Low-Cost, 2-Channel, ±14-Bit Serial ADCs

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Capacitive Loading Effects of XCLK in

Internal RC-Oscillator Mode

When using the internal RC oscillator, capacitive load-

ing effects on the XCLK pin must be minimized. Stray

capacitance, V is the supply voltage, and f is the fre-

quency of the internal RC oscillator.

External Reference

The reference inputs to the ADC are high impedance,

allowing both an external voltage reference and ratio-

metric applications without loading effects. The fully dif-

ferential analog signal and reference inputs are

advantageous for performing ratiometric conversions

(Figures 11 and 12). For example, when measuring

load cells, the bridge excitation and the ADC reference

input both share the same voltage source. As the exci-

tation changes with temperature or voltage, the output

of the load cell will change. But since the differential

reference voltage also changes, the conversion results

remain constant, all else remaining equal.

Weigh Scale Application

The fully differential analog signal and reference inputs

make the MAX111 easy to interface to transducers with

differential outputs, such as the load cell in Figure 11.

Because the ADC input is differential, the load cell only

requires differential gain, eliminating the need for the

difference amplifier (differential to single-ended con-

verter) of the standard three op-amp instrumentation-

amplifier realization.

The 30mV full-scale bridge output is amplified to 2V

full-scale and applied to the MAX111 channel-one

input. The reference voltage to the ADC is created by a

voltage divider connected to the +5V rail. The same 5V

provides excitation for the bridge; therefore, as the

excitation voltage varies, the reference voltage to the

ADC also varies, providing an ADC output that does

not depend on the supply voltage.

The two 121k

Ω resistors connected to the +5V supplies

shift the common-mode voltage from 2.5V (5V/2) to

1.5V to ensure linearity. Match these two resistors to

avoid introducing differential offset, or trim the resistor

mismatch with a potentiometer. In practice, the scale is

“zeroed” or “tared” by storing the average of several

conversions in a memory location while the scale is

+5V

30mV

FULL-SCALE

121k

2k

121k

49.9k

1k

22k

10k

1k

1k

1/2 MAX492

1/2 MAX492

1µF

1µF

REF+

REF-

IN1+

IN1-

AGND

CS

DIN

DOUT

SCLK

49.9k

+5V

0.1µF

MAX111

+5V

+5V

+5V

GND

Figure 11. Weigh Scale Application