Rainbow Electronics MAX1197 - Manual (Page 18)

MAX1197
modulation (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 upconversion can generate the QAM signal. The
result is an in-phase (I) and a quadrature (Q) carrier
component, where the Q component is 90° phase shift-
ed 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 modulation
process reversed. Figure 10 displays the demodulation
process performed in the analog domain, using the
dual matched 3V, 8-bit ADC MAX1197 and the
MAX2451 quadrature demodulator to recover and digi- tize the I and Q baseband signals. Before being digi-
tized by the MAX1197, the mixed-down signal compo-
nents may be filtered by matched analog filters, such
as Nyquist or pulse-shaping filters which remove
unwanted images from the mixing process, thereby
enhancing the overall signal-to-noise (SNR) perfor-
mance and minimizing intersymbol interference.
Grounding, Bypassing,
and Board Layout
The MAX1197 requires high-speed board layout design
techniques. Locate all bypass capacitors as close to
the device as possible, preferably on the same side as
the ADC, using surface-mount devices for minimum
Dual, 8-Bit, 60Msps, 3V, Low-Power ADC with
Internal Reference and Parallel Outputs
18 ______________________________________________________________________________________
1/4 MAX4252
MAX6066
1/4 MAX4252
1/4 MAX4252
1.47k Ω
21.5k Ω
21.5k Ω
21.5k Ω
21.5k Ω
21.5k Ω
47k Ω
3.3V
3.3V
11
2
2 3
4
1
1
REFOUT
REFP
REFIN
1 µF
10 µF
6V
MAX1197
N = 1 REFN
29 N.C.
N.C.
31
32
1
2
29
31
32
1
2
COM
REFOUT
NOTE: ONE FRONT-END REFERENCE CIRCUIT DESIGN MAY BE USED WITH UP TO 32 ADCs.
REFP
REFIN
MAX1197
N = 32
REFN
COM
2.0V AT 8mA
3
0.1 µF
0.1 µF
MAX4254 POWER SUPPLY
BYPASSING. PLACE CAPACITOR
AS CLOSE AS POSSIBLE TO
THE OP AMP.
3.3V
1.47k Ω
47k Ω
3.3V
1.5V
11
6
5
4
7
10 µF
6V
1.5V AT 0mA
1.47k Ω
47k Ω
3.3V
11
9
10
4
8
10 µF
6V
0.1 µF 0.1 µF 0.1 µF
0.1 µF
0.1 µF 2.2 µF
10V
0.1 µF 0.1 µF
1.0V AT -8mA
330 µF
6V
330 µF
6V
330 µF
6V
2.0V
1.0V
Figure 9. External Unbuffered Reference Drive with MAX4252 and MAX6066