Datasheet

MAX1280/MAX1281
400ksps/300ksps, Single-Supply, Low-Power, 8-Channel,
Serial 12-Bit ADCs with Internal Reference
______________________________________________________________________________________ 19
18kΩ. During conversion, an external reference at REF
must deliver up to 350µA DC load current and have 10Ω
or less output impedance. If the reference has a higher
output impedance or is noisy, bypass it close to the REF
pin with a 4.7µF capacitor.
Using the REFADJ input makes buffering the external
reference unnecessary. To use the direct REF input,
disable the internal buffer by connecting REFADJ to
V
DD1
.
Transfer Function
Table 5 shows the full-scale voltage ranges for unipolar
and bipolar modes. Figure 14 depicts the nominal,
unipolar input/output (I/O) transfer function, and Figure
15 shows the bipolar I/O transfer function. Code transi-
tions occur halfway between successive-integer LSB
values. Output coding is binary, with 1LSB = 610µV
for unipolar and bipolar operation.
Layout, Grounding, and Bypassing
For best performance, use printed circuit boards; wire-
wrap boards are not recommended. Board layout
should ensure that digital and analog signal lines are
separated from each other. Do not run analog and digi-
tal (especially clock) lines parallel to one another, or
digital lines underneath the ADC package.
Figure 16 shows the recommended system ground
connections. Establish a single-point analog ground
(star ground point) at GND. Connect all analog grounds
to the star ground. Connect the digital system ground
to star ground at this point only. For lowest-noise opera-
tion, the ground return to the star ground’s power sup-
ply should be low impedance and as short as possible.
High-frequency noise in the V
DD1
power supply may
affect the high-speed comparator in the ADC. Bypass
the supply to the star ground with 0.1µF and 10µF
capacitors, located close to pin 20 of the MAX1280/
MAX1281. Minimize capacitor lead lengths for best
supply-noise rejection. If the power supply is very
noisy, a 10Ω resistor can be connected as a lowpass
filter (Figure 16).
+3.3V
510kΩ
24kΩ
100kΩ
0.01µF
12
REFADJ
MAX1281
Figure 13. MAX1281 Reference-Adjust Circuit
OUTPUT CODE
FULL-SCALE
TRANSITION
11 . . . 111
11 . . . 110
11 . . . 101
00 . . . 011
00 . . . 010
00 . . . 001
00 . . . 000
123
0
(COM)
FS
FS - 3/2LSB
FS = V
REF
+ V
COM
ZS = V
COM
INPUT VOLTAGE (LSB)
1 LSB =
V
REF
4096
Figure 14. Unipolar Transfer Function, Full Scale (FS) = V
REF
+ V
COM
, Zero Scale (ZS) = V
COM
011 . . . 111
011 . . . 110
000 . . . 010
000 . . . 001
000 . . . 000
111 . . . 111
111 . . . 110
111 . . . 101
100 . . . 001
100 . . . 000
- FS
COM*
INPUT VOLTAGE (LSB)
OUTPUT CODE
ZS = COM
+FS - 1LSB
*V
COM
V
REF
/ 2
+ V
COM
FS
=
V
REF
2
-FS = + V
COM
-V
REF
2
1 LSB =
V
REF
4096
≤
Figure 15. Bipolar Transfer Function, Full Scale (FS) =
V
REF
/ 2 + V
COM
, Zero Scale (ZS) = V
COM