Rainbow Electronics ADC0820 - Manual (Page 12)

2 0 Analog Considerations
(Continued)
External Reference 2 5V Full-Scale
TL H 5501 – 21
Power Supply as Reference
TL H 5501 – 22
Input Not Referred to GND
TL H 5501 – 23
FIGURE 9 Analog Input Options
TL H 5501 – 24
FIGURE 10a
TL H 5501 – 25
FIGURE 10b
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 tran-
sients 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 nec-
essary to filter out these transients by putting an external
cap on the V
IN terminal
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
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 ms the time through which V
IN
must be 1 2 LSB 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
(Sections 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
IN 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 ms 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 ms 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
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