Specs for APFI0 on a 6251 DAC card

[MSU Spectrum Lab]

I have a new 6251 Multifunction DAQ and am trying to get up to speed on a couple of issues:

After downloading all the spec sheets and the M-series help I still cannot find a spec for the ranges on the APFI<0..1> for use as a voltage reference. The sheets say it can be used but not for what ranges and I'm still getting about 1mV of noise on a 1mV signal which seems to indicate that the output is refrenced to +-5V.

Also when I write a VI and simultaneously output a digital pulse and an analog pulse and compare them on a fast scope I see about 1ms of jitter in the analog signal rising edge. (I'm using the digital edge as a reference to trigger the scope.) Is there a way to use the fast time bases, 10MHz clock, 20MHz and 80MHz time bases to set the DAQ mx timing... no positive results so far.

Finally when connecting terminals and physical channels in LabView 7.1 many of the M series connections are not available. (For example APFI <0..1>) Or if I'm misunderstanding the use of these DAQ mx constants is there some help in English that explains what they are and how they are used? I have been through the help software for the last few weeks.

Thanks for the help.

Cal

[sergesl]

Hello MSU,

If you are wondering what the limits are on the signal that you can wire as reference voltage, beasically you can wire up to 10V. The problem though is that you need to keep the total voltage within the limits of the hardware. For example, if you had your reference at 5V, then you are only left with 5V to play with on the positive side before you exceed the 10V limit. This is valid on the negative side as well. So having a 5V reference means that you are left with -15V to play with from the reference.

Regarding your next question, the update rate for analog ouput has a limit of 2.8 MS/s for your device (6251). Using a faster timebase will not help you increase this rate beyond the specifications.

Your last question was regarding the channel constant in LabVIEW. Right-click on the edge of the drop-down and select I/O Name Filtering. Make sure that the option to include internal connections is checked. This will give you access to some internal signals. However, you will not be able to access the APFI lines from the software. These lines are meant to be driven by connecting an external signal to it.

Additionally, not all boards have APFI lines. Your board has only one (if it is a mono-connector board), APFI0. APFI0 is on pin 20 as you can see on the image below.

Hope I answered all your questions.

[MSU Spectrum Lab]

Serges,
Thanks for getting back to me. It sounds like you are talking about the output offset rather than the ouput reference. As I understand the specifications in the "NI 625x Specifications" document on p. 3 the output range of the 6251 is either +-10V, +-5V, or +-external reference on APFI <0..1>. The manual doesn't give me a range that is valid for those reference voltages. For example +-10V gives about 2mV/bit resolution, +-5V gives about 1mV/bit resolution... can I set the input refernence to 100mV and get about 50uV/bit resolution?

The other question about the timing jitter is still confusing me. If the sample update is at 2.8Ms/s then that corresponds to a minimum timing error of 1/(2.8e6)=... about 0.3 microseconds. I'm seeing about 1 milisecond which would correspond to a sample rate of 1ks/s. Much slower.

Thanks again.
Cal

[reddog]

Here are the specs for the external reference from the product manual:

External Reference
APFI <0..1> characteristics
Input impedance................................................ 10 kOhm
Coupling....................................................... DC
Protection
Power on.................................................... ±30 V
Power off .................................................. ±15 V
Range.......................................................... ±11 V
Slew rate ..................................................... 20 V/µs

[MSU Spectrum Lab]

Yes, I have that information in my copy of the manual. The specs for the Analog Out clearly state that the output range of the Analog Out can be set to +- external reference on APFI 0. What I need to know is what range of voltages, from 0V, will fix the output range on the Analog Out channels. The spec +-11V is no diffenent than the standard +-10V. I would get an accuracy of about 2mV/bit for that setting. Can I fix the Analog Out range to +-100mV by putting, say, 100mV on APFI 0 or is that out of the range that APFI 0 can use as a reference? Can it only take 1V to 11V, or 5V to 11V, or can it use a value less than 1V to set the Analog Output pins output range?

Thanks for your help, I'm afraid that I'm not clearly communicating the question I'm trying to have answered. I hope this is more clear.

[reddog]

You can use any reference voltage from 0 to 11 volts and the resolution of the device will be spread throughout that range. However, there are a couple of things to keep in mind. When using an external reference, the driver is still going to apply calibration coefficients as if you were using the internal reference source. Since this is generally a linear calibration, gain errors will likely not be a problem, but you could have offset errors. If this is the case, you have a couple of options. First, you can use raw writes through the driver and apply your own software calibration corrections to the data. Raw writes will bypass the scaling code that provide the calibration correction normally applied by the driver. Second, you can continue to use the scaled writes through the driver and apply a custom scale on top of this to provide the correct calibration constants for your external reference. To see what scaling coefficients are being applied by the driver, you can query the Device Scaling Coefficients property.