+2.7V to +5.25V, Low-Power, 10-Bit

Serial ADCs in SO-8

_______________________________________________________________________________________

7

AIN

TRACK INPUT

HOLD

GND

TRACK

HOLD

9k

16pF

-

+

COMPARATOR

ZERO

REF

CAPACITIVE DAC

AT THE SAMPLING INSTANT,

THE INPUT SWITCHES FROM

AIN TO GND.

SHUTDOWN

INPUT

ANALOG INPUT

+2.7V to +5.25V

1

2

3

4

AIN

SHDN

REF

8

7

6

5

SCLK

CS

DOUT

GND

SERIAL

INTERFACE

C*

*4.7

µF, MAX1242

0.1

µF, MAX1243

4.7

µF 0.1µF

REFERENCE

INPUT

MAX1243

MAX1242

MAX1243

Figure 3. Operational Diagram

Figure 4. Equivalent Input Circuit

ances below 4k

Ω do not significantly affect the ADC’s

AC performance.

Higher source impedances can be used if a 0.01µF

capacitor is connected to the analog input. Note that

the input capacitor forms an RC filter with the input

source impedance, limiting the ADC’s input signal

bandwidth.

Input Bandwidth

The ADC’s input tracking circuitry has a 2.25MHz

small-signal bandwidth, so it is possible to digitize

high-speed transient events and measure periodic sig-

nals with bandwidths exceeding the ADC’s sampling

rate by using undersampling techniques. To avoid

aliasing of unwanted high-frequency signals into the

frequency band of interest, anti-alias filtering is recom-

mended.

Analog Input Protection

Internal protection diodes, which clamp the analog

for accurate conversions near full scale, the input must

GND by 50mV.

If the analog input exceeds 50mV beyond the supplies,

limit the input current to 2mA.

Internal Reference (MAX1242)

The MAX1242 has an on-chip voltage reference

trimmed to 2.5V. The internal reference output is con-

nected to REF and also drives the internal capacitive

DAC. The output can be used as a reference voltage

source for other components and can source up to

400µA. Bypass REF with a 4.7µF capacitor. Larger

capacitors increase wake-up time when exiting shut-

down (see

The internal reference is enabled by pulling the SHDN

pin high. Letting SHDN float disables the internal refer-

ence, which allows the use of an external reference, as

described in the

External Reference section.

External Reference

The MAX1242/MAX1243 operate with an external refer-

ence at the REF pin. To use the MAX1242 with an

external reference, disable the internal reference by let-

ting SHDN float. Stay within the voltage range 1.0V to

impedance is 18k

Ω for DC currents. During conver-

sion, the external reference must be able to deliver up

to 250µA of DC load current and have an output

impedance of 10

Ω or less. The recommended mini-

mum value for the bypass capacitor is 0.1µF. If the ref-

erence has higher output impedance or is noisy,

bypass it close to the REF pin with a 4.7µF capacitor.