I have attached a drawing of a 4-20mA current loop circuit. This has been used by my colleagues in the past, however I am having some trouble with oscillations on my output.
I'll give a quick explanation of the circuit.
Anyways, I've made all sorts of attempts to stop the circuit from ringing. I've added bypass to components, added filter caps to the feedback, moved my transistor from the 12VDC rail to 24VDC (which helped significantly). I've changed the opamp and transistor to different components and have changed some ceramics over to tantalum capacitors. I am just wondering if anyone out there sees anything grossly wrong with the circuit or has any suggestions for improving things. I don't have a strong analog background and am not sure where to go from here.
On a side note, the circuit itself is on a 2 layer PCB that is roughly 2"x2". The opamp circuit has a tight feedback loop.
When applying a 3.0V to Vsignal, I'll get a >2V output oscillation in the 1-3kHz frequency. I am able to change the frequency and the shape of the oscillations, however they are still significantly large.
Please let me know what additional information is necessary to understand what is going on. Thanks in advance.
I'll give a quick explanation of the circuit.
- The circuit is designed using negative feedback to control a transistor.
- Let's say floating ground is at 0V for discussion purposes. Therefore earth ground will have a negative voltage.
- Based on ideal opamp priciples, connecting the negative input to 0V should create a virtual short and also connect the positive input to 0V.
- Setting Vsignal will generate a small current across Rset. (Vsignal - NULL_NODE) / Rset = Iset
- Assuming infinite input impedance, all the current will flow across Rbias.
- The voltage drop across Rbias will equal the voltage drop from floating ground across Rsense. (NULL_NODE - VOUT) / Rbias = -VOUT / Rbias
- Iout = Ibias + Ilimit + Iquiescent, where I quiescent is the current sunk into the floating ground.
Anyways, I've made all sorts of attempts to stop the circuit from ringing. I've added bypass to components, added filter caps to the feedback, moved my transistor from the 12VDC rail to 24VDC (which helped significantly). I've changed the opamp and transistor to different components and have changed some ceramics over to tantalum capacitors. I am just wondering if anyone out there sees anything grossly wrong with the circuit or has any suggestions for improving things. I don't have a strong analog background and am not sure where to go from here.
On a side note, the circuit itself is on a 2 layer PCB that is roughly 2"x2". The opamp circuit has a tight feedback loop.
When applying a 3.0V to Vsignal, I'll get a >2V output oscillation in the 1-3kHz frequency. I am able to change the frequency and the shape of the oscillations, however they are still significantly large.
Please let me know what additional information is necessary to understand what is going on. Thanks in advance.
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