AD9042
If no TTL source is available, a clean sine wave may be
substituted. In the case of the sine source, the matching net-
work is shown below. Since the matching transformer specified
is a 1:1 impedance ratio, R, the load resistor should be selected
to match the source impedance. The input impedance of the
AD9042 is negligible in most cases.
T1-1T
R
ENCODE
amplifier offset; this reference is designed to track internal cir-
cuit shifts over temperature.
250Ω
250Ω
AIN
SINE
SOURCE
ENCODE
AD9042
V
OFFSET
TIED TO
V
REF
THROUGH
50 OHMS
50Ω
+2.4V
REFERENCE
0.1µF
AD9042
Figure 33. Analog Input Offset by +2.4 V Reference
Figure 30. 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 below. The capacitors shown here should be chip
capacitors but do not need to be of the low inductance variety.
0.1µF
ENCODE
ECL
GATE
0.1µF
ENCODE
510Ω
510Ω
AD9042
Although the AD9042 may be used in many applications, it was
specifically designed for communications systems which must
digitize wide signal bandwidths. As such, the analog input was
designed to be ac-coupled. Since most communications products
do not down-convert to dc, this should not pose a problem. One
example of a typical analog input circuit is shown below. In this
application, the analog input is coupled with a high quality chip
capacitor, the value of which can be chosen to provide a low
frequency cutoff that is consistent with the signal being
sampled; in most cases, a 0.1
µF
chip capacitor will work well.
ANALOG
SIGNAL
SOURCE
R
T
0.1µF
AIN
V
OFFSET
50Ω
V
REF
0.1µF
AD9042
–V
S
Figure 31. 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.
AD96687 (1/2)
0.1µF
ENCODE
50Ω
510Ω
0.1µF
ENCODE
510Ω
Figure 34. AC-Coupled Analog Input Signal
AD9042
–V
S
Figure 32. ECL Comparator for Encode
Another option for ac-coupling is a transformer. The imped-
ance ratio and frequency characteristics of the transformer are
determined by examining the characteristics of the input signal
source (transformer primary connection), and the AD9042 in-
put characteristics (transformer secondary connection). “R
T
”
should be chosen to satisfy termination requirements of the
source, given the transformer turns ratio. A blocking capacitor
is required to prevent AD9042 dc bias currents from flowing
through the transformer.
BPF
ANALOG
SIGNAL
SOURCE
XFMR
R
T
50Ω
LO
0.1µF
V
REF
0.1µF
AIN
V
OFFSET
Care should be taken not to overdrive the encode input pin
when ac coupled. Although the input circuitry is electrically
protected from over or under voltage conditions, improper
circuit operations may result from overdriving the encode input
pins.
Driving the Analog Input
AD9042
Because the AD9042 operates off of a single +5 V supply, the
analog input range is offset from ground by 2.4 volts. The
analog input, AIN, is an operational amplifier configured in an
inverting mode (ref. Equivalent Circuits: Analog Input Stage).
V
OFFSET
is the noninverting input which is normally tied
through a 50 ohm resistor to V
REF
(ref. Equivalent Circuits:
2.4 V Reference). Since the operational amplifier forces its
inputs to the same voltage, the inverting input is also at 2.4 volts.
Therefore, the analog input has a Thevenin equivalent of 250 ohms
in series with a 2.4 volt source. It is strongly recommended
that the AD9042’s internal voltage reference be used for the
Figure 35. Transformer-Coupled Analog Input Signal
When calculating the proper termination resistor, note that the
external load resistor is in parallel with the AD9042 analog
input resistance, 250 ohms. The external resistor value can be
calculated from the following equation:
R
T
=
1
1
1
−
Z
250
where
Z
is desired impedance.
–12–
REV. A
AD [ ANALOG DEVICES ]
