Mosaic
Well-Known Member
Hi All:
This is an empirical test I just did with on-hand parts to address using the LM317 as a good voltage regulator that has minimized temp. drift.
The spec sheet data indicates a 1% tempco drift. I had a circuit using 5.12V as a VCC reference for an ADC. However, I can get a drift up to 0.04V down as the LM317 warms. which impacts 2 decimal precision. This is made worse as I am measuring up to 20v and have a 4:1 voltage divider which quadruples the apparent precision error with any Vref drift.
Used 100°C hot air from 2" away from the TO-220 regulator to test this.
The adjustment resistor is setup as a 150Ω from Vout to the adjustment leg node feeding a 10K trimmer pot paralleling a 510Ω to ground. Adjusting the trimmer delivers the 5.12V. A 0.1uf cap sits across the trimmer for 'noise' control.
I have found by adding a 47K SMT thermistor (3960 beta) close to the LM317 (TO220) legs in parallel with the 150Ω resistor cuts the drift to .005V or by nearly an order of magnitude. This is a cheap solution as compared to a LM4040 4.1AIZ part to supply a solid Vref. It also saves a pin on my PIC ADC project, which is in short supply.
This is an empirical test I just did with on-hand parts to address using the LM317 as a good voltage regulator that has minimized temp. drift.
The spec sheet data indicates a 1% tempco drift. I had a circuit using 5.12V as a VCC reference for an ADC. However, I can get a drift up to 0.04V down as the LM317 warms. which impacts 2 decimal precision. This is made worse as I am measuring up to 20v and have a 4:1 voltage divider which quadruples the apparent precision error with any Vref drift.
Used 100°C hot air from 2" away from the TO-220 regulator to test this.
The adjustment resistor is setup as a 150Ω from Vout to the adjustment leg node feeding a 10K trimmer pot paralleling a 510Ω to ground. Adjusting the trimmer delivers the 5.12V. A 0.1uf cap sits across the trimmer for 'noise' control.
I have found by adding a 47K SMT thermistor (3960 beta) close to the LM317 (TO220) legs in parallel with the 150Ω resistor cuts the drift to .005V or by nearly an order of magnitude. This is a cheap solution as compared to a LM4040 4.1AIZ part to supply a solid Vref. It also saves a pin on my PIC ADC project, which is in short supply.