MAX149ACAP(1998) 데이터 시트보기 (PDF) - Maxim Integrated

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MAX149ACAP
(Rev.:1998)

+2.7V to +5.25V, Low-Power, 8-Channel, Serial 10-Bit ADCs

Maxim Integrated 

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

Serial 10-Bit ADCs

COMPLETE CONVERSION SEQUENCE

DIN 1

00

FULLPD

REFADJ

1.21V

0V

2.50V

VREF

0V

(ZEROS)

1.6ms WAIT

1

01

FASTPD

CH1

1

11

NOPD

τ = RC = 20kΩ x CREFADJ

tBUFFEN ≈ 75µs

Figure 15. MAX149 FULLPD/FASTPD Power-Up Sequence

CH7

1

00

FULLPD

(ZEROS)

1

01

FASTPD

+3.3V

24k

100k

510k

0.01µF

MAX149

12 REFADJ

OUTPUT CODE

11 . . . 111

11 . . . 110

11 . . . 101

FULL-SCALE

TRANSITION

FS = VREF + COM

Figure 16. MAX149 Reference-Adjust Circuit

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.

00 . . . 011

00 . . . 010

00 . . . 001

00 . . . 000

01

(COM)

23

INPUT VOLTAGE (LSB)

ZS = COM

1LSB = VREF

1024

FS

FS - 3/2LSB

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

+ COM, Zero Scale (ZS) = COM

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 20kΩ for the

MAX149, and higher than 100kΩ for the MAX148. At

18 ______________________________________________________________________________________

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