+2 .7 V t o +5 .2 5 V, Lo w -P o w e r, 4 -Ch a n n e l,
S e ria l 1 0 -Bit ADCs in QS OP -1 6
COMPLETE CONVERSION SEQUENCE
1.6ms WAIT
0 1
(ZEROS)
CH1
CH7
(ZEROS)
DIN
1
0 0
FULLPD
1.21V
1
1
1 1
1
0 0
FULLPD
1
0 1
FASTPD
FASTPD
NOPD
REFADJ
VREF
0V
2.50V
0V
τ = RC = 20kΩ x C
REFADJ
t
≈ 75µs
BUFFEN
Figure 14. MAX1248 FULLPD/FASTPD Power-Up Sequence
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 20kΩ for
the MAX1248 and higher than 100kΩ for the MAX1249,
where the internal reference is omitted. At VREF, the
DC input resistance is a minimum of 18kΩ. During con-
version, an external reference at VREF must deliver
up to 350µA DC loa d c urre nt a nd ha ve a n outp ut
impedance of 10Ω or less. If the reference has higher
output impedance or is noisy, bypass it close to the
VREF pin with a 4.7µF capacitor.
+3.3V
8/MAX1249
24k
MAX1248
REFADJ
510k
9
100k
0.01µF
Figure 15. MAX1248 Reference-Adjust Circuit
Using the REFADJ input makes buffering the external
reference unnecessary. To use the direct VREF input,
OUTPUT CODE
FULL-SCALE
disable the internal buffer by tying REFADJ to V . In
DD
power-down, the input bias current to REFADJ can be
TRANSITION
11 . . . 111
a s muc h a s 25µA with REFADJ tie d to V . Pull
DD
11 . . . 110
11 . . . 101
REFADJ to AGND to minimize the input bias current in
power-down.
Tra n s fe r Fu n c t io n
Table 7 shows the full-scale voltage ranges for unipolar
and bipolar modes.
FS = VREF + COM
ZS = COM
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.
VREF
1024
1LSB =
00 . . . 011
00 . . . 010
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
ha lfwa y b e twe e n s uc c e s s ive -inte g e r LSB va lue s .
Output coding is binary, with 1LSB = 2.44mV (2.500V /
1024) for unip ola r op e ra tion a nd 1LSB = 2.44mV
[(2.500V / 2 - -2.500V / 2) / 1024] for bipolar operation.
00 . . . 001
00 . . . 000
0
1
2
3
FS
(COM)
FS - 3/2LSB
INPUT VOLTAGE (LSBs)
Figure 16. Unipolar Transfer Function, Full Scale (FS) = VREF
+ COM, Zero Scale (ZS) = COM
18 ______________________________________________________________________________________
MAXIM [ MAXIM INTEGRATED PRODUCTS ]
