Reading millivolts on Analog port

Hi, I am trying to read millivolts from a pH Probe on one of my analog ports.

pH probe produces voltages of between -440mV and +440mV depending on the pH of the solution it is in.

2 questions. Can the analog ports read negative voltages or only 0 to 3.3?

and is it sensitive enough to read millivoltes like this or should they be amplified. Typically I will be reading pH in the 8 range so the mV produced by the probe would be around 0 to 100mV

Thanks in advance
Martin

You need to add a circuit to scale the voltages to 0-3v

With that input voltage range you might want to look at ICs deigned for signal conditioning for current measurement. One method of measuring current is to use a very low value of ‘shunt’ resistor in the current path and to measure the voltage drop across the resistor. Since you want to use a small resistor so as to not waster a lot of power the voltage range being measured across the shunt resistor is in the millivolt range.

I know Avago makes some nifty instrumentation amps that might work.

What Gus said.

You need to use an opamp to scale the voltage to the full range values of the ADC.

Hello Martin,

the signal of a standard pH probe has an extrem high impedance. You need a voltage follower ( http://en.wikipedia.org/wiki/Operational_amplifier_applications#Voltage_follower_.28Unity_Buffer_Amplifier.29 ) build with a FET-OP (ie. LF351) first.
Do not route the input of the OP over the pcb. Connect IC-pin and the connector of the pH probe ( BNC ) direct with a short wire! A little bit of dirt at the pcb could ruin the sgnal.

The next is shifting and amplifying the signal from ± 400mV to 0 - 3V
( http://en.wikipedia.org/wiki/Operational_amplifier_applications#Summing_amplifier )

greetings,
Lutz

Please excuse my limited english…

Thanks for helping Lutz and welcome to the community.