Multirange, +5V, 8-Channel,

Serial 12-Bit ADCs

16

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Power-Down Mode

To save power, configure the converter into low-current

shutdown mode between conversions. Two program-

mable power-down modes are available in addition to a

hardware shutdown. Select STBYPD or FULLPD by pro-

gramming PD0 and PD1 in the input control byte

(Table 4). When software power-down is asserted, it

becomes effective only after the end of conversion. For

example, if the control byte contains PD1 = 0, then the

chip remains powered up. If PD1 = 1, then the chip

powers down at the end of conversion. In all power-

down modes, the interface remains active and conver-

sion results can be read. Input overvoltage protection is

active in all power-down modes.

The first logical 1 on DIN after CS falls is interpreted as

a start condition, and powers up the MAX1270/

MAX1271 from a software selected STBYPD or FULLPD

condition.

For hardware-controlled power-down (FULLPD), pull

SHDN low. When hardware shutdown is asserted, it

becomes effective immediately, and any conversion in

progress is aborted.

Choosing Power-Down Modes

The bandgap reference and reference buffer remain

active in STBYPD mode, maintaining the voltage on the

4.7µF capacitor at REF. This is a DC state that does not

degrade after power-down of any duration.

In FULLPD mode, only the bandgap reference is active.

Connect a 33µF capacitor between REF and AGND to

maintain the reference voltage between conversions

and to reduce transients when the buffer is enabled

and disabled. Throughput rates down to 1ksps can be

achieved without allotting extra acquisition time for ref-

erence recovery prior to conversion. This allows con-

version to begin immediately after power-up. If the

discharge of the REF capacitor during FULLPD

exceeds the desired limits for accuracy (less than a

fraction of an LSB), run a STBYPD power-down cycle

prior to starting conversions. Take into account that the

reference buffer recharges the bypass capacitor at an

80mV/ms slew rate, and add 50µs for settling time.

Auto-Shutdown

Selecting STBYPD on every conversion automatically

shuts down the MAX1270/MAX1271 after each conversion

without requiring any start-up time on the next conversion.

REF

10k

Ω

2.5V

2.5V

REFADJ

MAX1270

MAX1271

4.7

µF

0.01

µF

Figure 13c. External Reference—Reference at REFADJ

REF

10k

Ω

2.5V

4.096V

REFADJ

MAX1270

MAX1271

4.7

µF

Figure 13b. External Reference—Reference at REF

REF

10k

Ω

2.5V

REFADJ

0.01

µF

MAX1270

MAX1271

4.7

µF

Figure 13a. Internal Reference