Datasheet

MAX1196
Dual 8-Bit, 40Msps, 3V, Low-Power ADC with
Internal Reference and Multiplexed Parallel Outputs
______________________________________________________________________________________ 19
Typical QAM Demodulation Application
A frequently used modulation technique in digital com-
munications applications is quadrature amplitude mod-
ulation (QAM). Typically found in spread- spectrum-
based systems, a QAM signal represents a carrier fre-
quency modulated in both amplitude and phase. At the
transmitter, modulating the baseband signal with quad-
rature outputs, a local oscillator followed by subse-
quent up-conversion can generate the QAM signal. The
result is an in-phase (I) and a quadrature (Q) carrier
component, where the Q component is 90 degrees
phase-shifted with respect to the in-phase component.
At the receiver, the QAM signal is divided down into its
I and Q components, essentially representing the mod-
ulation process reversed. Figure 10 displays the
demodulation process performed in the analog domain,
using the dual matched 3V, 8-bit ADC MAX1196, and
the MAX2451 quadrature demodulator to recover and
digitize the I and Q baseband signals. Before being
digitized by the MAX1196, the mixed-down signal com-
ponents can be filtered by matched analog filters, such
1/4 MAX4252
MAX6066
1/4 MAX4252
1/4 MAX4252
1.47kΩ
21.5kΩ
21.5kΩ
21.5kΩ
21.5kΩ
21.5kΩ
47kΩ
3.3V
3.3V
11
2
2
3
4
1
1
REFOUT
REFP
REFIN
1µF
10µF
6V
MAX1196
N = 1
REFN
29N.C.
N.C.
31
32
1
2
29
31
32
1
2
COM
REFOUT
NOTE: ONE FRONT-END REFERENCE CIRCUIT DESIGN CAN BE USED WITH UP TO 32 ADCs.
REFP
REFIN
MAX1196
N = 32
REFN
COM
2.0V AT 8mA
3
0.1µF
0.1µF
MAX4254 POWER-SUPPLY
BYPASSING. PLACE CAPACITOR
AS CLOSE AS POSSIBLE TO
THE OP AMP.
3.3V
1.47kΩ
47kΩ
3.3V
1.5V
11
6
5
4
7
10µF
6V
1.5V AT 0mA
1.47kΩ
47kΩ
3.3V
11
9
10
4
8
10µF
6V
0.1µF0.1µF0.1µF
0.1µF
0.1µF
2.2µF
10V
0.1µF0.1µF
1.0V AT -8mA
330µF
6V
330µF
6V
330µF
6V
2.0V
1.0V
Figure 9. External Unbuffered Reference Drive With MAX4252 and MAX6066