Sampling• The data is acquired by an ADC using a process

called sampling. • Sampling an analog signal occurs at discrete

time intervals. • The rate at which the signal is sampled is known

as the sampling frequency.

Sampling• The sampling frequency determines the quality of the analog signal

conversion process.

• A Higher sampling frequency achieves better conversion of the analog signals.

• The minimum sampling frequency required to represent the signal should be at least twice the maximum frequency of the analog signal under test (this is called the Nyquist rate).

• If the sampling frequency is equal or less than twice the frequency of the input signal, a signal of lower frequency is generated from such a process (this is called aliasing).

Sampling

• For accurate frequency representation:– Sample at least 2x the highest frequency

signal being measured.

• For accurate shape representation– Sample 5–10x the highest frequency signal

being measured.

Sampling• # of Samples to Read is defines as how many data points the DAQ will

return

• Sampling Rate tells the DAQ how fast to acquire the data. T=1/(sampling rate)

• Divide the number of samples by the sample rate to determine thesample time (i.e. that last time in the graph), then you can determine how many cycle of input signal period will be shown.

• # of cycles= (sample time)*(input signal period)

• Note: The # of samples for 1 cycle= (sampling frequency)/ (input signal frequency)

Sampling Math. Example

• If the input signal has 2 Vp-p, 100 Hz and Sine Wave.• If you set the DAQ to 1000Hz sample rate, 50 Samples• What is the sample time? __(50mSec )_• How many wave cycles should we see? __(5)__

Aliasing-Varying Sample Rate• Now try sampling of– 500Hz and 25 samples– 300Hz and 15 samples– 210Hz and 10 samples

Aliasing-Varying Sample Rate• Now sample rates below Nyquist Frequency

Resolution

• Precision of the analog to digital conversion process is dependent upon the number of bits the ADC uses.

• The higher the resolution, the higher the number of divisions the voltage range is broken into, and therefore, the smaller the detectable voltage changes.

Resolution• An 8 bit ADC gives 256 levels (2^8) compared to

a 12 bit ADC that has 4096 levels (2^12).

Time (ms)100 200150500

0

1.25

5.00

2.50

3.75

6.25

7.50

8.75

10.00

Amplitude(Volts)

16-Bit versus 3-Bit Resolution(5 kHz Sine Wave)

16-bit

3-bit

000

001

010

011

100

101

110

111

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Digital to Analog Converter• The multifunction boards also have on-board digital to

analog converters (DAC). A DAC can generate an analog output from a digital input. This allows the board to generate analog signals, both dc and ac voltages.

• Like the ADC, the DAC's performance is limited by the number of samples it can process and the number of bits that is used in converting the digital code into an analog signal.

Sine wave generation from a 3-bit digital code