# Measuring voltage

Hi,

I’m looking to hook up a pressure transducer to a fez spider board. I had first thought to shop for a transducer that indicates pressure by varying output current and using the ACS712 to read the current, but I’m having a hard time getting a good reading from that module.

I’m now considering a breakout module and hooking a transducer like this one(http://www.omega.com/Pressure/pdf/PX309-5V.pdf) to one of the analog input ports.

Am I correct that the analog ports can measure a voltage of 0-5 volts?

Thanks,
John

Did you see my other posting on this?

You can read 4-20mA current output sensors with the addition of 1 precision resistor connected from ADC input and GND.

@ jhoge123

The EMX ADC’s high reference voltage is VCC (3.3V). It will not measure voltages above this voltage and will probably saturate (show 0x3FF) for any voltage above VCC.

If you need to measure voltage above VCC, you’ll need to scale down the measured voltage with a voltage divider.

One thing to remember when scaling down analog samples with a resistive divider is input impedance.

Too large of resistor value will cause wrong readings because the ADC’s holding capacitor will not fully charge. Good practice is to buffer the voltage divider with an op-amp.

S.B.D

1 Like

Thanks. I decided to go with a slightly bigger range on the pressure transducer so the measured voltage will not exceed 3.3v.

That’s not the way to do it. Going with a bigger range means your resolution will be lower. I hope your design can live with this reduced resolution?

Do what I suggested and use the existing current output transmitter and 1 simple 100 ohm resistor and you won’t have to worry about the 3.3V max input as the max will now only be 2.0 volts.

Even just to prove it works, find a basic 100 ohm resistor, any % will do and test it as I assume you still have the current output sensor?

@ Dave McLaughlin is right when using a current loop type sensor. So for a typical 4-20mA output sensor use a 3.3V/0.02A = 165 Ohm resistor.
I hope my post didn’t confuse too much as it referred to voltage output sensors such as 0-10V output.

@ S B D, good choice on that resistor. It gives a greater voltage swing which would be more ideal with the 12 bit ADC. I use a 16 bit ADC with 2.048 V max input so the 100 ohm is ideal with this.