OP777 4 to 20 ma

Hi Dica,

The circuit you have is a an example of a Howland Current Pump/Source. The approximate equation of the output current Io is:

                    Io = Vin x [(1/27kΩ)+(1/68Ω)]

The equation is true when your 27kΩ resistors (R13, R14, R15, R19) ansd 68Ω resistors (R18, R21) are tightly matched. To minimize errors due to mismatching of resistors, you may want to use an integrated Difference Amplifier like the AD8276. This solution also minimizes component count, thus reducing board space and probably cost as well. And as what Harry mentioned, the transistor Q1 boost the output current since the amplifier has a limited current drive.

For detailed discussion on AD8276 current sources, you can read page 22 of this article. Figure 5 (page 23) is similar to the circuit you presented.

I hope this helps.

Best regards,

Neil

Hi, NeilJ and Harryh, thanks so much, really.

Now i understand the circuit and i know the name hah.

Really Thank!