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HC55181DIM Datasheet(PDF) 11 Page - Intersil Corporation |
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HC55181DIM Datasheet(HTML) 11 Page - Intersil Corporation |
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11 / 19 page  38 Voice Transmission The feedback mechanism for monitoring the AC portion of the loop current consists of two amplifiers, the sense amplifier (SA) and the transmit amplifier (TA). The AC feedback signal is used for impedance synthesis. A detailed model of the AC feed back loop is provided below. The gain of the transmit amplifier, set by RS , determines the programmed impedance of the device. The capacitor CFB blocks the DC component of the loop current. The ground symbols in the model represent AC grounds, not actual DC potentials. The sense amp output voltage, VSA, as a function of Tip and Ring voltage and load is calculated using Equation 23. The transmit amplifier provides the programmable gain required for impedance synthesis. In addition, the output of this amplifier interfaces to the CODEC transmit input. The output voltage is calculated using Equation 24. Once the impedance matching components have been selected using the design equations, the above equations provide additional insight as to the expected AC node voltages for a specific Tip and Ring load. Transhybrid Balance The final step in completing the impedance synthesis design is calculating the necessary gains for transhybrid balance. The AC feed back loop produces an echo at the VTX output of the signal injected at VRX. The echo must be cancelled to maintain voice quality. Most applications will use a summing amplifier in the CODEC front end as shown below to cancel the echo signal. The resistor ratio, RF/RB, provides the final adjustment for the transmit gain, GTX. The transmit gain is calculated using Equation 25. Most applications set RF = RB, hence the device 2-wire to 4-wire equals the transmit gain. Typically RB is greater than 20k Ω to prevent loading of the device transmit output. The resistor ratio, RF/RA, is determined by the transhybrid gain of the device, G44. RF is previously defined by the transmit gain requirement and RA is calculated using Equation 26. Power Dissipation The power dissipated by the device during on hook transmission is strictly a function of the quiescent currents for each supply voltage during Forward Active operation. Off hook power dissipation is increased above the quiescent power dissipation by the DC load. If the loop length is less than or equal to RKNEE, the device is providing constant current, IA, and the power dissipation is calculated using Equation 28. If the loop length is greater than RKNEE, the device is operating in the constant voltage, resistive feed region. The power dissipated in this region is calculated using Equation 29. Since the current relationships are different for constant current versus constant voltage, the region of device operation is critical to valid power dissipation calculations. FIGURE 6. AC SIGNAL TRANSMISSION MODEL TIP RING + - -IN VFB VRX VTX R R R R + - + - 1:1 20 20 0.75R 3R 3R 3R 3R R/2 8K RS CFB TA VSA V SA V T V R – () – 10 Z L ------ = (EQ. 23) V VTX V SA R S 8e3 ---------- – = (EQ. 24) FIGURE 7. TRANSHYBRID BALANCE INTERFACE + - RF RB RA HC5518x CODEC +2.4V RX OUT TX IN -IN VRX VTX R R 1:1 RS TA G TX G – 24 R F R B -------- = (EQ. 25) R A R B G 44 ---------- = (EQ. 26) P FAQ V BH I × BHQ V BL I BLQ × V CC I CCQ × ++ = (EQ. 27) P FA IA () P FA Q () V BLxIA () R LOOPxI 2 A () – + = (EQ. 28) P FA IB () P FA Q () V BLxIB () R LOOPxI 2 B () – + = (EQ. 29) HC55180, HC55181, HC55182, HC55183, HC55184 |
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