Error Analog Output(sine wave) of PCI-6111E?

[PCI6111E]

Symptom:In MAX2.2, "Test Panel" is set as follow.
"Analog Output"
"Output Mode"---Sine Generation
"Channel"-------0 or 1
"Sinewave Amplitude"----5.00V

Update(Input in the pannel)
1===3750000
2===3500000
3===3250000
4===3000000
5===2500000
6===2000000
7===1500000
8===1250000
9===1000000
system help for "Update":Select the DAC update rate while in Sine Generator
mode in this selection box.The actual sine wave presention the output
is 1/1000 of this rate,since the sine wave is composed of 1000 points)

cycles of the output (computed from the measurment of oscillograph:Agilent 54641D)
1===2500Hz
2===2500Hz
3===2440Hz
4===2440Hz
5===2440Hz
6===1666Hz
7===1666Hz
8===1220Hz
9===1000Hz

The "actual" si
ne wave presention the output is "not always" 1/1000 of the update rate,
even not a direct proportion is found,that is
3750000 3500000 3250000 3000000 2500000 2000000 1500000 1250000 1000000
2500Hz 2500Hz 2440Hz 2440Hz 2440Hz 1666Hz 1666Hz 1220Hz 1000Hz
why?anything is wrong?

[Christian_L]

This is normal behaviour. The cause is the fact that the rate of the analog output generation is based on a reference clock and is determined by dividing down this reference clock by an integer value. Therefore the granularity of the output frequency at higher frequencies can be rather large.

I believe the base clock for the AO is 20 MHz so the update rate will be a value which is calculated by an integer division of that rate (e.g. 5 MHz, 4 MHz, 3.33 MHz, 2.5 MHz, 1.66 Mhz, 1.25 MHz, etc.). There is also a maximum output frequency which prevents output rates above a certain limit.

So you are seeing the maximum output rate at your faster rates and the effect of the granularity for the slower rates. In addition I think you have a bit of a measurement
error on a few of your measurements. I suspect the 2500 and 2440 Hz sine wave are the same, and the 1220 Hz sine wave is really 1225 Hz.

To generate sine waves at higher or different frequencies you will need to reduce or vary the number of samples per cycle used to generate the output.

Christian L
NI Consulting Services

[PCI6111E]

The Maximum update rate of the Analog Output is:
("MEASUREMENT and AUTOMATION 2003" Page-231)
1 channel===========4MS/s, system dependent
2 channel===========2.5MS/s, system dependent

In the "Test Panel" of MAX2.2, the board should work in "1 channel mode"
when it is set to generate sine wave .What is the definition of the
"Update rate" in "MEASUREMENT and AUTOMATION 2003"?Just the digits input
in the selection box of the "Analog Output" Panel? If yes, it seems that
it is a little meaningless to be informed that the maximum number can be
input is 4000000 not 3000000 while 3000000 already makes the output to
the limit. So I think something may be wrong.