National Instruments NI 5401 Manuel D’Utilisation

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 National Instruments Corporation

C-1

For DDS-based waveform generation, you must first load one cycle of the 
desired waveform into the lookup memory. The size of the DDS lookup 
memory is 16,384 samples. Each sample is 16 bits wide.

One cycle of the waveform buffer loaded into the memory should be exactly equal 

to the size of the DDS lookup memory.

F

c

 = update clock for the accumulator. 

For the NI 5401, F

c

 = 40 MHz.

F

a

 = desired frequency of the output signal

N = accumulator size in bits

For the NI 5401, N = 32.

FCW = frequency control word to be loaded into the accumulator to 
generate F

a

.

The frequency control word is calculated using the formula: 

FCW = (2

N

 * F

a

) / F

c

The frequency resolution is then given by:

frequency resolution = F

c

 / 2

N

 = (40 

×

 10

6

) / 2

32

 = 9.31322 mHz

For example, if you need to generate a frequency of 10 MHz, then the FCW 
is (2

32

 * 10E6)/40E6, which equals 1,073,741,824. If you need to generate 

a frequency of 1 Hz, then the FCW is (2

32

 * 1)/40E6, which equals 107.

On the NI 5401, the maximum frequency of a sine wave you can generate reliably 

is limited to 16 MHz. Other waveforms such as square or triangular waves are limited to 
1 MHz.