Rainbow Electronics MAX1249 - Manual (Page 18)

MAX1248/MAX1249
+2.7V to +5.25V, Low-Power, 4-Channel,
Serial 10-Bit ADCs in QSOP-16
External Reference
With both the MAX1248 and MAX1249, an external ref-
erence can be placed at either the input (REFADJ) or
the output (VREF) of the internal reference-buffer ampli-
fier. The REFADJ input impedance is typically 20k
Ω
for
the MAX1248 and higher than 100k
Ω
for the MAX1249,
where the internal reference is omitted. At VREF, the
DC input resistance is a minimum of 18k
Ω
. During con-
version, an external reference at VREF must deliver
up to 350µA DC load current and have an output
impedance of 10
Ω
or less. If the reference has 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
DD . In
power-down, the input bias current to REFADJ can be
as much as 25µA 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 coeffi-
cient of 20ppm/°C or less to achieve accuracy to within
1LSB over the commercial temperature range of 0°C to
+70°C.
Figure 16 depicts the nominal, unipolar input/output
(I/O) transfer function, and Figure 17 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.
OUTPUT CODE
FULL-SCALE
TRANSITION
11 . . . 111
11 . . . 110
11 . . . 101
00 . . . 011
00 . . . 010
00 . . . 001
00 . . . 000
1 2 3 0
(COM)
FS
FS - 3/2LSB
FS = VREF + COM
ZS = COM
INPUT VOLTAGE (LSBs)
1LSB =
VREF
1024
Figure 16. Unipolar Transfer Function, Full Scale (FS) = VREF
+ COM, Zero Scale (ZS) = COM
+3.3V
510k
24k
100k
0.01µF
9
REFADJ
MAX1248
Figure 15. MAX1248 Reference-Adjust Circuit
1 0 0 DIN
REFADJ
VREF
1.21V
0V
2.50V
0V
1 0 1 1 1 1 1 0 0 1 0 1
FULLPD FASTPD NOPD FULLPD FASTPD
1.6ms WAIT
COMPLETE CONVERSION SEQUENCE
t BUFFEN
≈ 75µs
τ = RC = 20k Ω x C REFADJ
(ZEROS)
CH1 CH7
(ZEROS)
Figure 14. MAX1248 FULLPD/FASTPD Power-Up Sequence
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