Datasheet

+2.7V to +5.25V, Low-Power, 8-Channel,
Serial 10-Bit ADCs
MAX148/MAX149
18
Lowest Power at Higher Throughputs
Figure 14b shows the power consumption with external-
reference compensation in fast power-down, with one
and eight channels converted. The external 4.7FF com-
pensation requires a 75Fs wait after power-up with one
dummy conversion. This graph shows fast multichannel
conversion with the lowest power consumption possible.
Full power-down mode may provide increased power
savings in applications where the MAX148/MAX149 are
inactive for long periods of time, but where intermittent
bursts of high-speed conversions are required.
Internal and External References
The MAX149 can be used with an internal or external
reference voltage, whereas an external reference is
required for the MAX148. An external reference can be
connected directly at V
REF
or at the REFADJ pin.
An internal buffer is designed to provide 2.5V at V
REF
for both the MAX149 and the MAX148. The MAX149’s
internally trimmed 1.21V reference is buffered with a
2.06 gain. The MAX148’s REFADJ pin is also buffered
with a 2.00 gain to scale an external 1.25V reference at
REFADJ to 2.5V at V
REF
.
Internal Reference (MAX149)
The MAX149’s full-scale range with the internal refer-
ence is 2.5V with unipolar inputs and Q1.25V with bipolar
inputs. The internal reference voltage is adjustable to
Q1.5% with the circuit in Figure 16.
External Reference
With both the MAX149 and MAX148, an external refer-
ence can be placed at either the input (REFADJ) or the
output (V
REF
) of the internal reference-buffer amplifier.
The REFADJ input impedance is typically 20kI for the
MAX149, and higher than 100kI for the MAX148. At
V
REF
, the DC input resistance is a minimum of 18kI.
During conversion, an external reference at V
REF
must
Figure 15. MAX149 FULLPD/FASTPD Power-Up Sequence
Figure 16. MAX149 Reference-Adjust Circuit
Figure 17. Unipolar Transfer Function, Full Scale (V
FS
) = V
REF
+ V
COM
, Zero Scale (ZS) = COM
1 0 0
DIN
REFADJ
V
REF
1.21V
0
2.50V
0
1 0 1 1 11 1 0 0 1 0 1
FULLPD FASTPD
NOPD
FULLPD
FASTPD
t
BUFFEN =
75Fs
H = RC = 20kI x C
REFADJ
(ZEROS)
CH1 CH7
(ZEROS)
COMPLETE CONVERSION SEQUENCE
1.6ms WAIT
+3.3V
REFADJ
510kI
24kI
100kI
0.01µF
12
MAX149
OUTPUT CODE
FULL-SCALE
TRANSITION
TRANSITION
11...111
11...110
11...101
00...011
00...010
00...001
00...000
1 2 3
0
(COM)
FS
FS - 3/2 LSB
V
FS
= V
REF
+ V
COM
ZS = COM
INPUT VOLTAGE (LSB)
1 LSB =
V
REF
1024