MAX148 Maxim, MAX148 Datasheet - Page 19

• Manufacturer Part Number: MAX148
• Description: +2.7V to +5.25V / Low-Power / 8-Channel / Serial 10-Bit ADCs
• Manufacturer: Maxim

Table 7. Full Scale and Zero Scale
Figure 18. Bipolar Transfer Function, Full Scale (FS) =
VREF / 2 + COM, Zero Scale (ZS) = COM
VREF, the DC input resistance is a minimum of 18kΩ.
During conversion, an external reference at VREF 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
VREF pin with a 4.7µF capacitor.
Using the REFADJ input makes buffering the external
reference unnecessary. To use the direct VREF input,
disable the internal buffer by tying REFADJ to V
power-down, the input bias current to REFADJ is typi-
cally 25µA (MAX149) with REFADJ tied to V
REFADJ to AGND to minimize the input bias current in
power-down.
*COM
011 . . . 111
011 . . . 110
000 . . . 010
000 . . . 001
000 . . . 000
111 . . . 111
111 . . . 110
111 . . . 101
100 . . . 001
100 . . . 000
VREF + COM
VREF / 2
OUTPUT CODE
Full Scale
-FS =
1LSB =
- FS
FS = VREF
ZS = COM
-VREF
______________________________________________________________________________________
2
2
VREF
UNIPOLAR MODE
1024
+2.7V to +5.25V, Low-Power, 8-Channel,
+ COM
+ COM
INPUT VOLTAGE (LSB)
COM*
Zero Scale
COM
+FS - 1LSB
DD
DD
. Pull
. In
Table 7 shows the full-scale voltage ranges for unipolar
and bipolar modes.
The external reference must have a temperature coeffi-
cient of 20ppm/°C or less to achieve accuracy to within
1LSB over the 0°C to +70°C 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 1LSB = 2.44mV (2.500V /
1024) for unipolar operation, and 1LSB = 2.44mV
[(2.500V / 2 - -2.500V / 2) / 1024] for bipolar operation.
Figure 19. Power-Supply Grounding Connection
R* = 10
Full Scale
VREF / 2
Positive
+ COM
*OPTIONAL
V
+3V
DD
Serial 10-Bit ADCs
AGND
BIPOLAR MODE
MAX148
MAX149
Scale
COM
Zero
SUPPLIES
COM
DGND
Transfer Function
Full Scale
-VREF / 2
Negative
+ COM
+3V
+3V
CIRCUITRY
DIGITAL
DGND
GND
19