+2.7V to +5.25V, Low-Power, 4-Channel,

Serial 10-Bit ADCs in QSOP-16

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19

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

011 . . . 111

011 . . . 110

000 . . . 010

000 . . . 001

000 . . . 000

111 . . . 111

111 . . . 110

111 . . . 101

100 . . . 001

100 . . . 000

- FS

COM*

OUTPUT CODE

ZS = COM

+FS - 1LSB

INPUT VOLTAGE (LSB)

+ COM

FS =

VREF

2

-FS =

+ COM

-VREF

2

1LSB =

VREF

1024

*COM

≥ VREF / 2

+3V

+3V

GND

SUPPLIES

DGND

+3V

DGND

COM

AGND

DIGITAL

CIRCUITRY

MAX1248

MAX1249

R* = 10

Ω

* OPTIONAL

Figure 17. Bipolar Transfer Function, Zero Scale (ZS) = COM,

Full Scale (FS) = VREF / 2 + COM

Figure 18. Power-Supply Grounding Connection

Table 7. Full Scale and Zero Scale

Layout, Grounding, and Bypassing

For best performance, use printed circuit boards.

Wire-wrap boards are not recommended. Board layout

should ensure that digital and analog signal lines are

separated from each other. Do not run analog and digi-

tal (especially clock) lines parallel to one another, or

digital lines underneath the ADC package.

Figure 18 shows the recommended system ground

connections. Establish a single-point analog ground

(star ground point) at AGND, separate from the logic

ground. Connect all other analog grounds and DGND

to the star ground. No other digital system ground

should be connected to this ground. For lowest noise

operation, the ground return to the star ground’s power

supply should be low impedance and as short as pos-

sible.

affect the ADC’s high-speed comparator. Bypass the

supply to the star ground with 0.1µF and 1µF capaci-

tors close to pin 1 of the MAX1248/MAX1249. Minimize

capacitor lead lengths for best supply-noise rejection.

If the +3V power supply is very noisy, a 10

Ω resistor

can be connected as a lowpass filter (Figure 18).

High-Speed Digital Interfacing with QSPI

The MAX1248/MAX1249 can interface with QSPI using

CPHA = 0). This QSPI circuit can be programmed to do a

conversion on each of the four channels. The result is

stored in memory without taxing the CPU, since QSPI

incorporates its own micro-sequencer.

The MAX1248/MAX1249 are QSPI compatible up to their

maximum external clock frequency of 2MHz.