External Reference

To use an external reference, set bit 2 (I/EREF) and bit

1 (REFSHDN) of control byte to 0 and connect the

at the REF pin. The DC input impedance at REF is

extremely high, consisting of leakage current only (typi-

cally 10nA). During a conversion, 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 conver-

sion clock frequency. If the reference has higher output

impedance or is noisy, bypass it close to the REF pin

with a 0.1µF capacitor. MAX1109 has an internal refer-

ence of +4.096V. To use the device with supply volt-

ages below 4.5V, external reference mode is required.

With an external reference voltage of less than +2.048V

(MAX1108) or +4.096V (MAX1109) at REF, the increase

in the ratio of the RMS noise to the LSB value (FS / 256)

results in performance degradation and decreased

dynamic range.

Internal Reference

To use the internal reference, set bit 2 (I/EREF) and bit 1

(REFSHDN) of the control byte to 1 and bypass REF with

a 1µF capacitor to ground. The internal reference can be

powered down after a conversion by setting bit 1 (REF-

SHDN) of the control byte to 0. When using the internal

reference, use MAX1108 and MAX1109 with supply volt-

age below 4.5V and above 4.5V, respectively.

Transfer Function

Table 4 shows the full-scale voltage ranges for unipolar

and bipolar modes. Figure 12a depicts the nominal,

unipolar I/O transfer function, and Figure 12b shows the

bipolar I/O transfer function. The zero scale is deter-

mined by the input selection setting and is either COM,

GND, or CH1.

Code transitions occur at integer LSB values. Output

coding is straight binary for unipolar operation and

two’s complement for bipolar operation. With a +2.048V

Layout, Grounding, and Bypassing

For best performance, use printed circuit boards. Wire-

wrap boards are not recommended. Board layout

should ensure that digital and analog signal lines are

separated from each other. Do not run analog and digi-

tal (especially clock) lines parallel to one another or run

digital lines underneath the ADC package.

Figure 13 shows the recommended system-ground

connections. A single-point analog ground (star-ground

point) should be established at the A/D ground.

Connect all analog grounds to the star ground. No digi-

tal-system ground should be connected to this point.

The ground return to the power supply for the star

ground should be low impedance and as short as pos-

sible for noise-free operation.

affect the comparator in the ADC. Bypass the supply to

the star ground with 0.1µF and 1µF capacitors close to

capacitor lead lengths for best supply-noise rejection. If

the power supply is very noisy, a 10

Ω resistor can be

connected to form a lowpass filter.

Single-Supply, Low-Power,

2-Channel, Serial 8-Bit ADCs

18

______________________________________________________________________________________

OUTPUT CODE

FULL-SCALE

TRANSITION

11111111

11111110

11111101

00000011

00000010

00000001

00000000

12

3

0

FS

FS - 1LSB

256

INPUT VOLTAGE (LSB)

(COM)

Figure 12a. Unipolar Transfer Function

01111111

OUTPUT CODE

01111110

00000010

00000001

00000000

11111111

11111110

11111101

10000001

10000000

-FS

COM

INPUT VOLTAGE (LSB)

+FS -

1

LSB

2

+FS =

+ COM

2

-FS =

+ COM

2

COM =

2

1LSB =

256

Figure 12b. Bipolar Transfer Function