	0.013436	839.7351
8.79	820.46	0.013292	830.75		851.7929	0.013425	839.0825
8.79	820.52	0.013293	830.8125		851.8273	0.013423	838.9606
8.79	820.52	0.013293	830.8125		851.6894	0.013417	838.5677
8.79	820.52	0.013293	830.8125		851.7731	0.013426	839.1226
8.79	820.53	0.013293	830.8125		851.8468	0.013426	839.1492
8.79	820.53	0.013293	830.8125		851.6481	0.013418	838.5979
8.79	820.53	0.013293	830.8125		851.8108	0.013422	838.8922
8.79	820.65	0.013295	830.9375		851.8403	0.013421	838.7907
8.79	820.65	0.013295	830.9375		851.6998	0.013429	839.296
8.79	820.65	0.013295	830.9375		851.7599	0.013426	839.1082
8.79	820.74	0.013296	831		851.8605	0.013452	840.7197
8.79	820.74	0.013296	831		851.7169	0.013452	840.7529
8.79	820.74	0.013296	831		851.7956	0.013433	839.5929
8.79	820.74	0.013296	831		851.8305	0.013425	839.0698

INA 228				DMM
Voltage Read	Current Read INA228	Shunt Voltage	Current by Shunt mA	Current read by DMM mA	Shunt read by DMM	current by shunt mA
8.8	819.48	0.013276	819.5062	850.5629	0.013415	828.066
8.8	819.48	0.013276	819.5062	850.8193	0.013411	827.8158
8.8	819.48	0.013276	819.5062	850.8242	0.013431	829.0639
8.8	819.37	0.013274	819.3827	850.7852	0.01342	828.3965
8.8	819.37	0.013274	819.3827	851.0354	0.013413	827.949
8.8	819.37	0.013274	819.3827	851.01	0.013423	828.5628
8.8	819.74	0.01328	819.7531	851.0544	0.013431	829.0964
8.8	819.74	0.01328	819.7531	851.2238	0.013442	829.7257
8.8	819.74	0.01328	819.7531	851.211	0.013454	830.4983
8.8	819.9	0.013282	819.8765	851.1479	0.013422	828.5197
8.8	819.9	0.013282	819.8765	851.2674	0.013424	828.6583
8.8	819.9	0.013282	819.8765	851.2189	0.013427	828.8139
8.8	819.99	0.013284	820	851.1889	0.013406	827.5108
8.8	819.99	0.013284	820	851.381	0.013408	827.6456
8.8	819.99	0.013284	820	851.3989	0.013403	827.3227
8.8	819.85	0.013282	819.8765	851.2842	0.013405	827.4822
8.8	819.85	0.013282	819.8765	851.5042	0.013414	828.0376
8.8	819.85	0.013282	819.8765	851.4465	0.01341	827.8007
8.8	819.85	0.013282	819.8765	851.3886	0.013415	828.0575
8.8	819.79	0.013281	819.8148	851.5431	0.013408	827.649
8.8	819.79	0.013281	819.8148	851.5157	0.013415	828.0738
8.8	819.79	0.013281	819.8148	851.4237	0.013413	827.9552
8.8	819.85	0.013282	819.8765	851.539	0.013386	826.2964
8.8	819.85	0.013282	819.8765	851.5828	0.013405	827.4516
8.8	819.85	0.013282	819.8765	851.4691	0.013398	827.0162
8.8	819.97	0.013283	819.9383	851.6177	0.013403	827.3198
8.8	819.97	0.013283	819.9383	851.6197	0.013414	828.0145
8.8	819.97	0.013283	819.9383	851.4496	0.013431	829.0706
8.8	820.19	0.013287	820.1852	851.6349	0.013418	828.2941
8.8	820.19	0.013287	820.1852	851.6473	0.013407	827.569
8.8	820.19	0.013287	820.1852	851.5227	0.013416	828.1527
8.8	820.27	0.013288	820.2469	851.7194	0.013409	827.7436
8.8	820.27	0.013288	820.2469	851.7229	0.013404	827.4314
8.8	820.27	0.013288	820.2469	851.4994	0.013406	827.5248
8.8	820.34	0.01329	820.3704	851.7288	0.01342	828.3959
8.8	820.34	0.01329	820.3704	851.7766	0.013423	828.5816
8.8	820.34	0.01329	820.3704	851.5989	0.013419	828.3083
8.8	820.34	0.01329	820.3704	851.8037	0.01344	829.6118
8.79	820.46	0.013292	820.4938	851.8261	0.013428	828.8682
8.79	820.46	0.013292	820.4938	851.642	0.013422	828.5389
8.79	820.46	0.013292	820.4938	851.8131	0.013424	828.6248
8.79	820.46	0.013292	820.4938	851.8047	0.013425	828.6819
8.79	820.46	0.013292	820.4938	851.6625	0.013436	829.368
8.79	820.46	0.013292	820.4938	851.7929	0.013425	828.7234
8.79	820.52	0.013293	820.5556	851.8273	0.013423	828.6031
8.79	820.52	0.013293	820.5556	851.6894	0.013417	828.215
8.79	820.52	0.013293	820.5556	851.7731	0.013426	828.7631
8.79	820.53	0.013293	820.5556	851.8468	0.013426	828.7893
8.79	820.53	0.013293	820.5556	851.6481	0.013418	828.2448
8.79	820.53	0.013293	820.5556	851.8108	0.013422	828.5355
8.79	820.65	0.013295	820.679	851.8403	0.013421	828.4353
8.79	820.65	0.013295	820.679	851.6998	0.013429	828.9343
8.79	820.65	0.013295	820.679	851.7599	0.013426	828.7488
8.79	820.74	0.013296	820.7407	851.8605	0.013452	830.3404
8.79	820.74	0.013296	820.7407	851.7169	0.013452	830.3733
8.79	820.74	0.013296	820.7407	851.7956	0.013433	829.2275
8.79	820.74	0.013296	820.7407	851.8305	0.013425	828.7109

0 Guang Zhou 5 months ago in reply to Nestor Ortinez

TI__Mastermind 35030 points

Hi Nestor,

The effective shunt resistance on the PCB matters as you found out. Using a DMM, the three quantities should be able to come together, namely voltage, resistance and current. The same applies to the INA of course.

Between DMM and INA, shunt voltage should agree when measured at the INA input, which you already confirmed.

We can say with some confidence that the discrepancy in current is due to the delta between ideal and actual shunt value. This can be compensated for by adjusting in calculation.

Regards

Guang

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INA228: does not reading correctly and i don't have a consistent error