Current to voltage converter using single supply OP-AMP

[Anantha Krishna]

Hi,

I am trying to interface a pressure sensor with 4-20 mA output with an ADC. So I have to convert this current to voltage.

But the problem is that I have a single supply of 5V and I want to use LM358 single supply OP-AMP for this conversion. I successfully implemented the circuit using dual supply(+5V and -5V) in inverting amplifier mode, but I was unable to implement using single supply(+5V).

Please suggest how to implement the same circuit in non-inverting mode, so that I can convert this 4-20 mA to +ve voltage.

Thank you.

 

[alexan_e]

Can you explain what was the problem with the configuration you tried?

You have a 4-20mA source, you can use a resistor to convert it to a voltage and a differential amplifier to apply an offset and amplify the voltage or maybe you don't need the offset if the output is going to a microcontroler ADC and can be compensated in the code.

Here is a differential amplifier
]

Alex

 

[godfreyl]

The opamp's input common-mode range includes ground, so you could use a circuit like the one below. The sensor's connections may need to to reversed, depending on whether it sources or sinks current.

 

[Anantha Krishna]

Hi,

I am thinking of two ways to implement the above circuit.

First is in the first image, where I use a resistor to convert current to voltage and use Op-Amp buffer.

Second is in the second and third image, where I use a differential amplifier to shift my output voltage to get better range for my adc, and use the formula mentioned in the image to calculate the output voltage. I think the circuit in Ck2 is much better than the circuit in Ck1.

Am I correct? Please suggest.

 

[FvM]

A typical 4-20 mA current loop configuration uses a positive supply of 15 - 30 V and a ground referenced shunt. Self powered 4-20 mA outputs are often ground referenced.

But its's not the preferred way to pass the full loop current through the input amplifier. Many PLCs are simply using a 250 or 500 ohm shunt as passive I-V converter and no amplifier at all. Alternatively you can use a lower resistance value (e.g. 50 ohm) to reduce shunt power dissipation and achieve better loop supply voltage margin. Then a single supply non-inverting amplifier can scale the shunt voltage to the intended range and optionally apply an offset to get zero output at 4 mA.

P.S.: I see, that your latest post is going the same way. All circuit variants are basically correct.

 

[godfreyl]

I think the type of circuit to use depends on the kind of sensor you have. Is it OK to just pass the current through a resistor to create a voltage drop, or should the sensor be connected to a virtual ground?

 

[Anantha Krishna]

The sensor has to be provided with a supply and ground.

I want to use the op-amp to reduce any non-linearity that might occur. The op-amp buffer or differential amplifier will reduce such possibilities and provides good output impedance.

 

[FvM]

The sensor has to be provided with a supply and ground.

You mean, it's a three-wire sensor: V+, Gnd and current out?

 

[godfreyl]

Yes, as shown in the pictures in post 4.