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

Serial 10-Bit ADCs

______________________________________________________________________________________ 19

deliver up to 350FA DC load current and have 10I or

less output impedance. If the reference has a higher out-

put impedance or is noisy, bypass it close to the VREF

pin with a 4.7FF capacitor.

Using the REFADJ input makes buffering the external

reference unnecessary. To use the direct VREF input,

In power-down, the input bias current to REFADJ is

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 coef-

ficient of 20ppm/NC or less to achieve accuracy to within

1 LSB over the 0NC to +70NC commercial temperature

range.

Figure 17 depicts the nominal, unipolar input/output

(I/O) transfer function, and Figure 18 shows the bipolar

input/output transfer function. Code transitions occur

halfway between successive-integer LSB values. Output

coding is binary, with 1 LSB = 2.44mV (2.500V/1024)

for unipolar operation, and 1 LSB = 2.44mV [(2.500V/2 -

-2.500V/2)/1024] for bipolar operation.

Table 7. Full Scale and Zero Scale

Figure 18. Bipolar Transfer Function, Full Scale (FS) = VREF/2

+ COM, Zero Scale (ZS) = COM

Figure 19. Power-Supply Grounding Connection

UNIPOLAR MODE

BIPOLAR MODE

Full Scale

Zero Scale

Positive Full Scale

Zero Scale

Negative Full Scale

VREF + COM

COM

VREF/2 + COM

COM

-VREF/2 + COM

011 . . . 111

011 . . . 110

000 . . . 010

000 . . . 001

000 . . . 000

111 . . . 111

111 . . . 110

111 . . . 101

100 . . . 001

100 . . . 000

- FS

COM*

INPUT VOLTAGE (LSB)

OUTPUT CODE

ZS = COM

+FS - 1LSB

*COM

≥ VREF/2

+ COM

FS = VREF

2

-FS =

+ COM

-VREF

2

1LSB = VREF

1024

+3V

+3V

GND

SUPPLIES

DGND

+3V

DGND

COM

AGND

DIGITAL

CIRCUITRY

R* = 10

Ω

*OPTIONAL

MAX148

MAX149