I am trying to convert 4-20 mA current loop signal to 0-3.3/0-3 V output for my 10 bit ADC IC that is connected to a Raspberry pi 3B+.

Schematic:



The input sensor is a 12 V loop powered 4-20 mA output humidity sensor. The op-amp is an MCP6002 rail-to-rail. The output of the op-amp is connected to a 10 bit ADC-MCP3008 (SPI enabled connected to RPi.)

The output voltage only varies from 0.6 to 2 V and I don't know where exactly the voltage drops.

The circuit works fine if I read the voltage using only a 150 Ω resistor (0-3 V.)

I can't understand what's wrong in this circuit.



Prior answers related to this question didn't give the solution. Please help me to find a reliable solution.

I also have same problem to convert 4-20ma into 0-3.3v but i am connecting it to stm32 and i want 0.1mv prescision so how can i get that result


Mod: link to old thread.
https://forum.allaboutcircuits.com/...-using-rail-to-rail-opamp.194166/post-1826317

 

Hi HT,
I guess you know, a 10 Bit ADC is only 1023 Counts?
So that is 2.9mV/Count resolution.

What is the maximum count of your ST32?
E

Checking, I see a 12Bit ADC for the ST32

 

i am connecting it to stm32 and i want 0.1mv prescision so how can i get that result

Hi HT,
For the ST32 with a 12 Bit ADC , 4095 Counts, that gives a resolution of 3V/4095 =0.7mV.

E

 

For 0.1mV resolution you need a 15 bit A/D converter, maybe a TC850 from Microchip. But I do wonder why you are looking for such high precision and whether the reading will be dominated by noise? Maybe oversampling with the 12 bit A/D would suffice?

 

For 0.1mV resolution you need a 15 bit A/D converter, maybe a TC850 from Microchip. But I do wonder why you are looking for such high precision and whether the reading will be dominated by noise? Maybe oversampling with the 12 bit A/D would suffice?

Thnks for the reply i know that stm32 has 12 bit adc and i am aware of it. My aim is to convert the 4-20ma or 0-20ma analog signal to the voltage level of adc of stm32. I want that much precision because i want to see the changes of every 0.1mv and i want that much precision with the help of op amp so can you please guide me about that how can i get that much precision.

 

hi HT,
This is the 4mA > 20mA A2D convertor, 0V through +3.3V

E

 

Thnks for your information but if i want to make this happen without the use of potentiometer how is this gonna make it?

 

Also i want to say that if we direct give 3.3v to opamp so we don't need voltage follower circuit am i right or wrong? if wrong please tell me

 

And If possible please someone give me the simulation for proteus

 

I want that much precision because i want to see the changes of every 0.1mv and i want that much precision with the help of op amp

Thnks for your information but if i want to make this happen without the use of potentiometer how is this gonna make it?

Hi HT,
You require 0.1mV precision, consider the tolerance values of the components you are using.
I would say even using all 0.1% tolerance you will not get a precise 0mV through +3.3V for a 4mA to 20mA drive input.
You need some method of trimming the vr Offset Voltage, also some way of trimming the Gain/Slope of the OPA, the 500R pot.

Also i want to say that if we direct give 3.3v to opamp so we don't need voltage follower circuit am i right or wrong? if wrong please tell me

The vr Offset voltage requires a stable, low output impedance source.

Your target of 0.1mV for the I2V >> ADC project requires a stable design.

E

 

Mcp6002 has input offset voltage around 4.5mv but in my information for 0.1mv precision input offset voltage have to low as 100uv around so can you suggest the op amp whose offset voltage is near to 100uv.

 

You need some method of trimming the vr Offset Voltage, also some way of trimming the Gain/Slope of the OPA, the 500R pot.



The vr Offset voltage requires a stable, low output impedance source.

Your target of 0.1mV for the I2V >> ADC project requires a stable design.

E

So you want to say that for my need of 0.1mv precision i have to trim the Vr offset voltage for stable design but with this triming method can i get this Vr offset voltage near to uv range if i am wrong please guide me

 

i get this Vr offset voltage near to uv range if i am wrong please guide me

hi HT,
I would say to maintain uV will not be feasible with a basic design.
E

 

You need some method of trimming the vr Offset Voltage, also some way of trimming the Gain/Slope of the OPA, the 500R pot.



The vr Offset voltage requires a stable, low output impedance source.

Your target of 0.1mV for the I2V >> ADC project requires a stable design.

E

So you want to say that for my need of 0.1mv precision i have to trim the Vr offset voltage for stable design but with this triming method can i get this Vr offset voltage near to uv range for 0.1mv precision

 

hi HT,
I would say to maintain uV will not be feasible with a basic design.
E

i try to make a circuit according to my information for my purpose can you please verify it and if some changes have to make can you please tell me

 

hi HT,
This is your circuit in LTSpice.
E

 

Ok so what changes i have to get for 3.3v with 0.1mv precision

 

hi HT,
This is your circuit in LTSpice.
E
View attachment 339798

Why you not connect capacitor for filtering can i change resistor value for feedback and input resistance at inverting amplifier

 

i want to see the changes of every 0.1mv

Do you need an absolute accuracy of 0.1mV or just a precision (difference between measurements) of 0.1mV?
Such accuracy is much harder to do than precision, since precision is not affected by a fixed offset voltage.

 

Do you need an absolute accuracy of 0.1mV or just a precision (difference between measurements) of 0.1mV?
Such accuracy is much harder to do than precision, since precision is not affected by a fixed offset voltage.

As we know stm32's adc has 0.8mv resolution i want that after converting 0/4-20ma to 0-3.3v it give me 0.1mv precision as i want to see changes at every 0.1mv and from above circuit is there any need or modification on that circuit to achieve my goal?