ADC0852 datasheet(13/20 Pages) NSC

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Functional Description (Continued)

40 ANALOG INPUTS

4 1 Differential Inputs

The serial interface of the ADC0852 and ADC0854 allows

them to be located right at the analog signal source and to

communicate with a controlling processor via a few fairly

noise immune digital lines This feature in itself greatly re-

duces the analog front end circuitry often needed to main-

tain signal integrity Nevertheless a few words are in order

with regard to the analog inputs should the input be noisy to

begin with or possibly riding on a large common mode volt-

age

The differential input of the comparator actually reduces the

effect of common-mode input noise ie signals common to

both selected ‘‘a’’ and ‘‘b’’ inputs such as 60 Hz line

noise The time interval between sampling the ‘‘a’’ input

and then the ‘‘b’’ input is

of a clock period (see

Figure

The change in the common-mode voltage during this short

time interval can cause comparator errors For a sinusoidal

common-mode signal this error is

VERROR (MAX) e VPEAK (2q fCM 2 fCLK)

where fCM is the frequency of the common-mode signal

Vpeak is its peak voltage value and fCLK is the DAC clock

frequency

For example 1 VPP 60 Hz noise superimposed on both

sides of a differential input signal would cause an error (re-

ferred to the input) of 075 mV This amounts to less than

of an LSB referred to the threshold DAC

assuming

VREF e V and fCLK e

kHz

4 2 Input Currents and Filtering

Due to the sampling nature of the analog inputs short

spikes of current enter the ‘‘a’’ input and leave the ‘‘b’’ at

the clock edges during a comparison These currents decay

rapidly and do not cause errors as the comparator is

strobed at the end of the clock period (see

Figure 5 )

The source resistance of the analog input is important with

regard to the DC leakage currents of the input multiplexer

The worst-case leakage currents of g1 mA over tempera-

ture will createa1mV input error with a 1 kX source

resistance An op-amp RC active low pass filter can provide

both impedance buffering and noise filtering should a high

impedance source be required

4 3 Arbitrary Analog InputReference Range

The total span of the DAC output and hence the compara-

tor’s threshold voltage is determined by the DAC reference

For example if VREF is set to 1 volt then the comparator’s

threshold can be programmed overa0to1 volt range with

8 bits of resolution From the analog input’s point of view

this span can also be shifted by applying an offset potential

to one of the comparator’s selected analog input lines (usu-

ally ‘‘b’’) This gives the designer greater control of the

ADC08524’s input range and resolution and can help sim-

plify or eliminate expensive signal conditioning electronics

An example of this capability is shown in the ‘‘Load Cell

Limit Comparator’’ of

Figure 15 In this circuit the ADC0852

allows the load-cell signal conditioning to be done with only

one dual op-amp and without complex multiple resistor

matching

50 POWER SUPPLY

A unique feature of the ADC0854 is the inclusion of a 7 volt

zener diode connected from the ‘‘Va’’ terminal to ground

(

Figures 2 and 8 ) ‘‘Va’’ also connects to ‘‘VCC’’ via a silicon

diode The zener is intended for use as a shunt voltage

regulator to eliminate the need for additional regulating

components This is especially useful if the ADC0854 is to

be remotely located from the system power source

An important use of the interconnecting diode between Va

and VCC is shown in Figures 10 and 11 Here this diode is

used as a rectifier to allow the VCC supply for the converter

to be derived from the comparator clock The low device

current requirements and the relatively high clock frequen-

cies used (10 kHz – 400 kHz) allows use of the small value

filter capacitor shown The shunt zener regulator can also

be used in this mode however this requires a clock voltage

swing in excess of 7 volts Current limiting for the zener is

also needed either built into the clock generator or through

a resistor connected from the clock to Va

Typical Applications

TLH5521 – 17

FIGURE 8 An On-Chip Shunt Regulator Diode

TLH5521 – 18

FIGURE 9 Using the ADC0854 as the

System Supply Regulator

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