# DC load when ADC enabled **URL:** https://community.particle.io/t/dc-load-when-adc-enabled/4885 **Category:** Hardware **Created:** [June 5, 2014, 8:53pm UTC](https://community.particle.io/t/dc-load-when-adc-enabled/4885 "2014-06-05T20:53:41Z") **Posts on this page:** 1 **Showing post:** 2
### Author: ![bko](https://sea2.discourse-cdn.com/flex026/user_avatar/community.particle.io/bko/32/20704_2.png) [@bko](https://community.particle.io/u/bko) #### Post date: [June 5, 2014, 9:20pm UTC](https://community.particle.io/t/dc-load-when-adc-enabled/4885/2 "2014-06-05T21:20:21Z")
First off, you should be using a resistor voltage divider with a fixed resistor and the thermistor, or better yet a bridge to get good measurements. In order to speed up the ADC, the internal capacitor is charged to the middle of the supply rail. That way it takes half the time to do the conversion, either up or down from that point, than it would otherwise. This input impedance of the ADC is not that simple to model since it is switched, but 40k-100k ohm is a number to keep in mind. You have a couple of options: - Use a [10K](http://www.adafruit.com/products/372) rather than a 100K thermistor with another resistor in a divider. - Use an op-amp or instrumentation amp before the ADC input to buffer your high-impedance sensor (This is essentially built-in on the Arduino ATMEL parts). - Use a special purpose thermistor/thermocouple IC - Use an external ADC that you like better. Here is another thread were this was discussed with some links to op-amp circuits. > **[Low ADC Readings](https://community.particle.io/t/low-adc-readings/3385/2)** > > HI @carlvon Because of the way the ARM processor chip on the Spark core implements the ADC, the input impedance of analog input depends on how fast you run the ADC. More samples per seconds means a lower input impedance so there is a trade-off. ... --- _[View the full topic](https://community.particle.io/t/dc-load-when-adc-enabled/4885)._