ADC0820-N

SNAS529C – JUNE 1999 – REVISED MARCH 2013

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Figure 27. ADC0820-N Input Circuit

Figure 28. Analog Input, RC Network Model

INPUT FILTERING

It should be made clear that transients in the analog input signal, caused by charging current flowing into VIN, will

not degrade the A/D's performance in most cases. In effect the ADC0820-n 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 VIN terminal.

INHERENT SAMPLE-HOLD

Another benefit of the ADC0820-N'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 remain at least ½ LSB stable throughout the conversion process 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 switching, already accomplish this function to a large degree

(see The Sampled Data Comparator). Although the conversion time for the ADC0820-N is 1.5 µs, the time

through which VIN must be ½ LSB stable is much smaller. Since the MS flash ADC uses VIN as its “compare”

input and the LS ADC uses VIN as its “zero” input, the ADC0820-N only “samples” VIN when WR is low (see

Architecture and Input Current). Even though the two flashes are not done simultaneously, the analog signal is

measured at one instant. The value of VIN approximately 100 ns after the rising edge of WR (100 ns due to

internal logic prop delay) will be the measured 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-N'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-N, with no such help, can typically measure 5V, 7 kHz waveforms.

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