MAX1184 Maxim Integrated Products, MAX1184 Datasheet - Page 15

• Manufacturer Part Number: MAX1184
• Description: Low-Power ADC
• Manufacturer: Maxim Integrated Products

half the signal swing compared to a single-ended
mode.
Figure 7 shows an AC-coupled, single-ended applica-
tion. Amplifiers like the MAX4108 provide high-speed,
high-bandwidth, low noise, and low distortion to main-
tain the integrity of the input signal.
The most frequently used modulation technique for dig-
ital communications applications is probably the
Quadrature Amplitude Modulation (QAM). Typically
found in spread-spectrum based systems, a QAM sig-
nal represents a carrier frequency modulated in both
amplitude and phase. At the transmitter, modulating the
baseband signal with quadrature outputs, a local oscil-
lator followed by subsequent up-conversion can gener-
ate the QAM signal. The result is an in-phase (I) and a
quadrature (Q) carrier component, where the Q compo-
nent is 90 degree phase-shifted with respect to the in-
phase component. At the receiver, the QAM signal is
divided down into it’s I and Q components, essentially
representing the modulation process reversed. Figure 8
displays the demodulation process performed in the
analog domain, using the dual matched +3V, 10-bit
ADC (MAX1184), and the MAX2451 quadrature demod-
ulator to recover and digitize the I and Q baseband sig-
nals. Before being digitized by the MAX1184, the mixed
down-signal components may be filtered by matched
analog filters, such as Nyquist or pulse-shaping filters
which remove any unwanted images from the mixing
process, thereby enhancing the overall signal-to-noise
(SNR) performance and minimizing intersymbol interfer-
ence.
Table 1. MAX1184 Output Codes For Differential Inputs
*V
REF
Dual 10-Bit, 20Msps, +3V, Low-Power ADC with
DIFFERENTIAL INPUT
Typical QAM Demodulation Application
= V
Single-Ended AC-Coupled Input Signal
-V
-V
V
- V
REFP
V
REF
VOLTAGE*
REF
REF
REF
REF
x 511/512
x 511/512
x 512/512
- V
x 1/512
0
x 1/512
REFN
______________________________________________________________________________________
Internal Reference and Parallel Outputs
- FULL SCALE + 1 LSB
+FULL SCALE - 1LSB
DIFFERENTIAL
- FULL SCALE
Bipolar Zero
+ 1 LSB
- 1 LSB
INPUT
The MAX1184 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
inductance. Bypass V
two parallel 0.1µF ceramic capacitors and a 2.2µF
bipolar capacitor to GND. Follow the same rules to
bypass the digital supply (OV
boards with separated ground and power planes pro-
duce the highest level of signal integrity. Consider the
use of a split ground plane arranged to match the phys-
ical location of the analog ground (GND) and the digital
output driver ground (OGND) on the ADCs package.
The two ground planes should be joined at a single
point such that the noisy digital ground currents do not
interfere with the analog ground plane. The ideal loca-
tion of this connection can be determined experimental-
ly at a point along the gap between the two ground
planes, which produces optimum results. Make this
connection with a low-value, surface-mount resistor (1Ω
to 5Ω), a ferrite bead, or a direct short. Alternatively, all
ground pins could share the same ground plane, if the
ground plane is sufficiently isolated from any noisy, dig-
ital systems ground plane (e.g., downstream output
buffer or DSP ground plane). Route high-speed digital
signal traces away from the sensitive analog traces of
either channel. Make sure to isolate the analog input
lines to each respective converter to minimize channel-
to-channel crosstalk. Keep all signal lines short and
free of 90 degree turns.
STRAIGHT OFFSET
11 1111 1111
10 0000 0001
10 0000 0000
01 1111 1111
00 0000 0001
00 0000 0000
BINARY
T/B = 0
Grounding, Bypassing, and
DD
, REFP, REFN, and COM with
DD
TWO’S COMPLEMENT
) to OGND. Multilayer
Board Layout
01 1111 1111
00 0000 0001
00 0000 0000
11 1111 1111
10 0000 0001
10 0000 0000
T/B = 1
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