电压检测MAX110ACPE[1](18)

Low-Cost, 2-Channel, ±14-Bit Serial ADCsMAX110/MAX111

Figure 11. Weigh Scale Application

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. Straycapacitance causes the VDDpower consumption toincrease by an amount p = 1 2CV2f, where C = straycapacitance, V is the supply voltage, and f is the fre-quency of the internal RC oscillator.

Weigh Scale Application

The fully differential analog signal and reference inputsmake the MAX111 easy to interface to transducers withdifferential outputs, such as the load cell in Figure 11.Because the ADC input is differential, the load cell onlyrequires differential gain, eliminating the need for thedifference 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 2Vfull-scale and applied to the MAX111 channel-oneinput. The reference voltage to the ADC is created by avoltage divider connected to the +5V rail. The same 5Vprovides excitation for the bridge; therefore, as theexcitation voltage varies, the reference voltage to theADC also varies, providing an ADC output that doesnot depend on the supply voltage.

The two 121k resistors connected to the +5V suppliesshift the common-mode voltage from 2.5V (5V/2) to1.5V to ensure linearity. Match these two resistors toavoid introducing differential offset, or trim the resistormismatch with a potentiometer. In practice, the scale is“zeroed” or “tared” by storing the average of severalconversions in a memory location while the scale is

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 areadvantageous for performing ratiometric conversions(Figures 11 and 12). For example, when measuringload cells, the bridge excitation and the ADC referenceinput both share the same voltage source. As the exci-tation changes with temperature or voltage, the outputof the load cell will change. But since the differentialreference voltage also changes, the conversion resultsremain constant, all else remaining equal.

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