+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,

power-down, the input bias current to REFADJ can be

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

(ZEROS)

CH1

CH7

(ZEROS)

Figure 14. MAX1248 FULLPD/FASTPD Power-Up Sequence

18

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