Hardware Recommendation: Signal conditioning, Constant Current Excitation Voltage Needed

[mschmit]

I have a hardware recommendation question.  Here is my current setup:

PXI 1040 Chassis with:
- PXI-8187 controller running RT
- PXI-6259 M-Series DAQ (32 AI, 48DIO, 4AO, 1.25MS/s)
- PXI-6132 S-Series DAQ (4AI, 3MS/s)

I have slots for both PXI and SCXI modules in the chassis.

I am using an ICP Accelerometer (W356B11) that requires a constant-current excitation voltage:

Excitation Voltage	18 to 30 VDC	18 to 30 VDC
Constant Current Excitation	2 to 20 mA	2 to 20 mA

I would like to use this sensor with the PXI-6259 card.  As far as I can tell, my hardware cannot provide this sort of output.  Can anyone recommend an NI solution that can utilize my existing hardware, minimize cost, and provide the adequite specifications.  I know that a separate SCXI-1100 module exists that will work, but this has an onboard DAQ system that is unessesary with my current setup.

Thanks for the feedback.

-Mike

[gwd]

Sure enough that's what it SAYS (2-20 mA) but it doesn't make sense (or isn't complete) as written.

If it requires power it must have some internal electronics. The manufacturer expects you to use this with his own power supply/ electronics. Doesn't mean you need to, but they may not be particularly helpful in giving you enough info to do this yourself.

I'm GUESSING that you supply 18-30 VDC to the X,Y,Z pins and that the output signal is the current drawn, which will vary from 2-20 mA.

With common NI DAQ boards, you can only supply 10 V with respect to ground.

[mschmit]

Yeah, I think you're right that the PCB people want me to use their own proprietary power source.

Digging through some of the documentation, I came across this wiring diagram:

The sensor is attached at the "Sensor Output", which is a BNC connection.  I assume the "To Readout" is where my DAQ would be connected.  I personally think that shelling out for a SCXI-1100 is overkill here.

Hopefully this will give a little more insight into what they are thinking.  I sent email to the support contact at PCB to see what they said, but I figured I'd ask the NI Community since I have found that the best help is usually here.

Message Edited by mschmit on 04-02-2007 09:42 AM

[gwd]

That C.C. diode...seems likely that it IS constant current. There are no constant current diodes but a depletion mode FET with gate connected to source would approximate one.

I would try this circuit configuration with a 25- 30 V source and with a resistor replacing the "CC diode". Choose the resistor value to get about 5 V drop. And lose the meter unless you care about the DC voltage.

[johnsold]

You can buy current regulating "diodes." Internally they are FETs connected to provide constant current over a fairly wide range of applied voltages. Typical currents are from 0.2 mA to about 5 mA. Siliconix J502 .. J510 for example.

Lynn

[gwd]

Didn't know that, but it makes sense. The potential liability with using such a device... the voltage output would depend on both the output resistance of the device and the output resistance of the accelerometer. However they spec the output resistance as 200 ohms...which is very low...so this shouldn't be a problem.