adc0820ccwmx National Semiconductor Corporation, adc0820ccwmx Datasheet - Page 16

• Manufacturer Part Number: adc0820ccwmx
• Description: 8-bit High Speed ?p Compatible A/d Converter With Track/hold Function
• Manufacturer: National Semiconductor Corporation

www.national.com
2.0 Analog Considerations
2.3 INPUT FILTERING
It should be made clear that transients in the analog input
signal, caused by charging current flowing into V
degrade the A/D’s performance in most cases. In effect the
ADC0820 does not “look” at the input when these transients
FIGURE 14.
FIGURE 15.
00550125
(Continued)
00550124
IN
, will not
16
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
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
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
much smaller. Since the MS flash ADC uses V
“compare” input and the LS ADC uses V
the ADC0820 only “samples” V
1.3 and 2.2). Even though the two flashes are not done
simultaneously, the analog signal is measured at one instant.
The value of V
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’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.
1
⁄
IN
2
LSB stable throughout the conversion pro-
approximately 100 ns after the rising edge
IN
IN
when WR is low (Sections
must be
IN
as its “zero” input,
1
⁄
2
LSB stable is
IN
IN
termi-
as its