XTR111: Current loop referenced to VSP rather than 0v

Kevin Krause

Part Number: XTR111

Tool/software:

Is the XTR111 capable of driving a current loop which connects between VSP and the driver output stage, rather than between output and 0v?

I am upgrading a very old design where the customer prefers to sense the voltage across a resistor between o/p & 0v and the loop o/p is on the high-side of the drive circuit.

I was thinking of using the 15-ohm low-side resistor in your examples as the sense resistor - perhaps.

over 1 year ago

0 Robert Clifton56 over 1 year ago

TI__Mastermind 28160 points

Hi Kevin,

What is the supply voltage for VSP?

Also would VSP be loaded to the output stage as shown?

Best Regards,

Robert Clifton

0 Kevin Krause over 1 year ago in reply to Robert Clifton56

Prodigy 30 points

Hi,

VSP is probably going to be +24v.

As for VSP connections: probably as per your drawing.

We just need a way of maintaining correct current mirror/driver functionality.

Rgds,

Kevin

0 Robert Clifton56 over 1 year ago in reply to Kevin Krause

TI__Mastermind 28160 points

Hi Kevin,

I don't believe that will work. The Is pin can only go up to 22V max. That would leave the load at a higher voltage potential, preventing the current from flowing in the correct direction.

Kevin Krause said:
I am upgrading a very old design where the customer prefers to sense the voltage across a resistor between o/p & 0v and the loop o/p is on the high-side of the drive circuit.

Can you please help clarify this for me. Does the customer still wants to see the load go to 0V?

Best Regards,

Robert Clifton

0 Kevin Krause over 1 year ago in reply to Kevin Krause

Prodigy 30 points

Hi Robert, The original circuit had the loop connecting between V+ (actually +15v) and the drain of the original output stage's upper node. The lower node (source pin) ran through a sense resistor to 0V

If I can't do it this way (and the customer is happy to), I'll just connect a current sense amplifier across the sense resistor - wherever it sits in the final circuit.

Regards,

Kevin

0 Robert Clifton56 over 1 year ago in reply to Kevin Krause

TI__Mastermind 28160 points

Hi Kevin,

Kevin Krause said:
The original circuit had the loop connecting between V+ (actually +15v) and the drain of the original output stage's upper node. The lower node (source pin) ran through a sense resistor to 0V

Do you happen to have the schematic of this circuit? It'll make it easier for me to understand.

Thanks,

Robert Clifton

0 Kevin Krause over 1 year ago in reply to Robert Clifton56

Prodigy 30 points

I will scan it when I'm back in the office tomorrow. It goes back to the 1990's - lots of individual DACs, op-amps, discretes!

Regards,

Kevin

0 Kevin Krause over 1 year ago in reply to Robert Clifton56

Prodigy 30 points

AE892_7_3_schematic.pdf

0 Robert Clifton56 over 1 year ago in reply to Kevin Krause

TI__Mastermind 28160 points

Hi Kevin,

You weren't kidding with that when you said there's lots of discrete devices!

Actually this implementation looks very similar to the 2-wire design compared to a 3-wire, which is what the XTR111 is. You're making a lot of changes so it's possible that a 3-wire 4-20mA current transmitter fits in their design but I wanted to make sure you were aware of this!

Best Regards,

Robert Clifton

0 Kevin Krause over 1 year ago in reply to Robert Clifton56

Prodigy 30 points

Hi Robert,

Thanks for highlighting the 3-wire variant. I had skipped over that - I'll have another look at it.

I'll mark this as resolved for now, and see where I end up. I'll probably purchase an evaluation board to experiment with before I commit to a particular design.

Thanks and regards,

Kevin

+1 Robert Clifton56 over 1 year ago in reply to Kevin Krause

TI__Mastermind 28160 points

Hi Kevin,

I'm glad I could help! Let me know if you have any additional questions!

Best Regards,

Robert Clifton

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XTR111: Current loop referenced to VSP rather than 0v