MAX1090/MAX1092
400ksps, +5V, 8-/4-Channel, 10-Bit ADCs
with +2.5V Reference and Parallel Interface
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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.
( ) ARE FOR MAX1090 ONLY.
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