TEC current is incorrect #13

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sb10q · 2024-06-09T17:41:30+08:00

sb10q commented

2024-06-09 17:41:30 +08:00

Set TEC to 100mA constant current in GUI
Short TEC pins with ammeter. Reading is about 60mA.

* Set TEC to 100mA constant current in GUI * Short TEC pins with ammeter. Reading is about 60mA.

linuswck commented

2024-07-10 17:31:02 +08:00

I cannot reproduce the large error with the above test environment. But, I observe that MAX1968 has an unspecified offset error and a specified gain error.

Measurement

Data is measured with Agilent 34401A with the probe connecting directly to the TEC+ and TEC- pins headers like the above mentioned test condition.

Gain (Ammeter Reading / Value Set) = 0.9657
Offset = 21.73mA

Value Set (mA)	Ammeter Reading (mA)	Line Fitted Result(mA)
-1000	-943.38	-943.97
-900	-847.11	-847.4
-800	-750.61	-750.83
-700	-654.21	-654.26
-600	-557.65	-557.69
-500	-461	-461.12
-400	-364.45	-364.55
-300	-268.05	-267.98
-200	-171.61	-171.41
-100	-72.668	-74.84
0	20.364	21.73
100	113.415	118.3
200	214.82	214.87
300	311.65	311.44
400	408.37	408.01
500	505.13	504.58
600	601.84	601.15
700	698.34	697.72
800	794.76	794.29
900	891.15	890.86
1000	987.31	987.43

Discussion

The same problem also exists on Thermostat. I think this is related to MAX1968 more than a software related issue.

Offset Error

Currently, our firmware can calibrate zero current point up to VREF accurately.
VCTLI - VREF = 1.4989V - 1.4986V < 1 LSB
Expected Offset Current = 600uA
Actual Offset Current = 20.364mA

This should be an offset within MAX1968. This offset error is not specified anywhere in the datasheet.

Gain Error

The dominant gain error should come from MAX1968, which is specified to be +-5%. The measured gain error is within spec.

I cannot reproduce the large error with the above test environment. But, I observe that MAX1968 has an unspecified offset error and a specified gain error. # Measurement Data is measured with Agilent 34401A with the probe connecting directly to the TEC+ and TEC- pins headers like the above mentioned test condition. Gain (Ammeter Reading / Value Set) = 0.9657 Offset = 21.73mA | Value Set (mA) | Ammeter Reading (mA) | Line Fitted Result(mA) | |----------------|----------------------|------------------------| | -1000 | -943.38 | -943.97 | | -900 | -847.11 | -847.4 | | -800 | -750.61 | -750.83 | | -700 | -654.21 | -654.26 | | -600 | -557.65 | -557.69 | | -500 | -461 | -461.12 | | -400 | -364.45 | -364.55 | | -300 | -268.05 | -267.98 | | -200 | -171.61 | -171.41 | | -100 | -72.668 | -74.84 | | 0 | 20.364 | 21.73 | | 100 | 113.415 | 118.3 | | 200 | 214.82 | 214.87 | | 300 | 311.65 | 311.44 | | 400 | 408.37 | 408.01 | | 500 | 505.13 | 504.58 | | 600 | 601.84 | 601.15 | | 700 | 698.34 | 697.72 | | 800 | 794.76 | 794.29 | | 900 | 891.15 | 890.86 | | 1000 | 987.31 | 987.43 | # Discussion The same problem also exists on Thermostat. I think this is related to MAX1968 more than a software related issue. ## Offset Error Currently, our firmware can calibrate zero current point up to VREF accurately. VCTLI - VREF = 1.4989V - 1.4986V < 1 LSB Expected Offset Current = 600uA Actual Offset Current = 20.364mA This should be an offset within MAX1968. This offset error is not specified anywhere in the datasheet. ## Gain Error The dominant gain error should come from MAX1968, which is specified to be +-5%. The measured gain error is within spec.

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Reference: M-Labs/kirdy#13