LTC1275 LINER [Linear Technology], LTC1275 Datasheet - Page 21

• Manufacturer Part Number: LTC1275
• Description: 12-Bit, 300ksps Sampling A/D Converters with Reference
• Manufacturer: LINER [Linear Technology]

A
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.
Figure 24. 455kHz Input Voltage Modulated by a 5kHz Signal
PPLICATI
MODULATED
AMPLITUDE
455kHz
INPUT
Figure 23. A 455kHz Amplitude Modulated Input
Undersampled by the LTC1275
–100
–110
–80
–90
–10
–20
–30
–40
–50
–60
–70
0
0
O
A
20
IN
U
S
FREQUENCY (kHz)
40
LTC1275
I FOR ATIO
5V
U
60
–5V
f
f
f
SAMPLE
IN
MOD
D11
RD
D0
= 454.8kHz
80
= 5.03kHz
= 212.5kHz
LTC1273/75/76 • F24
W
100
RD
120
SAMPLE RATE
DATA OUTPUT
227.5kHz
LTC1273/75/76 • F23
U
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.
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.
DEMODULATED
5kHz OUTPUT
AM SIGNAL
Figure 25. 455kHz AM Signal Demodulated to 10.5 ENOBs
455kHz
LTC1275/LTC1276
50 s/DIV
LTC1273
LTC1273/75/76 • F27
21
1V/DIV
1V/DIV