400ksps, +5V, 8-/4-Channel, 12-Bit ADCs

with +2.5V Reference and Parallel Interface

_______________________________________________________________________________________

9

_______________Detailed Description

Converter Operation

The MAX1290/MAX1292 ADCs use a successive-

approximation (SAR) conversion technique and an input

track-and-hold (T/H) stage to convert an analog input

signal to a 12-bit digital output. Their parallel (8 + 4) out-

put format provides an easy interface to standard micro-

processors (µPs). Figure 2 shows the simplified internal

architecture of the MAX1290/MAX1292.

Single-Ended and

Pseudo-Differential Operation

The sampling architecture of the ADC’s analog com-

parator is illustrated in the equivalent input circuits in

Figures 3a and 3b. In single-ended mode, IN+ is inter-

nally switched to channels CH0–CH7 for the MAX1290

(Figure 3a) and to CH0–CH3 for the MAX1292 (Figure

3b), while IN- is switched to COM (Table 3). In differen-

tial mode, IN+ and IN- are selected from analog input

pairs (Table 4).

In differential mode, IN- and IN+ are internally switched to

either of the analog inputs. This configuration is pseudo-

differential in that only the signal at IN+ is sampled. The

return side (IN-) must remain stable within ±0.5 LSB

(±0.1 LSB for best performance) with respect to GND

during a conversion. To accomplish this, connect a

0.1µF capacitor from IN- (the selected input) to GND.

During the acquisition interval, the channel selected as

the end of the acquisition interval, the T/H switch

signal at IN+.

The conversion interval begins with the input multiplex-

negative input (IN-). This unbalances node ZERO at the

comparator’s positive input. The capacitive digital-to-

analog converter (DAC) adjusts during the remainder of

the conversion cycle to restore node ZERO to 0V within

the limits of 12-bit resolution. This action is equivalent to

to the binary-weighted capacitive DAC, which in turn

forms a digital representation of the analog input signal.

T/H

THREE-STATE, BIDIRECTIONAL

I/O INTERFACE

12

17k

Ω

8

8

4

8

4

8

SUCCESSIVE-

APPROXIMATION

REGISTER

MUX

( ) ARE FOR MAX1290 ONLY.

CHARGE REDISTRIBUTION

12-BIT DAC

CLOCK

ANALOG

INPUT

MULTIPLEXER

CONTROL LOGIC

&

LATCHES

REF

REFADJ

1.22V

REFERENCE

D0–D7

8-BIT DATA BUS

(CH5)

(CH4)

CH3

CH2

CH1

CH0

COM

CLK

CS

WR

RD

INT

HBEN

GND

MAX1290

MAX1292

2.05

COMP

(CH7)

(CH6)

Figure 2. Simplified Functional Diagram of 8-/4-Channel MAX1290/MAX1292