Detailed description – Rainbow Electronics MAX1092 User Manual

Page 9

MAX1090/MAX1092

400ksps, +5V, 8-/4-Channel, 10-Bit ADCs

with +2.5V Reference and Parallel Interface

_______________________________________________________________________________________

Detailed Description

Converter Operation

The MAX1090/MAX1092 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 10-bit digital output. Their parallel 8+2
output format provides an easy interface to standard
microprocessors (µPs). Figure 2 shows the simplified
internal architecture of the MAX1090/MAX1092.

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 MAX1090
(Figure 3a) and to CH0–CH3 for the MAX1092 (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 ±0.5LSB
(±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 positive input (IN+) charges capacitor C

HOLD

. At

the end of the acquisition interval, the T/H switch
opens, retaining the charge on C

HOLD

as a sample of

the signal at IN+.

The conversion interval begins with the input multiplex-
er switching C

HOLD

from the positive input (IN+) to the

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 10-bit resolution. This action is equivalent to
transferring a 12pF [(V

IN+

) - (V

IN-

)] charge from C

HOLD

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

17k

SUCCESSIVE-

APPROXIMATION

MUX

( ) ARE FOR MAX1090 ONLY.

CHARGE REDISTRIBUTION

10-BIT DAC

CLOCK

ANALOG

INPUT

MULTIPLEXER

CONTROL LOGIC

AND

LATCHES

REF

REFADJ

1.22V

REFERENCE

D0–D7

8-BIT DATA BUS

(CH5)

(CH4)

CH3

CH2

CH1

CH0

COM

CLK

INT

HBEN

GND

LOGIC

MAX1090
MAX1092

2.05

COMP

(CH7)

(CH6)

Figure 2. Simplified Functional Diagram of 8-/4-Channel MAX1090/MAX1092