ADS1292R: Resistance is not accurate and non-linear across various values

Travis Jett

Part Number: ADS1292R

Measuring resistance using ADS1292R. Application is AED device. Using a variable resistor to simulate the PADS resistance when connected to human body.

Software Impedance = (Average ADC Counts - 525500)/3070

The pads are connected through IN2P and IN2N.

Decade Box (ohms)	Average ADC Counts	DMM (ohms)	Software (ohms)
10	534492	10.2	2
20	565304	20.6	12
30	596400	30.6	23
40	626930	40.8	33
50	657710	50.5	43
60	688572	60.1	53
70	719186	70.6	63
80	749833	80.3	73
90	780217	90.6	82
100	810349	100.1	92
110	841161	110.4	102
120	871205	120.2	112
130	882326	129.8	116
140	909453	140.2	125
150	936528	149.8	133
160	963449	159.4	142
170	990381	170.2	151
180	1016690	179.4	159
190	1042996	189.8	168
200	1070591	200.1	177
210	1096322	210	185
220	1121881	220.1	194
230	1147459	229.9	202
240	1172401	240	210
250	1197390	250.1	218
300	1319643	300.1	258
350	1436212	349.7	296
400	1548883	399.9	333
450	1656890	448	368
500	1760219	498	402
550	1860201	547	434
600	1957423	598	466
650	2050351	647	496
700	2139468	698	525
750	2225924	747	553
800	2309680	797	581
850	2390416	847	607
880	2443559	880	624
890	2458888	890	629
900	2469292	897	633
950	2544756	947	657
1000	2613666	993	680
1050	2684663	1044	703
1100	2762476	1100	728
1150	2828826	1150	750
1200	2892593	1200	771
1250	2955100	1250	791
1300	3014740	1300	810
1350	3078340	1350	831
1400	3134460	1400	849
1450	3190124	1450	867
1500	3243871	1500	885
1550	3296406	1550	902
1600	3346765	1600	918
1650	3396728	1650	935
1700	3446938	1700	951
1750	3493156	1750	966
1800	3539526	1800	981
1850	3584458	1850	996
1900	3628168	1900	1010
1950	3670932	1950	1024
2000	3712686	2000	1038
2050	3752483	2050	1051
2100	3792619	2100	1064
2150	3833009	2150	1077
2200	3870168	2200	1089
2250	3907515	2250	1101
2300	3943747	2300	1113
2350	3978335	2350	1124
2400	4012755	2400	1135
2450	4046337	2450	1146
2500	4080440	2500	1157
2550	4112773	2550	1168
2600	4144331	2600	1178
2650	4175332	2650	1188
2700	4206293	2700	1198
2750	4236075	2750	1208
2800	4265185	2800	1218
2850	4293129	2850	1227
2900	4321436	2900	1236
2950	4348383	2950	1245
3000	4375290	3000	1254
3050	4401432	3049	1262
3100	4429666	3099	1271
3150	4455106	3150	1280
3200	4479771	3200	1288
3250	4504703	3250	1296
3300	4528831	3301	1304

CONFIG REgisters

Final Configuration prior to receiving impedance and ECG data	CONFIG1	0x1	0x01	Continuous Converstion Mode, 250 SPS
	CONFIG2	0x2	0xE0	Lead-off comparators enabled, Reference buffer enabled, 2.42V reference, OSC out disabled, Test signal off, Test signal DC
	LOFF	0x3	0x10	Lead-off 15% - 85%, Lead-off current 6 nA, Lead-off frequency DC
	CH1SET	0x4	0x00	Channel enabled, PGA gain is 6, Normal electrode input
	CH2SET	0x5	0x40	Channel enabled, PGA gain is 4, Normal electrode input
	RLD_SENS	0x6	0x20	Chop frequency Fmod/16, RLD buffer enabled, RLD lead-off sense disabled, CH2 RLD negative inputs not connected, CH2 RLD positive inputs not connected, CH1 RLD negative inputs not connected, CH1 RLD positive inputs not connected
	LOFF_SENS	0x7	0x00	FLIP2 disabled, FLIP1 disabled, LOFF2N Disabled, LOFF2P Disabled, LOFF1N disabled, LOFF1P disabled
	LOFF_STAT	0x8	0x40	Fmod divider is 16, RLD is connected, IN2N_OFF connected, IN2P_OFF connected, IN1N_OFF connected, IN1P_OFF connected
	RESP1	0x9	0xEA	RESP_DEMOD_EN1 ON, RESP_MOD_EN ON, RESP_PH is 112.5 degrees, RESP_CTRL internal clock
	RESP2	0xa	0x01	Calibration off, RESP_FREQ is 32kHz, RLDREF external

over 1 year ago

0 Praveen Aroul over 1 year ago

TI__Mastermind 49956 points

Hello Travis,

Can you kindly clarify the software impedance equation? How did you come up with the values?

0 Travis Jett over 1 year ago in reply to Praveen Aroul

Prodigy 10 points

Hello Praveen,

We actually did not develop the code we are helping out on the investigation. We aren't sure how they are coming up with the values. Can you point me to the conversion calculation in the datasheet?

Below is our schematic where ECG_L and ECG_R are connected to the patient. We are connecting a resistive load there to simulate the patient resistance.

Thank you!

Travis

0 Praveen Aroul over 1 year ago in reply to Travis Jett

TI__Mastermind 49956 points

Hello Travis,

There is no conversion calculation provided in the datasheet.

Let me check the schematics and the register configuration and shall let you know if I see any concerns.

Meanwhile, please check with your team regarding the equation.

0 Travis Jett over 1 year ago in reply to Praveen Aroul

Prodigy 10 points

Ok, thanks Praveen. Looking forward to hearing back from you on any help you can provide.

0 Travis Jett over 1 year ago

Prodigy 10 points

Hello Praveen, did you get a chance to look at this yet? Many thanks!

0 Travis Jett over 1 year ago in reply to Travis Jett

Prodigy 10 points

Hi Praveen - just following up if you had a chance to look at this? We are still getting incorrect values when measuring impedance. Many thanks

0 Stuart McCutchen over 1 year ago in reply to Travis Jett

TI__Prodigy 560 points

Hi Travis,

Praveen is currently out of office for the next couple of weeks. I apologize of the inconvenience. Please allow a few weeks delay.

0 Travis Jett over 1 year ago in reply to Stuart McCutchen

Prodigy 10 points

Hello - Just following up on this. Is there someone I can talk to related to the ADS1292R impedance measurement function?

0 Praveen Aroul over 1 year ago in reply to Travis Jett

TI__Mastermind 49956 points

Hello Travis,

Apologize for the delay in my response.

Based on the register configuration settings, you are using DC lead off detection to measure the electrode contact impedance.

Refer to Figure 3 in the link below

https://www.planetanalog.com/signal-chain-basics-149-how-to-accurately-measure-electrode-impedance-for-lead-off-detection-in-ecg-systems/

Based on the schematics, look like the RLD is used as voltage follower and the RLDREF is connected to VREFP / 2

When you compare Figure 3 and your schematics, there is no return path to GND to measure the resistance. Both IN2P and IN2N are pulled up to the RLDOUT voltage. Hence I am not sure how you can measure the resistance using this configuration.

Let me know if I am missing anything.

Also did you find out how this equation came to be?

Software Impedance = (Average ADC Counts - 525500)/3070

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ADS1292R: Resistance is not accurate and non-linear across various values