Rainbow-electronics MAX149 Manual de usuario descargar pdf (Pagina 19)

MAX148/MAX149

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

Serial 10-Bit ADCs

______________________________________________________________________________________ 19

VREF, the DC input resistance is a minimum of 18kΩ.

During conversion, an external reference at VREF must

deliver up to 350µA DC load current and have 10Ω 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,

disable the internal buffer by tying REFADJ to V

. In

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

cally 25µA (MAX149) with REFADJ tied to V

. Pull

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

VREF

-FS = + COM

-VREF

1LSB =

VREF

1024

≤

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

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

+3V

GND

SUPPLIES

DGND+3VDGNDCOM

AGNDV

DIGITAL

CIRCUITRY

MAX148

MAX149

R* = 10Ω

*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

1 2 ... 14 15 16 17 18 19 20 21 22 23 24

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Rainbow-electronics MAX149 Manual de usuario Pagina 19

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