Preliminary Technical Data

ADN2849

Rev. Pr. G | Page 11 of 17

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 20KΩ

potentiometer generates the required differential voltage within

the specified input voltage range.

CPAP

GND

Cross

Point

Adjust

100uA

100uA

CPAN

20k

Ω

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 100µΑ. 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

Modulation voltage

Low

Enabled

High

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

50Ω 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.

VEE

MODP

R

R

GND

ADN2849

From cross

point adjust

block

50Ω

EAM

+

-

VBIAS_SET

+

-

VMOD_SET

100nF

GND

IMOD

Figure 25. Output stage of the ADN2849