reading and logging external mV source

Hi, sorry if this will have a simple solution, but I looked in the reference section and couldn't find something similar. Also I'm not very good at electronics so I may get something wrong, please bear with me.

I need to read and log an output from a pachometer sensor that outputs a signal variable from 300 to 1300 mV DC. I need precision of at least less than 40 mV (I must be able to distinguish 320 mV from 360 mV). The output is through a 3.5 mm mono jack (so I have 2 wires). I am aware of the Vref pin but I have only this signal on output so I don't have a max V to use, so I suppose I must read the deltaV between the 2 wires.

How do I do that? My first thought was of connecting the lower potential to the Arduino GND and then read the positive with an analog pin but it sounds a very bad thing to do (mixing the gnd of the 2 circuits I mean).

So my second guess is reading the lower potential with pin A0 and the higher with pin A1 and then by difference get the delta V I need.

Can this be done? My main concern is the time it may take to read the 2 pin thus becoming not simultaneous and giving incorrect values.

So I am asking if this can be done and how, I can then experiment with the output values to see if they'll fit my use.

thanks.

Please post a link to the product page or the data sheet for the sensor. Note that voltage measurements cannot be made unless there is a common ground between a sensor and the measuring device.

i don't have a datasheet or I'd have posted it before, but the product is this: http://www.orpheusweb.co.uk/protovale/brochures/cm529.html

I have some data on the user manual, but unfortunately only in italian, here: https://www.dropbox.com/s/badatfscm9wm9ck/Pacometro%20manuale%20uso.pdf?dl=0[manual](https://www.dropbox.com/s/badatfscm9wm9ck/Pacometro%20manuale%20uso.pdf?dl=0). the notes are on page 37-38 of the printed pages (39-40 of the pdf). 766666

I'll try to give a roughly translated abstract here: "The instrument can be connected to any portable data logger unit [...]. The plug for the analog input to the data logger is on the back panel [...]. The logger output increases proportionally to the signal intensity - in this case the distance to the bar - but is elaborated to be directly proportional to the cover by an inverse law, so that the product of the cover (in mm) times the signal (in mV) is constant. So to convert the signal from mV to mm it must be applied: cover in mm = constant / signal in mV. The constant is a value between 32.000 and 32.768 V-mm and is set by the producer. The output signal therefore varies between 300 mV and 1.27 V and is suitable to be managed by any data-logger with analogic input normally available on the market, included models suitable for reinforcement corrosion surveys (potential measurements). [...] The conversion table below shows some typical values: cover in mm output in mV

That's all I have on the subject.

How does the commercial data logger read the signal? Can I replicate that with an Arduino?

Could the differential reading capabilities of the Leonardo and MEGA be useful?

If the datalogger is portable (i.e. battery powered) then only two terminals from the measuring device are needed. One terminal defines "ground" or 0V and then the voltage difference to the other can be measured. An Arduino would work for this.

The central concern is that either or both the measuring device and the logger must be completely isolated from AC line voltages. Otherwise you run into serious difficulties with so-called ground loops.

Since the Arduino can measure only positive voltages, you would connect the output terminal at the lower potential to the Arduino ground, and the other to the ADC input. If you have available a reference voltage source that is slightly higher than the maximum expected sensor output voltage, then you can connect that to AREF on the Arduino and maximize the precision of the ADC conversion.

jremington: If the datalogger is portable (i.e. battery powered)

You mean the measuring device? Otherwise I don't understand what you mean. The measuring device is battery powered and since it's used for field surveys it will be while in use.

jremington: then only two terminals from the measuring device are needed. One terminal defines "ground" or 0V and then the voltage difference to the other can be measured. An Arduino would work for this.

The central concern is that either or both the measuring device and the logger must be completely isolated from AC line voltages. Otherwise you run into serious difficulties with so-called ground loops.

So you mean I'll have to operate the Arduino on battery when measuring? That was one of the goals of the projects (to be able to use it on the field) and shouldn't be a problem, apart from some problem in the testing (disconnecting USB after every upload, connecting batteries, etc.).

What if I use the Arduino powered from a portable device USB (MS surface)? Will that count as "battery power" in terms of grounding?

jremington: Since the Arduino can measure only positive voltages, you would connect the output terminal at the lower potential to the Arduino ground, and the other to the ADC input.

OK, that'd be great.

jremington: If you have available a reference voltage source that is slightly higher than the maximum expected sensor output voltage, then you can connect that to AREF on the Arduino and maximize the precision of the ADC conversion.

Yes, that part was more clear to me. Thanks for the help.

If EITHER the measuring device or the datalogger is portable (battery powered) you are OK.