Single-Supply, Low-Power,

Serial 8-Bit ADCs

14

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When operating at speeds below the maximum sam-

pling rate, the MAX1106/MAX1107’s power-down mode

can save considerable power by placing the converter

in a low-current shutdown state between conversions.

Pull SHDN low after the conversion byte has been read

to shut down the device completely.

CONVST should remain low most of the time and tog-

gled high for 1µs (100µs max) for the autozero adjust-

ment. An external reference is recommended for best

accuracy when using the shutdown feature. This

requires only 20µs for the internal biasing circuit to sta-

bilize before starting a new conversion. Alternatively,

the internal reference can be used, but additional time

is required for the reference to stabilize (when

bypassed by a 1µF capacitor; at data rates above

1ksps, the reference stabilizes within 1LSB in 200µs). If

the reference is completely discharged it requires

12ms to settle. No conversions should be performed

until the reference voltage has stabilized.

Internal or External Voltage Reference

connected directly at the REFIN pin. To use the internal

reference, connect REFOUT directly to REFIN and

bypass REFOUT with a 1µF capacitor. The DC input

impedance at REFIN is extremely high, consisting of

leakage current only (typically 10nA). During a conver-

sion, the reference must be able to deliver up to 20µA

average load current and have an output impedance of

1k

Ω or less at the conversion clock frequency. If the

reference has higher output impedance or is noisy,

bypass it close to the REFIN pin with a 0.1µF capacitor.

The internal reference is active as long as SHDN is high

and powers down when SHDN is low.

Transfer Function

Figure 9 depicts the input/output transfer function.

Code transitions occur at integer LSB values. Output

coding is binary; with a 2.048V reference 1LSB = 8mV

and give a zero output code. Voltages greater than full-

scale give an all ones output code.

OUTPUT CODE

FULL-SCALE

TRANSITION

11111111

11111110

11111101

00000011

00000010

00000001

00000000

12

3

0

FS

FS - 1LSB

256

INPUT VOLTAGE (LSB)

(IN-)

Figure 9. Input/Output Transfer Function