LTC1273ACS 查看數據表（PDF） - Linear Technology

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LTC1273ACS

12-Bit, 300ksps Sampling A/D Converters with Reference

Linear Technology 

LTC1273

LTC1275/LTC1276

APPLICATI S 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.

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.

455kHz

AMPLITUDE

MODULATED

INPUT

5V

RD

AIN LTC1275

D11

D0

227.5kHz

SAMPLE RATE

RD

DATA OUTPUT

– 5V

LTC1273/75/76 • F23

455kHz

AM SIGNAL

DEMODULATED

5kHz OUTPUT

1V/DIV

1V/DIV

Figure 23. A 455kHz Amplitude Modulated Input

Undersampled by the LTC1275

0

– 10

– 20

– 30

– 40

– 50

– 60

– 70

– 80

– 90

– 100

– 110

0

fSAMPLE = 212.5kHz

fIN = 454.8kHz

fMOD = 5.03kHz

20 40 60 80 100 120

FREQUENCY (kHz)

LTC1273/75/76 • F24

Figure 24. 455kHz Input Voltage Modulated by a 5kHz Signal

50µs/DIV

LTC1273/75/76 • F27

Figure 25. 455kHz AM Signal Demodulated to 10.5 ENOBs

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.

21

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