+2.7V to +5.25V, Low-Power, 8-Channel,

Serial 10-Bit ADCs

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

4.7µF compensation requires a 75µs wait after

power-up with one dummy conversion. This graph

shows fast multi-channel 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 peri-

ods 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 VREF or at the REFADJ pin.

An internal buffer is designed to provide 2.5V at

VREF for both the MAX149 and the MAX148. The

MAX149’s internally trimmed 1.21V reference is buf-

fered 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 VREF.

Internal Reference (MAX149)

The MAX149’s full-scale range with the internal refer-

ence is 2.5V with unipolar inputs and ±1.25V with bipo-

lar inputs. The internal reference voltage is adjustable

to ±1.5% 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 (VREF) of the internal reference-buffer amplifier.

The REFADJ input impedance is typically 20k

Ω for the

MAX149, and higher than 100k

Ω for the MAX148. At

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 (LSB)

1LSB =

VREF

1024

Figure 17. Unipolar Transfer Function, Full Scale (FS) = VREF

+ COM, Zero Scale (ZS) = COM

+3.3V

510k

24k

100k

0.01µF

12

REFADJ

MAX149

Figure 16. MAX149 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

(ZEROS)

CH1

CH7

(ZEROS)

Figure 15. MAX149 FULLPD/FASTPD Power-Up Sequence