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### Topic: Analog reading negative voltage... (Read 4176 times)previous topic - next topic

#### rmaurer

##### Jan 05, 2013, 05:09 am
My load cell unfortunately starts at a negative voltage (-8 to 0 volts depending on amplifier gain).  I dont believe the Arduino UNO will read negative voltages (i just get a value of 0).  My issue is compunded by the fact that when force is applied the voltage goes ABOVE 0 (up to +8v depending on amp gain).  So, my question is, how can i "reverse" a circuit while the value is a negative and then "unreverse" the circuit when it becomes positive again?

Thanks,
Ryan

#### Magician

#1
##### Jan 05, 2013, 06:16 am
Put 2 resistors as voltage divider, +5V - R1 - R2 - Ground. Connect arduino analog input to middle point R1 and R2.  Than take 3-rd resistor and connect to same middle point. Input is other end R3. Voltage divider form by R3 and (R1||R2). Example: R1 =  R2 = 2k. So (R1||R2) = 1k. Picking 9.1k as R3 and you have 10:1 divider. Now when input goes negative, arduino reads less than 2.5V, nothing connected = 2.5V and positive +8 - more than 2.5V Do a math.

#### dc42

#2
##### Jan 05, 2013, 03:38 pm
What amplifier are you using? If it's under your control, the easiest option is to adjust the gain in the amplifier and add an offset voltage to get the output in the range 0 to +5V.
Formal verification of safety-critical software, software development, and electronic design and prototyping. See http://www.eschertech.com. Please do not ask for unpaid help via PM, use the forum.

#### rmaurer

#3
##### Jan 05, 2013, 04:13 pm
Using the TI INA125P Amplifier...

Magician, can you post a diagram of that?

#4
Here you are.

#### dc42

#5
##### Jan 05, 2013, 04:48 pm

Using the TI INA125P Amplifier...

You probably have the IAref pin connected to ground. Connect it to the +2.5V reference output instead, as shown on page 13 of the datasheet http://www.ti.com/lit/ds/symlink/ina125.pdf.
Formal verification of safety-critical software, software development, and electronic design and prototyping. See http://www.eschertech.com. Please do not ask for unpaid help via PM, use the forum.

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