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Reading millivolts on Analog port


#1

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


#2

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


#3

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.


#4

What Gus said.

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


#5

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…


#6

Thanks for helping Lutz and welcome to the community.