Timeline for Temperature sensor reading too high and heavily coupled to input voltage
Current License: CC BY-SA 3.0
24 events
| when toggle format | what | by | license | comment | |
|---|---|---|---|---|---|
| Aug 2, 2016 at 3:41 | vote | accept | bitshift | ||
| Aug 1, 2016 at 16:45 | answer | added | Chupacabras | timeline score: 1 | |
| Jul 31, 2016 at 18:55 | comment | added | Andy aka | Maybe you can evaluate and leave your findings as the correct answer for posterity? | |
| Jul 31, 2016 at 18:02 | comment | added | bitshift | @Andyaka One across the attiny 85 supply and one across the MCP 9700. I did both at the same time though so didn't evaluate the MCP 9700 without one. | |
| Jul 31, 2016 at 16:22 | comment | added | Andy aka | Where did you put them? | |
| Jul 31, 2016 at 14:41 | comment | added | bitshift | @Chupacabras Bodged on some caps and all is working great. Thank you | |
| Jul 31, 2016 at 11:48 | comment | added | Chupacabras | Use some bodge wires and connect some 100nF cap to that sensor. Just to check if it helps, or not. | |
| Jul 31, 2016 at 11:34 | comment | added | bitshift | @Chupacabras That's good to know, thank you. Damn, should have included some.... | |
| Jul 31, 2016 at 11:27 | comment | added | Chupacabras | I looked to some datasheet now, and 100nF cap has insulation resistance 5Gohm. So it is 1nA at 5V. Way less than your mentioned 1uA... | |
| Jul 31, 2016 at 11:11 | comment | added | Chupacabras | Where exactly did you read it? Could you give some link? I'd like to read what cap are you talking about. Standard 100nF cap has about 500Mohm insulation resistance. Apply 5V to that and you'll get 10nA leakage. Just read datasheet for cap. | |
| Jul 31, 2016 at 10:54 | comment | added | bitshift | @Chupacabras Just a typical value I've read. I'm not at all well versed in this sort of thing. Even if it's 200nA that basically halves the battery life. | |
| Jul 31, 2016 at 10:53 | comment | added | bitshift | @Andyaka I measured the output voltage using my trusty Fluke 87V. Ignoring the micro completely, the output voltage is around 100-150mV too high. | |
| Jul 31, 2016 at 10:52 | comment | added | Andy aka | What is the offset error and gain error of the ADC inside the ATTiny? What is the drift in the 1.1 volt reference versus power rail and temperature? How many LSbs of the ADC equates to 1 degC change in the output of the MCP9700? | |
| Jul 31, 2016 at 10:51 | comment | added | bitshift | @Asmyldof fair enough. COuld this cause an extra 100-150mV on the output of the temperature sensor though? | |
| Jul 31, 2016 at 10:48 | comment | added | Chupacabras | Where did you take that number from? Leakage 1uA? | |
| Jul 31, 2016 at 10:46 | comment | added | bitshift | @Chupacabras The attiny spends something like 99% of its time sleeping. Current during sleep is around 200nA. Leakage of even 1uA will swamp this and significantly decrease the battery life. | |
| Jul 31, 2016 at 10:39 | comment | added | Asmyldof | You are afraid the nA, if not pA of a well chosen ceramic are going to win, at all, from any active component, let alone the self discharge inherent in any battery chemistry? You need to think again. Not to mention home etched and hand soldered PCBs, their contamination and trace-to-trace resistance being well below the giga-Ohms you seem to hope they are. | |
| Jul 31, 2016 at 10:39 | comment | added | Chupacabras | Leakage of ceramic caps is negligible compared to power consumption of MCU, sensor and 7-segment LED display. | |
| Jul 31, 2016 at 10:36 | comment | added | bitshift | @Andyaka the attiny may need decoupling caps... I don't sink or source any significant current through it though so I left them out (probably a mistake). I'm connecting an oscilloscope as I type this so will include some screenshots of everything soon. | |
| Jul 31, 2016 at 10:34 | comment | added | Andy aka | And what about the ATiny - does that also say decouplers are optional? | |
| Jul 31, 2016 at 10:32 | comment | added | bitshift | No, I explicitly avoided capacitors as the leakage of the caps would completely ruin the battery life. The datasheet does state that decoupling caps are optional and only required in a noisy environment. | |
| Jul 31, 2016 at 10:27 | comment | added | Chupacabras | Did you try to put some decoupling capacitor next to that sensor? | |
| Jul 31, 2016 at 10:17 | history | edited | bitshift | CC BY-SA 3.0 |
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| Jul 31, 2016 at 10:10 | history | asked | bitshift | CC BY-SA 3.0 |