
Preliminary Technical Data
ADN2849
The single-ended voltage at CPAP and CPAN pins must be
within the –0.8V to-1.85V range for proper operation of the
cross point adjust block. The cross point will be controlled by
the differential voltage obtained from the single-ended voltages
applied to CPAP and CPAN pins. A simple implementation of a
cross point adjust circuit is shown in figure 23 where a 20K
potentiometer generates the required differential voltage within
the specified input voltage range.
Rev. Pr. G | Page 11 of 17
CPAP
GND
Cross
Point
Adjust
100uA
100uA
CPAN
20k
VEE
VEE
Figure 23. Cross point adjust control circuit
An alternative implementation is to use a voltage DAC and a
single-ended to differential conversion amplifier such as the
AD138 that will allow digital control of the cross point. When
designing the circuitry that will drive the voltages at the CPAP
and CPAN pins the user should take in account that each pin is
sinking 100
μΑ
. If the cross point adjust feature is not required
both the CPAP and CPAN pins should be connected to GND.
This will automatically set the cross point to 50%. Once the
cross point adjust has been calibrated under nominal conditions
it has very low drift over temperature and supply voltage
variations.
MODULATION ENABLE
The modulation voltage generated by the ADN2849 can be
enabled or disabled under the control of the MOD_ENB pin.
When the modulation is disabled, the input data is ignored and
the voltage at the output of the ADN2849 will place the EA
modulator in a high absorption (low transparency) state. The
relationship between the logic state of the MOD_ENB input and
the modulation voltage is described in table 8.
Table 8.
MOD_ENB logic level
Low
High
Modulation voltage
Enabled
Disabled
The MOD_ENB pin is a 5V TTL and CMOS compatible logic
input. Its equivalent circuit of the MOD_ENB pin is shown in
figure 24.
180K
MOD_ENB
60K
40K
VEE
VEE
GND
GND
GND
Figure 24. Equivalent circuit of the MOD_ENB pin
OUTPUT STAGE
The output stage of the ADN2849 can provide up to 2V bias
offset and up to 3V modulation voltage across a single-ended
50
load. Both the bias offset and the modulation voltage are
made available at a single pin (MODN) eliminating the need for
external bias inductors as shown in figure 25.
50
VEE
MODP
M
B
V
G
M
R
R
GND
ADN2849
From cross
point adjust
block
50
EAM
+
-
VBIAS_SET
+
-
VMOD_SET
100nF
GND
IMOD
Figure 25. Output stage of the ADN2849