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LTC1273ACSW 数据表(PDF) 21 Page - Linear Technology

部件名 LTC1273ACSW
功能描述  12-Bit, 300ksps Sampling A/D Converters with Reference
PDF  24 Pages
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制造商  LINER [Linear Technology]
网页  http://www.linear.com
标志 LINER - Linear Technology

LTC1273ACSW 数据表(HTML) 21 Page - Linear Technology

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21
LTC1273
LTC1275/LTC1276
127356fa
S
APPLICATI
I FOR ATIO
Demodulating a Signal by Undersampling
with LTC1275
Figure 23 shows a 455kHz amplitude modulated input
undersampled by the LTC1275. With a 227.5kHz sample
rate, the converter provides a 100dB noise floor and 68dB
distortion when digitizing the 455kHz AM input.
Figure 24 shows an FFT of the AM signal digitized at
212.5kHz.
AIN
LTC1275
D11
D0
455kHz
AMPLITUDE
MODULATED
INPUT
RD
RD
–5V
227.5kHz
SAMPLE RATE
5V
LTC1273/75/76 • F23
DATA OUTPUT
Figure 23. A 455kHz Amplitude Modulated Input
Undersampled by the LTC1275
FREQUENCY (kHz)
0
–110
–90
–70
–50
–30
20
40
60
80
LTC1273/75/76 • F24
100
120
–10
–100
–80
–60
–40
–20
0
fSAMPLE = 212.5kHz
fIN = 454.8kHz
fMOD = 5.03kHz
Figure 24. 455kHz Input Voltage Modulated by a 5kHz Signal
A time domain view of the demodulation is shown in Figure
25. The top trace shows the 455kHz waveform modulated
by a – 6dB, 5kHz signal. The bottom trace shows the
demodulated signal produced by the LTC1275 recon-
structed through a 12-bit DAC. The resultant frequency is
5kHz with a sample rate of 227.5kHz. There are roughly 45
points per cycle.
Figure 25. 455kHz AM Signal Demodulated to 10.5 ENOBs
DEMODULATED
5kHz OUTPUT
455kHz
AM SIGNAL
50
µs/DIV
LTC1273/75/76 • F27
1V/DIV
1V/DIV
100ps Resolution
∆Time Measurement with LTC1273
Figure 26 shows a circuit that precisely measures the
difference in time between two events. It has a 400ns full
scale and 100ps resolution. The start signal releases the
ramp generator made up of the PNP current source and
the 250pF capacitor. The circuit ramps until the stop
signal shuts off the current source. The final value of the
ramp represents the time between the start and stop
events. The LTC1273 digitizes this final value and outputs
the digital data.



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