PXI 6733 Slew Rate is slow

[Bob_Albern]

PXI-6733 claims a slew rate of 15V/us. I am seeing 2mV/us (that's right: 0.002V/us). I am generating a square wave using the Square Waveform.vi function and 1M samples per second generation. The hardware includes a 2 meter cable from the 6733 to a scope probe and another 2 meter cable going to a PXI-6220. The scope and 6220 agree and the displayed signals do not change when I remove one if the measurement instruments.

[Lorne H.]

Bob,

I do not think this a problem with your hardware.  I believe your VI is causing the issue.  Currently, your VI starts and stops the analog output task and the analog input task every iteration of your loop,  this will really mess up your signal.  Take a look at the attached VI were I do essentially the same thing you were doing, but I only start and stop the tasks once per task.

Lorne Hengst
Application Engineer
National Instruments

[Bob_Albern]

Lorne,

The sub-VI "Waveform Buffer Generation.vi" was not found when I loaded the program. It is close to but not quite the same as the square wave waveform generation vi I used. Is it a custom vi? If so, could you forward it along?

Thanks

Bob

[Lorne H.]

Bob,

Sure the VI is attached.

Lorne Hengst
Application Engineer
National Instruments

[Bob_Albern]

Lorne,

I'm using an event structure to control the instruments. As such, the 6733 is only stopped when I change a setting, otherwise it runs continuously. While changing settings I see a 100ms discontinuity in the output which is about what I would expect to reprogram the instrument.

Your program resulted in the same 2mV/us slew rate. For convenience I had plugged the cables into my load board which has no components in the signal path (I had removed the terminating resistors to my socket). When I removed the cable from the load board and plugged the scope probe directly into the end of the cable, I measured 4.5V/us. Much improved but still 1/3 the spec. I suspect if I measured the slew rate at the 6733 connector with no cable, I would get a result close to the spec.

This leads to the question: What termination does the 6733 expect? The data sheet gives only a stability range, not an expected termination. The scope load is 10 MOhms and 10 pF. Since the 6733 has a 0.1 Ohm output resistance, the scope is essentially an open. What termination would you recommend to maintain something close to the spec'ed slew rate at the end of the cable?

Bob

[Lorne H.]

Bob,

Your right, I should have looked at your code closer (your code should work properly).

Anyway,  could you give me a bit more information about the square wave you are generating?  (Buffer Size, Frequency of Square Wave, Amplitude of Square Wave, etc.)

Can you clarify what hardware you have hooked up to your board.  Are you using a National Instruments cable? do you have a National Instruments breakout box?

I am continuing to work on your problem.

Thanks,
Lorne Hengst
Application Engineer
National Instruments

[Bob_Albern]

Lorne, Travis,

After talking to Travis on the phone, I realized the 6733 output is an opamp. I changed my 2Vpp signal to a 10Vpp and the peak slew rate increased to 18V/us which comfortably beats the spec. As for the extremely slow slew rate I originally reported, the load board designer pointed out an additional cap I was not aware of on the signal line. After removing this cap, the slew rate on the board matched what I measured on the cable. From my point of view, this issue has been resolved.

Thanks for your help,

Bob