I using an Arduino uno analog read function to read a voltage divider circuit for a voltage range from 0-70 VDC. On the voltage divider, I am using 2.7Kohm and 36kohm to reduce to voltage range from 0-70VDC to 0-5VDC for arduino analog input.
The circuit works fine with the USB power to the arduino, and as soon as the I disconnect the USB, and the arduino switches to the backup dual switching power supply (120 VAC to 12VDC and 5VDC), the analog values change.
The values are still steady and I dont see any garbage values, but there is a big offset between the original values and the values after disconnecting the USB power to arduino.
Can anyone comment with a solution to this? I checked multiple times and the ground is common through out the circuit.
Electronic – Arduino ADC values change after USB disconnect
adcarduinoswitch-mode-power-supplyvoltage divider
Related Solutions
In this answer I showed that the optimal series resistor value is
\$ R_S = \sqrt{R_{MIN} \cdot R_{MAX}} \$
where \$R_{MIN}\$ and \$R_{MAX}\$ are the minimum and maximum resistances of your sensor. The Sparkfun page speaks of between infinity and 300 kΩ. If we assume 10 MΩ as maximum value then your series resistor should be 1.8 MΩ (rounded to nearest E12 value).
These are pretty high values, too high for an ADC, which, like Mathieu says, likes a impedance of less than 10 kΩ. So you'll need to buffer the divider with an unity gain buffer opamp:
Note that a common opamp may have an input bias current as high as 1 µA, and this could distort your reading. A CMOS opamp like the MCP600x has a much lower input bias current, 1 pA typical for the MCP600x, which won't deteriorate your reading.
The capacitor is there to "AC-couple" the input. This removes any DC component from it, allowing only the AC in the waveform to pass through.
This circuit isn't feeding the 5V supply, it's drawing from it. The voltage divider biases the input at 2.5V, and the AC-coupled signal causes it to vary from that voltage in either direction.
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Best Answer
Read the Atmel Datasheet for the '328
Section 28.2 cover the A/D The inputs look like this:
By default on the Arduino Uno the ref voltage is taken from AVCC which is the 5 V VCC used by the MCU.
The Arduino schematic is here.
You can program the unit to not use AVCC/1.1ref and feed in a more stable reference ....or fix your supply voltage.
You could also switch to the 1.1 V reference and change your input divider.