LT1191 UUWUAPPLICATIO S I FOR ATIOCable TerminationsUsing the Shutdown Feature The LT1191 operational amplifier has been optimized as a The LT1191 has a unique feature that allows the amplifier low cost video cable driver. The ±50mA guaranteed output to be shut down for conserving power or for multiplexing current enables the LT1191 to easily deliver 7.5VP-P into several amplifiers onto a common cable. The amplifier will 100Ω, while operating on ±5V supplies or 2.6VP-P on a shut down by taking Pin 5 to V–. In shutdown, the amplifier single 5V supply. dissipates 15mW while maintaining a true high impedance output state of 15kΩ in parallel with the feedback resis- When driving a cable it is important to terminate the cable tors. The amplifiers must be used in a noninverting con- to avoid unwanted reflections. This can be done in one of figuration for MUX applications. In inverting configurations two ways: single termination or double termination. With the input signal is fed to the output through the feedback single termination, the cable must be terminated at the components. The following scope photos show that with receiving end (75Ω to ground) to absorb unwanted en- very high R ergy. The best performance can be obtained by double L, the output is truly high impedance; the output slowly decays toward ground. Additionally, when termination (75Ω in series with the output of the amplifier, the output is loaded with as little as 1kΩ the amplifier shuts and 75Ω to ground at the other end of the cable). This off in 400ns. This shutoff can be under the control of HC termination is preferred because reflected energy is ab- CMOS operating between 0V and –5V. sorbed at each end of the cable. When using the double termination technique it is important to note that the signal Output Shutdown is attenuated by a factor of 2, or 6dB. This can be compen- sated for by taking a gain of 2, or 6dB in the amplifier. The cable driver has a –3dB bandwidth of 100MHz while 0V VSHDN driving the 150Ω load. Note the response can be improved – 5V by lowering the impedance of the feedback elements. VOUT Double Terminated Cable Driver 5V 3 + 7 75Ω CABLE 6 LT1191 2 – 4 R 75 FB Ω RG –5V LT1191 • TA09 1MHz SINE WAVE GATED OFF WITH SHUTDOWN PIN, AV = 1, RL = ∞ Cable Driver Voltage Gain vs FrequencyOutput Shutdown 10 AV = 2 VS = ±5V RFB = 1k TA = 25°C RG = 330Ω 0V 5 VSHDN R = 1k – 5V FB R = 1k G AV = 1 0 R V FB = 300Ω OUT RG = 300Ω –5 CLOSED-LOOP VOLTAGE GAIN (dB) –10 100k 1M 10M 100M FREQUENCY (Hz) LT1191 • TA10 LT1191 • TA08 1MHz SINE WAVE GATED OFF WITH SHUTDOWN PIN, AV = 1, RL = 1kΩ 9
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