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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".

schematic

simulate this circuit – Schematic created using CircuitLab

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    \$\begingroup\$ 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. \$\endgroup\$ – Maple Jun 28 '18 at 19:11
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    \$\begingroup\$ Replace the thermistor with a 10K resistor and test again. \$\endgroup\$ – Spehro Pefhany Jun 28 '18 at 21:20
  • \$\begingroup\$ 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. \$\endgroup\$ – AP Shwarts Jul 2 '18 at 15:44
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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.

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