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

Serial 10-Bit ADCs

______________________________________________________________________________________

19

VREF, the DC input resistance is a minimum of 18k

Ω.

During conversion, an external reference at VREF must

deliver up to 350µA DC load current and have 10

Ω or

less output impedance. If the reference has a 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 is typi-

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 0°C to +70°C 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 1LSB = 2.44mV (2.500V /

1024) for unipolar operation, and 1LSB = 2.44mV

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

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

≤

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

VREF / 2 + COM, Zero Scale (ZS) = COM

+3V

+3V

GND

SUPPLIES

DGND

+3V

DGND

COM

AGND

DIGITAL

CIRCUITRY

MAX148

MAX149

R* = 10

Ω

*OPTIONAL

Figure 19. Power-Supply Grounding Connection

UNIPOLAR MODE

BIPOLAR MODE

Full Scale

Zero Scale

Positive

Zero

Negative

Full Scale

Scale

Full Scale

VREF + COM

COM

VREF / 2

COM

-VREF / 2

+ COM

+ COM

Table 7. Full Scale and Zero Scale