5V, 8-Channel, Serial, 12-Bit ADCs

with 3V Digital Interface

10

______________________________________________________________________________________

Track/Hold

The T/H enters tracking mode on the falling clock edge

after the fifth bit of the 8-bit control word is shifted in. The

T/H enters hold mode on the falling clock edge after the

eighth bit of the control word is shifted in. IN- is con-

nected to GND if the converter is set up for single-ended

inputs, and the converter samples the “+” input. IN- con-

nects to the “-” input if the converter is set up for differen-

tial inputs, and the difference of

|N+ - IN- is sampled.

The positive input connects back to IN+, at the end of

The time required for the T/H to acquire an input signal

is a function of how quickly its input capacitance is

charged. If the input signal’s source impedance is high,

acquisition time increases and more time must be

allowed between conversions. The acquisition time,

the signal, and is also the minimum time needed for the

signal to be acquired. It is calculated by the following:

impedances below 1k

Ω do not significantly affect the

ADC’s AC performance. Higher source impedances can

be used if an input capacitor is connected to the analog

inputs, as shown in Figure 5. Note that the input capaci-

tor forms an RC filter with the input source impedance,

limiting the ADC’s signal bandwidth.

Figure 5. Quick-Look Circuit

CH0

CH1

CH2

CH3

CH4

CH5

CH6

CH7

GND

TRACK

T/H

SWITCH

9k

HOLD

12-BIT CAPACITIVE DAC

REF

ZERO

COMPARATOR

–

+

16pF

SINGLE-ENDED MODE: IN+ = CHO–CH7, IN- = GND.

DIFFERENTIAL MODE:

AT THE SAMPLING INSTANT,

THE MUX INPUT SWITCHES

FROM THE SELECTED IN+

CHANNEL TO THE SELECTED

IN- CHANNEL.

INPUT

MUX

IN+ AND IN- SELECTED FROM PAIRS OF

CH0/CH1, CH2/CH3, CH4/CH5, CH6/CH7.

0.1µF

4.7µF

GND

CS

SCLK

DIN

DOUT

SSTRB

SHDN

+3V

N.C.

0.01µF

CH7

VL

REFADJ

+2.5V

REFERENCE

REF

C2

0.01µF

C1

4.7µF

+2.5V

**

0V TO

4.096V

ANALOG

INPUT

0.1µF

+3V

OSCILLOSCOPE

CH1

CH2

CH3

CH4

*FULL-SCALE ANALOG INPUT, CONVERSION RESULT = $FFF (HEX).

**REQUIRED FOR MAX1203 ONLY.

MAX1202

MAX1203

+5V

2MHz

OSCILLATOR

SCLK

SSTRB

DOUT*

Figure 4. Equivalent Input Circuit