| Electronics > Beginners |
| PIC16F1827 and LM35 ADC impedance |
| (1/1) |
| PerranOak:
I'm trying to design a simple circuit to use a PIC16F1827 and LM35 to create a simple thermometer. The PIC datasheet says that the maximum recommended impedance of the analogue device is 10kOhms. I measured the resistance of the LM35 (output to ground) which was 43kOhms. Is this recommendation one I can just ignore? Or ... could I simply delay the start of measurement to accommodate the slower charging of the sample/hold capacitor? [the datasheet calc gives acquisition time as about 1us with 10kOhm so the LM35 would give about 3us) Or ... could I put a 10kOhm resistor in parallel to the LM35 output: this would give a total resistance of about 8kOhm? Thank you. |
| tecman:
The LM35 has an active driver. The datasheet has a load regulation of 0.5 mv/ma, which implies an output impedance of 0.5 ohms. Measuring the resistance of the output pin to ground when powered off is not a meaningful number. So adding a series R to the output is no problem. However the max recommended is not a value you would use. You should be able to connect it directly to the PIC. You could add a 100 ohm for some noise isolation if you wish. paul |
| PerranOak:
Brilliant, thank you very much. |
| StillTrying:
It's the large amount of negative feedback that gives the LM35's output less than 1R output impedance, which only works when it's powered up, and sourcing a small current. |
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