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LT1190CS8 Datasheet(PDF) 9 Page - Linear Technology |
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LT1190CS8 Datasheet(HTML) 9 Page - Linear Technology |
9 / 12 page 9 LT1190 Output Shutdown Output Shutdown Cable Terminations The LT1190 operational amplifier has been optimized as a low cost video cable driver. The ±50mAguaranteedoutput current enables the LT1190 to easily deliver 7.5VP-P into 100 Ω, while operating on ±5V supplies or 2.6VP-P on a single 5V supply. APPLICATIO S I FOR ATIO FREQUENCY (Hz) 10 8 6 4 2 0 –2 –4 –6 –8 –10 LT1190 • TA08 100M 10M 1M 100k VS = ±5V TA = 25°C AV = 2 RFB = 1k RG = 330Ω AV = 1 RFB = 1k RG = 1k CABLE 2 + – 5V 7 LT1190 –5V 4 3 6 Cable Driver Voltage Gain vs Frequency Double Terminated Cable Driver RG 75 Ω 75 Ω RFB When driving a cable it is important to terminate the cable to avoid unwanted reflections. This can be done in one of two ways: single termination or double termination. With single termination, the cable must be terminated at the receiving end (75 Ω to ground) to absorb unwanted en- ergy. The best performance can be obtained by double termination (75 Ω in series with the output of the amplifier, and 75 Ω to ground at the other end of the cable). This termination is preferred because reflected energy is ab- sorbed at each end of the cable. When using the double termination technique it is important to note that the signal 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 in excess of 30MHz while driving the 150 Ω load. Using the Shutdown Feature The LT1190 has a unique feature that allows the amplifier to be shut down for conserving power or for multiplexing several amplifiers onto a common cable. The amplifier will shut down by taking Pin 5 to V –. In shutdown, the amplifier dissipates 15mW while maintaining a true high impedance output state of 15k Ω in parallel with the feedback resistors. The amplifiers must be used in a noninverting configuration for MUX applications. In in- verting configurations the input signal is fed to the output through the feedback components. The following scope photos show that with very high RL, the output is truly high impedance; the output slowly decays toward ground. Additionally, when the output is loaded with as little as 1k Ω the amplifier shuts off in 400ns. This shutoff can be under the control of HC CMOS operating between 0V and – 5V. LT1190 • TA09 1MHz SINE WAVE GATED OFF WITH SHUTDOWN PIN, AV = 1, RL = SCOPE PROBE VOUT –5V 0V VSHDN LT1190 • TA10 1MHz SINE WAVE GATED OFF WITH SHUTDOWN PIN, AV = 1, RL = 1kΩ VOUT –5V 0V VSHDN |
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