May 8, 1997

Circuit adapts differential input to drive coax

Donald Whitney Jr, Harris Semiconductor, Melbourne, FL

ICs designed for analog and digital signal processing are migrating to lower voltage processes. Analog signals must therefore use differential lines to achieve adequate dynamic range. However, cable-modem and video applications require single-ended signal transmission through a coaxial cable. Moreover, the signal levels are higher than the level a typical op-amp driver with a series termination resistor can provide. The line driver in Figure 1 uses an HFA1100 or HFA1105 to convert a balanced input signal to a single-ended output signal. The circuit synthesizes the back-termination impedance to provide a matched termination to the coax load.

Positive feedback synthesizes the desired back-termination impedance, using the series resistor, R_M, which has a lower value than does Z_O. The voltage drop, or signal loss, across R_M is less than it would be for a line-driver circuit that uses a series back-termination resistor of value Z_O. A larger voltage swing is thus possible at the load. The positive feedback causes the amplifier to make up the difference between R_M and Z_O. Moreover, the addition of R₆ makes it possible to achieve equal gains from V _IN+ to V_OUT and from V_IN­ to V_OUT. Assuming that the amplifier has high open-loop gain, you can determine the resistor values by using the following equations and parameters. First, define the parameters:

• A_OF is the gain from V_IN+ to V_OUT or V_IN­ to V_OUT when Z_O=Z_L;

• Z_O is the desired back-termination impedance of the positive-feedback circuit;

• Z_L is the load impedance;

• V_OUTP is the peak desired output voltage at V_OUT;

• V_OP is the peak op-amp output voltage at V_O.

The input resistance at V_IN­ is a function of R₃, so make this resistance as high as the bandwidth considerations allow--usually 300 to 700 ohms for the HFA1100 or HFA1105. Set R₆ as low as possible to keep the input resistance at V_IN close to the value at V_IN­--usually 300 to 1000V. You can calculate the remaining resistors from the following equations:

The peak output voltage, V_OUTP, cannot be higher than the peak output current times the load impedance, Z_L. Also, as the gain, A_OF, and the ratio of Z_O to R_M increase, the required closed-loop gain, A_CL, increases, thereby limiting the attainable bandwidth of the line driver. Table 1 gives calculated A_CL values for some given R_M and A_OF values. Table 2 gives component values for A_OF=2 (differential gain of 4) and Z_O=75 ohms. Figure 2 shows both the amplifier voltage and the load voltage. The positive feedback increases the available load voltage from 1.4V_PK (2.8V_PK/2) to 2.1V_PK. (DI #2024)

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