ADC0820BCWM datasheet(15/22 Pages) NSC

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2.0 Analog Considerations (Continued)

2.3 INPUT FILTERING

It should be made clear that transients in the analog input

signal, caused by charging current flowing into V

IN, will not

degrade the A/D’s performance in most cases. In effect the

ADC0820 does not “look” at the input when these transients

occur. The comparators’ outputs are not latched while WR is

low, so at least 600 ns will be provided to charge the ADC’s

input capacitance. It is therefore not necessary to filter out

these transients by putting an external cap on the V

IN termi-

nal.

2.4 INHERENT SAMPLE-HOLD

Another benefit of the ADC0820’s input mechanism is its

ability to measure a variety of high speed signals without the

help of an external sample-and-hold. In a conventional SAR

type converter, regardless of its speed, the input must re-

main at least 1⁄2 LSB stable throughout the conversion pro-

cess if full accuracy is to be maintained. Consequently, for

many high speed signals, this signal must be externally

sampled, and held stationary during the conversion.

Sampled-data comparators, by nature of their input switch-

ing, already accomplish this function to a large degree (Sec-

tion 1.2). Although the conversion time for the ADC0820 is

1.5 µs, the time through which V

IN must be

⁄2 LSB stable is

much smaller. Since the MS flash ADC uses V

IN as its “com-

pare” input and the LS ADC uses V

IN as its “zero” input, the

ADC0820 only “samples” V

IN when WR is low (Sections 1.3

and 2.2). Even though the two flashes are not done simulta-

neously, the analog signal is measured at one instant. The

value of V

IN approximately 100 ns after the rising edge of

WR (100 ns due to internal logic prop delay) will be the mea-

sured value.

Input signals with slew rates typically below 100 mV/µs can

be converted without error. However, because of the input

time constants, and charge injection through the opened

comparator input switches, faster signals may cause errors.

Still, the ADC0820’s loss in accuracy for a given increase in

signal slope is far less than what would be witnessed in a

conventional successive approximation device. An SAR type

converter with a conversion time as fast as 1 µs would still

not be able to measure a 5V 1 kHz sine wave without the aid

of an external sample-and-hold. The ADC0820, with no such

help, can typically measure 5V, 7 kHz waveforms.

DS005501-24

FIGURE 14.

DS005501-25

FIGURE 15.

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