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ADN2849ACP-RL7 Datenblatt(PDF) 11 Page - Analog Devices |
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ADN2849ACP-RL7 Datenblatt(HTML) 11 Page - Analog Devices |
11 / 10 page 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. 50Ω 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 |
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Ähnliche Beschreibung - ADN2849ACP-RL7 |
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