ADC0820 Datasheet, PDF (13/14 Pages) National Semiconductor (TI) – 8-Bit High Speed μP Compatible A/D Converter with Track/Hold Function

Philips Semiconductors Linear Products
8-Bit, high-speed, µP-compatible A/D converter with
track/hold function
Product specification
ADC0820
therefore not necessary to filter out these transients by putting an
external cap on the V_IN terminal, if an input amplifier that can settle
within 600ns is used to drive the input. The NE530 is a suitable op
amp for driving the input of the ADC0820.
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 remain at least 1/2LSB
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 (Section 1.2).
Although the conversion time for the ADC0820 is 1.5µs, the time
through which V_IN must be 1/2LSB stable is much smaller. Since
the MS flash ADC uses V_IN as its “compare” input and the LS ADC
uses V_IN as its “zero” input, the ADC0820 only “samples” V_IN when
WR is Low. Even though the two flashes are not done
simultaneously, the analog signal is measured at one instant. The
value of V_IN approximately 100ns after the rising edge of WR (100ns
due to internal logic propagation delay) will be the measured value.
Input signals with slew rates typically below 100mV/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 1µs would still not be able to measure a 5V, 1kHz
sine wave without the aid of an external sample-and-hold. The
ADC0820, with no such help, can typically measure 5V, 7kHz
waveforms.
V_DD
+5V
V_DD
INT
RDY
V_REF
0.1µF 47µF _VIN
V_REF(+) CS
V_IN
RD
MODE
DB7
V_REF(–)
GND
DB0
Figure 11. 8-Bit Resolution Configuration
25k
40k
V_IN (+4V_CC, 3kHz MAX)
27k
+5V
–
+
12k
V_IN
CS
WR
RD
+5V
V_REF(+)
INT
+5V
0.1µF
47µF
V_DD
V_REF(–)
GND
MODE
DB7
DB0
Figure 12. Telecom A/D Converter
August 31, 1994
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