Grounding and Layout
For best performance, use printed circuit boards with
separate analog and digital ground planes. Wire-wrap
boards are not recommended.
Design the printed circuit board so that the analog and
digital sections are separated and confined to different
areas of the board. Join the digital and analog ground
planes at only one point. If the MAX1400 is the only
device requiring an AGND to DGND connection, then
the ground planes should be connected at the AGND
and DGND pins of the MAX1400. In systems where
multiple devices require AGND to DGND connections,
the connection should still be made at only one point.
Make the star ground as close to the MAX1400 as pos-
sible.
Avoid running digital lines under the device, because
these may couple noise onto the die. Run the analog
ground plane under the MAX1400 to minimize coupling
of digital noise. Make the power-supply lines to the
MAX1400 as wide as possible to provide low-imped-
ance paths and reduce the effects of glitches on the
power-supply line.
Shield fast switching signals, such as clocks, with digi-
tal ground to avoid radiating noise to other sections of
the board. Avoid running clock signals near the analog
inputs. Avoid crossover of digital and analog signals.
Traces on opposite sides of the board should run at
right angles to each other. This will reduce the effects
of feedthrough on the board. A microstrip technique is
best, but is not always possible with double-sided
boards. In this technique, the component side of the
board is dedicated to ground planes while signals are
placed on the solder side.
Good decoupling is important when using high-resolu-
tion ADCs. Decouple all analog supplies with 10µF tan-
talum capacitors in parallel with 0.1µF HF ceramic
capacitors to AGND. Place these components as close
to the device as possible to achieve the best decou-
pling.
See the MAX1402 evaluation kit manual for recom-
mended layout. The evaluation board package
includes a fully assembled and tested evaluation
board.
Optical Isolation
For applications that require an optically isolated
interface, refer to Figure 19. With 6N136-type optocou-
plers, maximum clock speed is 4MHz. Maximum clock
speed is limited by the degree of mismatch between
the individual optocouplers. Faster optocouplers allow
faster signaling at a higher cost.
MAX1400
+5V, 18-Bit, Low-Power, Multichannel,
Oversampling (Sigma-Delta) ADC
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