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

Serial ADCs in SO-8

8

_______________________________________________________________________________________

Serial Interface

Initialization after Power-Up and

Starting a Conversion

When power is first applied, and if SHDN is not pulled

low, it takes the fully discharged 4.7µF reference

bypass capacitor up to 20ms to provide adequate

charge for specified accuracy. With an external refer-

ence, the internal reset time is 10µs after the power

supplies have stabilized. No conversions should be

performed during these times.

To start a conversion, pull CS low. At CS’s falling edge,

the T/H enters its hold mode and a conversion is initiat-

ed. After an internally timed conversion period, the end

of conversion is signaled by DOUT pulling high. Data

can then be shifted out serially with the external clock.

Using

S

SH

HD

DN

N to Reduce Supply Current

Power consumption can be reduced significantly by

shutting down the MAX1242/MAX1243 between con-

versions. Figure 6 shows a plot of average supply cur-

rent vs. conversion rate. Because the MAX1243 uses

an external reference voltage (assumed to be present

continuously), it “wakes up” from shutdown more quick-

ly, providing lower average supply currents. The wake-

the time when a conversion may be initiated (Figure 5).

For the MAX1242, this time depends on the time in

shutdown (Figure 7) because the external 4.7µF refer-

ence bypass capacitor loses charge slowly during

shutdown. The MAX1243’s wake-up time is largely

dependent on the external reference’s power-up time. If

the external reference is not shut down, the wake-up

time is approximately 4µs.

10,000

100

1000

1

0.1

1

10

100

1k

10k

100k

10

CONVERSIONS/SEC

CODE = 0101010100

MAX1242

MAX1242

MAX1243

Figure 6. Average Supply Current vs. Conversion Rate

1.0

0.0

0.001

0.01

0.1

1

10

0.8

0.6

0.4

0.2

TIME IN SHUTDOWN (sec)

Figure 7. Typical Reference-Buffer Power-Up Delay vs. Time in

Shutdown

COMPLETE CONVERSION SEQUENCE

POWERED UP

POWERED DOWN

POWERED UP

CONVERSION 0

CONVERSION 1

DOUT

CS

SHDN

Figure 5. Shutdown Sequence