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AD10242TZ Datasheet(PDF) 12 Page - Analog Devices

Part No. AD10242TZ
Description  MCM A/D Converter
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Maker  AD [Analog Devices]
Homepage  http://www.analog.com
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AD10242TZ Datasheet(HTML) 12 Page - Analog Devices

 
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AD10242
–11–
If a logic threshold other than the nominal 1.6 V is required,
the following equations show how to use an external resistor,
Rx, to raise or lower the trip point (see Figure 4, R1 = 17 k
Ω,
R2 = 8 k
Ω).
V
RRx
RR
RRx
R Rx
1
52
12
1
2
=
++
to lower logic threshold.
0.01 F
ENCODE
SOURCE
ENCODE
ENCODE
AD10242
Rx
Vl
5V
R1
R2
Figure 7. Lower Threshold for Encode
V
R
R
RRx
RRx
1
52
2
1
1
=
+
+
to raise logic threshold.
0.01 F
ENCODE
SOURCE
ENCODE
ENCODE
AD10242
Rx
Vl
5V
R1
R2
AVCC
Figure 8. Raise Logic Threshold for Encode
While the single-ended encode will work well for many applica-
tions, driving the encode differentially will provide increased
performance. Depending on circuit layout and system noise, a
1 dB to 3 dB improvement in SNR can be realized. It is recom-
mended that the encode signal be ac-coupled into the ENCODE
and
ENCODE pins.
The simplest option is shown below. The low jitter TTL signal
is coupled with a limiting resistor, typically 100
Ω, to the primary
side of an RF transformer (these transformers are inexpensive
and readily available; part number in Figures 9 and 10 is from
Mini-Circuits). The secondary side is connected to the ENCODE
and
ENCODE pins of the converter. Since both encode inputs
are self-biased, no additional components are required.
TTL
ENCODE
ENCODE
AD10242
100
T1–1T
Figure 9. TTL Source—Differential Encode
If no TTL source is available, a clean sine wave may be substi-
tuted. In the case of the sine source, the matching network is
shown below. Since the matching transformer specified is a 1:1
impedance ratio, the load resistor R should be selected to match
the source impedance. The input impedance of the AD9042
is negligible in most cases.
ENCODE
ENCODE
AD10242
R
T1–1T
SINE
SOURCE
Figure 10. Sine Source—Differential Encode
If a low jitter ECL clock is available, another option is to ac-couple
a differential ECL signal to the encode input pins, as shown
in Figure 11. The capacitors shown here should be chip capaci-
tors but do not need to be of the low inductance variety.
ENCODE
ENCODE
AD10242
ECL
GATE
0.1 F
0.1 F
–VS
510
510
Figure 11. Differential ECL for Encode
As a final alternative, the ECL gate may be replaced by an ECL
comparator. The input to the comparator could then be a logic
signal or a sine signal.
ENCODE
ENCODE
AD10242
0.1 F
0.1 F
–VS
50
AD96687 (1/2)
510
510
Figure 12. ECL Comparator for Encode
Care should be taken not to overdrive the encode input pin when
ac-coupled. Although the input circuitry is electrically protected
from overvoltage or undervoltage conditions, improper circuit
operations may result from overdriving the encode input pin.
REV. D


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