Detailed Description

Converter Operation

The MAX1291/MAX1293 ADCs use a successive-

approximation (SAR) conversion technique and an

input track/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

microprocessors (µPs). Figure 2 shows the simplified

internal architecture of the MAX1291/MAX1293.

Single-Ended and

Pseudo-Differential Operation

The sampling architecture of the ADC’s analog com-

parator is illustrated in the equivalent input circuit in

Figure 3. In single-ended mode, IN+ is internally

switched to channels CH0–CH7 for the MAX1291

(Figure 3a) and to CH0–CH3 for the MAX1293 (Figure

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

In differential mode IN+ and IN- are selected from ana-

log input pairs (Table 4) and are internally switched to

either of the analog inputs. This configuration is pseu-

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

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

(±0.1LSB 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.

250ksps, +3V, 8-/4-Channel, 12-Bit ADCs

with +2.5V Reference and Parallel Interface

_______________________________________________________________________________________

9

T/H

THREE-STATE, BIDIRECTIONAL

I/O INTERFACE

12

17k

8

8

4

8

4

8

SUCCESSIVE-

APPROXIMATION

REGISTER

MUX

CHARGE REDISTRIBUTION

12-BIT DAC

CLOCK

( ) ARE FOR MAX1291 ONLY.

ANALOG

INPUT

MULTIPLEXER

CONTROL LOGIC

&

LATCHES

REF

REFADJ

1.22V

REFERENCE

D0–D7

8-BIT DATA BUS

(CH5)

(CH4)

(CH7)

(CH6)

CH3

CH2

CH1

CH0

COM

CLK

CS

WR

RD

HBEN

GND

MAX1291

MAX1293

2.05

COMP

INT

Figure 2. Simplified Internal Architecture for 8-/4-Channel MAX1291/MAX1293