Hi guys! I want to measure 4-20mA and 0-10V analog signals with my microcontrollers ADC wich can measure in the range of 0-3.3V. This is my circuit, but since I have no previous experience in analog circuitries I am not sure at all whether this is correct or not. Can you guys help me in this matter. I know analog domain is not isolated. I would like to measure slow processes, like tempretaure.
I

 

hi GT,
I would recommend you remove the series diodes, you will get linearity problems.
On the 4-20mA remove the 3v3 Zener and add a series 10K on the non inverting input.
If you are concerned about over voltage inputs on the 0-10V, again add a series 10K.
E

 

In the 10V circuit, the forward voltage of D7 is going to seriously degrade accuracy. You could rely on the forward conduction of D8 to protect against negative voltages. I can't read the resistor values so I can't check them.

Note that for both circuits you will need an amplifier which is happy with 0V inputs and an output that can get to 0V.

 

I would recommend you remove the series diodes, you will get linearity problems.

The series diode won't be a problem in the 4-20mA circuit as it is a current input so the voltage drop of the diode doesn't affect the voltage reaching the amplifier.

 

In the 10V circuit, the forward voltage of D7 is going to seriously degrade accuracy. You could rely on the forward conduction of D8 to protect against negative voltages. I can't read the resistor values so I can't check them.

Note that for both circuits you will need an amplifier which is happy with 0V inputs and an output that can get to 0V.

Thank you! Divider should be fine. As for the 0V reading, is it a possible solution if I offset somehow the non-inverting input, so that at 0V the opamp will receive say 1V?

 

You might be able to do that but there are op-amps available with rail to rail inputs and outputs.

 

You might be able to do that but there are op-amps available with rail to rail inputs and outputs.

Well, that's a good point, thanks!

 

hi GT,
I would recommend you remove the series diodes, you will get linearity problems.
On the 4-20mA remove the 3v3 Zener and add a series 10K on the non inverting input.
If you are concerned about over voltage inputs on the 0-10V, again add a series 10K.
E

Thank you! However, I find op amp circuits a bit confusing yet, I need to digest how circuits work. Why is the series 10k resistor used instead of a zener?

 

You really do not need the OpAmp.
Just the termination resistor for the 4-20mA. Scale it so you get full ADC reading at 22mA, not 20mA, so you can tell if there is a fault. Add a 50mA Polyswitch or fuse to protect from shorted senders. Then from your termination resistor, have a series resistor, maybe 10K (check your ADC specs), to the ADC input with diode clamps for protection. Also some 10nF filter caps would be a good idea on the ADC inputs. Smaller if you need faster readings.
Similar for 0-10V.
Always have your ADC read a bit more than the expected input for error checking.
I have used that for many years quite successfully.