How To Remove the Offset Voltage of Ethernet Shield?

Projects Networking, Protocols, and Devices
1

Hello all.

I am trying to make my Arduino to work as an online multimeter. At the beginning, I'm trying to make it a voltmeter. I'm simply trying this tutorial: http://tiny.cc/93515/.

I've done exactly the same as the tutorial I'm following, with three exceptions:

  1. I'm not using an LCD monitor.

  2. I'm using R1 = 25 kilo-ohms and R2 = 5 kilo-ohms.

  3. I'm trying to make it an online voltmeter, so there is an Ethernet shield attached to it.

  4. Instead of a 9 V battery, I've given Arduino's own 5 V source.

Now, whenever I'm measuring the voltage across 5 kilo-ohms with a regular multimeter, it is showing the regular voltage, nearly 0.8 volts. Therefore, the voltage of the Arduino 5 volts is measured almost correctly (0.8X6 = 4.8 volts).

But, as soon as I connect the point to the analog pin A1, the voltage across 5 kilo-ohms jumps to 1.9 volts! So, the serial monitor is showing 1.9X6 = nearly 12 volts!!!

So, I think that there is some offset to Arduino Ethernet shield, which is causing this problem.

Could anyone help me with this please?

Thanks in advance.

2

Did you measure the voltage between A1 and GND with the voltmeter?

3

SurferTim:
Did you measure the voltage between A1 and GND with the voltmeter?

Thanks for the tip. I powered on the Arduino with the USB, and it is showing 4.75 volts on the voltmeter.