How to supply a 1mA input and read a mv output proportional to the input current

Mooneyguy

Is there a way to create a constant 1 ma input to a sensor and then read a mv output that is proportional to the input current? I am wanting to run a pressure sensor and the supplier says the nominal input is 1.0 mA (1.5 ma max) and the output is 0 to 50 mv but it proportional to the input current. I would like to store the input and output data to make decisions for control of another device. I would like to store the last 10 readings in such a way that they will be in memory when the board is disconnected from power and then powered back up again. The schematic of the pressure is located at this website: http://meas-spec.com/downloads/23_33_Std.pdf

Phil Pilgrim (PhiPi)

Hey Mooneyguy,

I know you're pretty new to the forum -- welcome, by the way! -- so you get a bye this time. But, in the future, please be aware that posting the same question in more than one location is against forum rules. M'kay?

Thanks,
-Phil

Mooneyguy

Ok, sorry about that. I put it in the first location and then thought it would be better in the sensor location but did not know how to transfer it over along with your comment. Thank you for both comments: LM713L from National Semi for the constant current input and the heads up on the double posting. I will do better next time. I did look at the LM713L and did not see it speced to go as low as 1.0 to 1.5 ma (I could very likely be missing something though). Can I just put the 5V from my Stamp into this device and get 1.5 ma from it?

Phil Pilgrim (PhiPi)

I stand corrected regarding the LM317L. It works reliably as a current regulator only down to 5 mA, apparently. Here's what might work. Use two LM317L's: One to source 6.5 mA; the other, to sink 5 mA of it to ground. That leaves 1.5 mA for your bridge circuit.

BTW, if you ever want to have one of your threads moved, just click the little triangular "!" below your first post, and send a request to the moderators to move it for you.

-Phil

Tracy Allen

The venerable LM334 is an adjustable current source. To make it temperature stable you add a diode and two resistors. The data sheet has the exact circuit for 1 mA on page 7. The LT3092 is another possibility, but I think it is only available in surface mount. My choice would be to make the ground referenced current source for this sensor from an op-amp/reference, the LM10.

Mooneyguy

Thank you. I think the LM334 just might work out for me. I will give it a try.

Tracy Allen

The resistance of that bridge is 4400Ω typical or 6000Ω max, so with 1.5V thru it it will develop between 6.6 and 9 volts. An LM334 current source takes about 2V of overhead, so the power supply will need to be something like 10 or 12 volts. Maybe you knew that, but I thought I'd mention it just in case...

If you end up with the quad op-amp that came up in your other thread, then it is possible to use that with a voltage source to regulate the 1.5 mA current for the bridge. Circuit attached.