# Voltage divider : using board voltage vs Power supply

I am using an ncd relay power switch that is capable of reading 8 sensors.

When I use the 5V and GND provided by the board I get a Vo=3.46V. If i switch R1 to 100k I actually get Vo= 2.7.

When i use an external 5V and GND provided by a power supply I get Vo=2.6, still using R1=10k here.

Question: Why am I getting different voltage values from the two different power sources. Why am I having to switch R1 to 100k to get the proper Vo when using the relay board? Is there some voltage reference I need to take into account?

According to curstomer support "the ADC inputs on that board float(they are not pulled either direction".

simulate this circuit – Schematic created using CircuitLab

• Are you sure your thermistor stays at the same temperature between measurements? Because if you do any soldering to replace that R1 you most likely affect thermistor too. Commented Jun 28, 2018 at 19:11
• Replace the thermistor with a 10K resistor and test again. Commented Jun 28, 2018 at 21:20
• I did that and my Vout is 3.34. If i dont connect to the AD input i get the proper voltage of 2.5v. Commented Jul 2, 2018 at 15:44

The input impedance of an ADC with an analog mux can be pretty low. Typical microcontrollers recommend a source impedance no more than tens of kilohms. Changing R1 to 100k$\Omega$ may lead to a situation where the input impedance of the ADC changes the measured voltage. Either stick with low value resistors or add a unity-gain buffer between the thermistor and the ADC.
The "5V" provided by a typical board can easily vary by $\pm$5%. If you are relying on voltage dividers for precise measurements you need to give them a precise voltage to begin with. Consider using a precision voltage regulator rather than either of the 5V sources you've mentioned.