# Voltage drop when fed to ADC in MSP 430

We are trying to implement a battery monitor circuit in our project. We have a regulator in our design that supplies 3.3V. The logic we want to implement is to monitor the voltage and glow an LED if the voltage is less than 2.7V.

We are using a MSP 430 controller and we use the internal default reference for ADC which happens to be 2.5V. Now this became a challenge and so we put a voltage divider with a 1K and a 300 ohm resistor in series. We are trying to tap the voltage across 1K and feed the ADC through that. When the battery is full, the voltage drop across 1K was 2.5V as expected in the open circuit condition but as soon as we tap the voltage from across 1K resistor and connect to the ADC pin ,there is a substantial drop of 0.8V across 1K and hence my detection algorithm is not working as expected. Am I missing something here?

• This question belongs on Electrical Engineering.
– CL.
Mar 7, 2016 at 10:43
• I do not know, if I understood you correctly. One reason could be that if you use the same ground (GND) for the MSP and the volt. reg. and you tap the voltage around the 1k resistor then, you shorten the 300Ohms Resistor and your voltage devider losses it functionallity. Perhaps ?
– Peter Paul Kiefer
Mar 11, 2016 at 15:41
• Hello Peter, Thanks for the response. I will check if this is the reason but the controller should have a very high resistance and hence ideally should not disturb the voltage divider circuit. Apr 20, 2016 at 2:30

## 1 Answer

First, your resistors are very small and will suck your battery dry. Second, you need a capacitor near the lower resistor. It will provide enough current for sample and hold circuit of your ADC. Although 300R is fairly low, i would start with that cap.

• Thanks for the feedback. I will check by adding a capacitor to the lower resistor. Apr 20, 2016 at 2:31