XTR110 question

David,

Sorry, but it is not possible to make the XTR110 control two external MOSFETs to produce two identical currents. There is no way that the two discrete MOSFETs will match closely enough to provide the same current output. The variation in threshold voltage and transconductance of these transistors is certain to cause the two output currents to be significantly different.

Regards, Bruce.

David,

Sorry, I should have considered this approach using an additional op amp:

The input should connect directly to pin 1 of the XTR110. The +Vcc side of R10 should connect directly to pin 16.More distant connections may degrade accuracy. The op amp must have a common-mode range that extends to +Vcc and operating voltage equal or greater than the power supply for the XTR110. OPA137 is a possible choice. C1 and R11 might not be required depending on the characteristics of the op amp, transistor and load conditions.

Regards, Bruce.

I did a simulation of this circuit, and the current though AM2 is a stable 89.9mA.  Pin 1 is the built in source resister, is that the correct connection?

I must have missed the point of the circuit, I dont see how this helps?

David Prince 

David,

Yes, the input of this circuit comes from pin 1, connected to the internal source resistor.

This is a second current output circuit. The existing current output transistor is still connected and providing the same current output.  Is this clear?

Regards, Bruce.

Pin 1 does not vary at all as far as I know, so the opamp circuit would be a constant current.

How will the opamp circuit be the same current value as the current going though the current output transister hooked to pin 14?

David Prince

David,

The voltage on pin 1 (measured with respect to pin 16) varies in proportion to the output current that flows through the internal resistor, R9. The voltage on this resistor is replicated on the external resistor, R10, with the external op amp circuit. So the second output current will be the same as the first.

Is this clear?

Regards, Bruce.

I built the circuit, on my bread board. The secondary output starts out putting at 18mA or above, or .965v drop between pin 1 and pin 16. The opamp output 22.9v until 18mA, then the output drops. I am using an LM358AN for my opamp, and IRF9510 P Mosfets.

Am I missing the low end because of the opamp? 

David Prince

David,

As I indicated in the posting with the schematic, the op amp must have a common-mode range that extends to the positive power supply. The LM358 does not have the required common-mode range. I recommend the OPA137. Most common op amps that operate from a 24V power supply do not have this special common-mode range so the selection of this op amp is important.

Regards, Bruce.

I built the circuit with the OPA2137, the dual version of the OPA137, and it worked. The accuracy of the secondary current was off a little. So I put a pot in place of the 50 ohm resister, and tuned it to 52.2 ohms. At 52.2 ohms, the current tracks the primary within 2 decimal places. I did let the circuit warm up before tuning it.

Thanks for help

David Prince

David,

Thanks for the update. I was curious how this circuit worked for you. Variation of the two currents comes mostly from two sources:  1) mismatch of the internal and external 50 ohm resistors,  and 2) Offset voltage of the external op amp.

A constant offset in the current value over the output range is caused by offset voltage of the op amp.

A constant percentage error in the output current is caused by mismatch of the internal and external resistor. This can be minimized by making careful connections to the XTR110. The +Vcc side of R10 should connect directly to pin 16.More distant connections may degrade accuracy. Or, you can trim the resistor value as you have commented.

Regards, Bruce.