Hello ADI,
I have trouble setting up the LTM2985 with two PT100 RTDs. They have a shared RSense of 1 kΩ and are configured in 2-Wire-Mode. My schematic is very close to figure 12, "Shared 2-Wire RTD Example" in the LTC2985 Datasheet (Rev. A) on pages 39 and 40. To illustrate my problem, I have copied the two pages and put my values in.
Two thermocouples are already working without a cold junction. The RTDs will be used for that. At the moment I use the minimal example with the RTDs and the RSense only.
The data I get is extremely wrong and continuously drifting away. I searched for a very long time and don't get the problem. Please help.
Kind regards,
Mario
Here is the Data I get.| Time(sec) | CH5 ResultReg | CH5 Status | CH5 Wert | CH9 ResultReg | Status | Wert |
|-----------|---------------|------------|----------|---------------|--------|----------|
| 0 | 05FCC043 | 5 | 16176,07 | 01FEEB86 | 1 | 16314,88 |
| 2,564 | 05FCAF99 | 5 | 16171,9 | 01FEC3CE | 1 | 16304,95 |
| 3,065 | 05FCADC3 | 5 | 16171,44 | 01FEBECF | 1 | 16303,7 |
| 3,589 | 05FCA993 | 5 | 16170,39 | 01FEB568 | 1 | 16301,35 |
| 4,092 | 05FCA7B8 | 5 | 16169,93 | 01FEB0EB | 1 | 16300,23 |
| 4,553 | 05FCA5BD | 5 | 16169,43 | 01FEAC02 | 1 | 16299 |
| 5,008 | 05FCA18D | 5 | 16168,39 | 01FEA292 | 1 | 16296,64 |
| 5,459 | 05FC9F9C | 5 | 16167,9 | 01FE9E0F | 1 | 16295,51 |
| 5,906 | 05FC9D9D | 5 | 16167,4 | 01FE9980 | 1 | 16294,38 |
| 6,369 | 05FC99E8 | 5 | 16166,48 | 01FE9042 | 1 | 16292,06 |
| 6,874 | 05FC97E6 | 5 | 16165,97 | 01FE8BB8 | 1 | 16290,93 |
| 7,365 | 05FC93B8 | 5 | 16164,93 | 01FE82F3 | 1 | 16288,74 |
| 7,885 | 05FC921D | 5 | 16164,53 | 01FE7E46 | 1 | 16287,57 |
| 8,325 | 05FC8FD3 | 5 | 16163,96 | 01FE79FC | 1 | 16286,5 |
| 8,842 | 05FC8C7F | 5 | 16163,12 | 01FE7176 | 1 | 16284,37 |
| 9,326 | 05FC8A31 | 5 | 16162,55 | 01FE6D0A | 1 | 16283,26 |
| 9,831 | 05FC8682 | 5 | 16161,63 | 01FE6413 | 1 | 16281,02 |
| 10,346 | 05FC849B | 5 | 16161,15 | 01FE6037 | 1 | 16280,05 |
| 10,825 | 05FC80DD | 5 | 16160,22 | 01FE57A9 | 1 | 16277,92 |
| 11,296 | 05FC7F1F | 5 | 16159,78 | 01FE53D8 | 1 | 16276,96 |
| 11,796 | 05FC7B53 | 5 | 16158,83 | 01FE4B26 | 1 | 16274,79 |
| 12,246 | 05FC7994 | 5 | 16158,39 | 01FE4735 | 1 | 16273,8 |
| 12,678 | 05FC77AF | 5 | 16157,92 | 01FE4357 | 1 | 16272,83 |
| 13,176 | 05FC742F | 5 | 16157,05 | 01FE3AE2 | 1 | 16270,72 |
| 13,678 | 05FC72AF | 5 | 16156,67 | 01FE3782 | 1 | 16269,88 |
| 14,113 | 05FC70F0 | 5 | 16156,23 | 01FE33D6 | 1 | 16268,96 |
| 14,633 | 05FC6D73 | 5 | 16155,36 | 01FE2BC7 | 1 | 16266,94 |
| 15,126 | 05FC6BAC | 5 | 16154,92 | 01FE2835 | 1 | 16266,05 |
| 15,576 | 05FC6844 | 5 | 16154,07 | 01FE209B | 1 | 16264,15 |
| 16,093 | 05FC66C2 | 5 | 16153,69 | 01FE1CCE | 1 | 16263,2 |
| 16,555 | 05FC649F | 5 | 16153,16 | 01FE18E2 | 1 | 16262,22 |
| 17,034 | 05FC614F | 5 | 16152,33 | 01FE1172 | 1 | 16260,36 |
| 17,542 | 05FC5F89 | 5 | 16151,88 | 01FE0DB4 | 1 | 16259,43 |
| 18,017 | 05FC5C52 | 5 | 16151,08 | 01FE0649 | 1 | 16257,57 |
| 18,481 | 05FC5A9F | 5 | 16150,66 | 01FE02E6 | 1 | 16256,72 |
| 18,969 | 05FC5725 | 5 | 16149,79 | 01FDFBDC | 1 | 16254,96 |
| 19,452 | 05FC557D | 5 | 16149,37 | 01FDF849 | 1 | 16254,07 |
| 19,974 | 05FC53C8 | 5 | 16148,95 | 01FDF472 | 1 | 16253,11 |
| 20,446 | 05FC50E3 | 5 | 16148,22 | 01FDEDF0 | 1 | 16251,48 |
| 20,936 | 05FC4F12 | 5 | 16147,77 | 01FDEA77 | 1 | 16250,62 |
| 21,407 | 05FC4BBC | 5 | 16146,93 | 01FDE32C | 1 | 16248,79 |
| 21,894 | 05FC4A6E | 5 | 16146,61 | 01FDE017 | 1 | 16248,02 |
| 22,314 | 85F06400 | 133 | 15385 | 01FDDCB5 | 1 | 16247,18 |
| 22,806 | 85F06400 | 133 | 15385 | 01FDD624 | 1 | 16245,54 |
| 23,281 | 85F06400 | 133 | 15385 | 01FDD30B | 1 | 16244,76 |
| 23,783 | 85F06400 | 133 | 15385 | 01FDCC41 | 1 | 16243,06 |
| 24,262 | 85F06400 | 133 | 15385 | 01FDC901 | 1 | 16242,25 |
| 24,781 | 85F06400 | 133 | 15385 | 01FDC29B | 1 | 16240,65 |
| 25,523 | 85F06400 | 133 | 15385 | 01FDBC33 | 1 | 16239,05 |
| 26,011 | 85F06400 | 133 | 15385 | 01FDB906 | 1 | 16238,26 |
| 26,796 | 85F06400 | 133 | 15385 | 01FDB2A9 | 1 | 16236,67 |
The plotted data looks like this.
