ADC1034 datasheet(12/18 Pages) NSC

12 / 18 page

30 Analog Considerations (Continued)

External Reference 25V Full Scale

TLH10556 – 20

Power Supply as Reference

TLH10556 – 21

Input Not Referred to GND

TLH10556 – 22

Current path must still exist from VIN(b) to

ground

FIGURE 3 Analog Input Options

33 REFERENCE AND INPUT

The two VREF inputs of the ADC103148 are fully differen-

tial and define the zero to full-scale input range of the A to D

converter This allows the designer to easily vary the span

of the analog input since this range will be equivalent to the

voltage difference between VREFa and VREFb By reduc-

ing VREF (VREF e VREFab VREFb) to less than 5V the

sensitivity of the converter can be increased (ie if VREF e

2V then 1 LSB e 195 mV) The inputreference arrange-

ment also facilitates ratiometric operation and in many

cases the chip power supply can be used for transducer

power as well as the VREF source

This reference flexibility lets the input span not only be var-

ied but also offset from zero The voltage at VREFb sets the

input level which produces a digital output of all zeros

Though VIN is not itself differential the reference design

allows nearly differential-input capability for many measure-

ment applications

Figure 3 shows some of the configura-

tions that are possible

The ADC1031 has no VREFb pin VREFb is internally tied to

GND

Power Supply Bypassing

TLH10556 – 23

TLH10556 – 24

ADC1034 Datasheet(PDF) 12 Page - National Semiconductor (TI)