LT1195 UUWUAPPLICATIO S I FOR ATIOPower Supply Bypassing The LT1195 is quite tolerant of power supply bypassing. receiving end (75Ω to ground) to absorb unwanted en- In some applications a 0.1µF ceramic disc capacitor ergy. The best performance can be obtained by double placed 0.5 inches from the ampifier is all that is required. termination (75Ω in series with the output of the amplifier, In applications requiring good settling time, it is important and 75Ω to ground at the other end of the cable). This to use multiple bypass capacitors. A 0.1µF ceramic disc in termination is preferred because reflected energy is ab- parallel with a 4.7µF tantalum is recommended. sorbed at each end of the cable. When using the double termination technique it is important to note that the signal Cable Terminations 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 LT1195 operational amplifier has been optimized as a low cost video cable driver. The ±20mA guaranteed output Using the Shutdown Feature current enables the LT1195 to easily deliver 6VP-P into 150Ω, while operating on ±5V supplies. The LT1195 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 Double-Terminated Cable Driver shutdown by taking Pin 5 to V –. In shutdown, the amplifier 5V dissipates 15mW while maintaining a true high impedance 3 7 output state of 15k in parallel with the feedback resistors. + CABLE 6 75Ω The amplifiers must be used in a noninverting configura- LT1195 2 tion for MUX applications. In inverting configurations the – 4 R 75 FB Ω input signal is fed to the output through the feedback RG –5V components. The following scope photos show that with very high RL, the output is truly high impedance; the 1195 AI01 output slowly decays toward ground. Additionally, when the output is loaded with as little as 1k the amplifier shuts Cable Driver Voltage Gain vs Frequency off in 700ns. This shutoff can be under the control of HC 8 CMOS operating between 0V and –5V. 6 4 AV = 2 2 RFB = 1k R Output Shutdown G = 330Ω 0 A –2 V = 1 RFB = 1k –4 RG = 1k VOLTAGE GAIN (dB) –6 – 8 VS = ±5V –10 TA = 25°C –12 100k 1M 10M 100M FREQUENCY (Hz) 1195 AI02 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 1MHz SINE WAVE GATED OFF WITH SHUTDOWN PIN single termination, the cable must be terminated at the AV = 1, RL = SCOPE PROBE 1195 AI03 1195fa 8
