Rainbow Electronics MAX1195 - Manual (Page 19)

Grounding, Bypassing,
and Board Layout
The MAX1195 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
DD , REFP, REFN, and COM with
two parallel 0.1µF ceramic capacitors and a 2.2µF
bipolar capacitor to GND. Follow the same rules to
bypass the digital supply (OV
DD ) to OGND. Multilayer
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
physical location of the analog ground (GND) and the
digital output driver ground (OGND) on the ADC’s
package. The two ground planes should be joined at a
single point so the noisy digital ground currents do not
interfere with the analog ground plane. The ideal loca-
tion for this connection can be determined experimen-
tally 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 (1
Ω
to 5
Ω), 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, digital 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
° turns.
Static Parameter Definitions
Integral Nonlinearity
Integral nonlinearity (INL) is the deviation of the values
on an actual transfer function from a straight line. This
straight line can be either a best-straight-line fit or a line
drawn between the endpoints of the transfer function,
once offset and gain errors have been nullified. The
static linearity parameters for the MAX1195 are mea-
sured using the best-straight-line-fit method.
Differential Nonlinearity
Differential nonlinearity (DNL) is the difference between
an actual step width and the ideal value of 1LSB. A
DNL error specification of less than 1LSB guarantees
no missing codes and a monotonic transfer function.
MAX1195
Dual, 8-Bit, 40Msps, 3V, Low-Power ADC with
Internal Reference and Parallel Outputs
______________________________________________________________________________________ 19
0 °
90 °
÷8
DOWNCONVERTER
MAX2451
INA+
MAX1195
INA-
INB+
INB-
DSP
POST-
PROCESSING
Figure 10. Typical QAM Application Using the MAX1195
HOLD
ANALOG
INPUT
SAMPLED
DATA (T/H)
T/H
t
AD
t AJ
TRACK TRACK
CLK
Figure 11. T/H Aperture Timing