Datasheet

+2.7V to +5.25V, Low-Power, 8-Channel,
Serial 10-Bit ADCs
MAX148/MAX149
19
deliver up to 350FA DC load current and have 10I or
less output impedance. If the reference has a higher
output impedance or is noisy, bypass it close to the V
REF
pin with a 4.7FF capacitor.
Using the REFADJ input makes buffering the external
reference unnecessary. To use the direct V
REF
input,
disable the internal buffer by tying REFADJ to V
DD
.
In power-down, the input bias current to REFADJ is
typically 25FA (MAX149) with REFADJ tied to V
DD
. Pull
REFADJ to AGND to minimize the input bias current in
power-down.
Transfer Function
Table 7 shows the full-scale voltage ranges for unipolar
and bipolar modes.
The external reference must have a temperature coef-
ficient of 20ppm/NC or less to achieve accuracy to within
1 LSB over the 0NC to +70NC commercial temperature
range.
Figure 17 depicts the nominal, unipolar input/output
(I/O) transfer function, and Figure 18 shows the bipolar
input/output transfer function. Code transitions occur
halfway between successive-integer LSB values. Output
coding is binary, with 1 LSB = 2.44mV (2.500V/1024)
for unipolar operation, and 1 LSB = 2.44mV [(2.500V/2 -
-2.500V/2)/1024] for bipolar operation.
Table 7. Full Scale and Zero Scale
Figure 18. Bipolar Transfer Function, Full Scale (V
FS
) = V
REF
/2
+ V
COM
, Zero Scale (ZS) = COM
Figure 19. Power-Supply Grounding Connection
UNIPOLAR MODE BIPOLAR MODE
Full Scale Zero Scale Positive Full Scale Zero Scale Negative Full Scale
V
REF
+ V
COM
V
COM
V
REF
/2 + V
COM
V
COM
- V
REF
/2 + 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 - 1 LSB
*V
COM
≥ V
REF
/2
+ V
COM
V
FS
=
V
REF
2
-V
FS
= + V
COM
-V
REF
2
1 LSB =
V
REF
1024
+3V
+3V
GND
SUPPLIES
DGND+3VDGND
COM
AGNDV
DD
DIGITAL
CIRCUITRY
R* = 10Ω
*OPTIONAL
MAX148
MAX149