LT1194 UUWUAPPLICATIO S I FOR ATIODouble Terminated Cable DriverVoltage Controlled Current Source 5V 5V 3 7 + 3 7 CABLE ±V + 75Ω IN 5 6 LT1194 6 LT1191 2 – 2 75Ω 4 – 8 4 CC 1 –5V –5V 5V VC 7 3 + 2k 6 5 R LT1194 5.1Ω Voltage Gain vs Frequency – 4 2 I 16 O = ±20mA T –5V A = 25°C RL 14 100Ω LT1194 • TA09 12 10 Output Current Response 8 VOLTAGE GAIN (dB) 6 4 CC = 1pF 2 100k 1M 10M 100M FREQUENCY (Hz) LT1194 • TA08 CC = 3pF will appear at the (–) input if the feedback loop is properly CC = 20pF closed. In steady state the input signal appears at the output of the LT1194, and 1/10 of this signal is applied across the sense resistor. Thus the output current is LT1194 • TA10 ±20mA CURRENT SOURCE WITH DIFFERENT simply: COMPENSATION CAPACITORS V I IN O = Differential Video Loop Thru Amplifier R • 10 for Power-Down Applications The compensation capacitor CC forces the LT1191 to be the dominate pole for the loop, while the LT1194 is fast VIN enough to be transparent in the feedback path. The ratio of 5V 1.5k the load resistor to the sense resistor should be approxi- 15k 3 + 7 mately 10:1 or greater for easy compensation. For the CABLE 15k 2 6 LT1194 OUTPUT example shown the load resistor is 100Ω, the sense 5 1.5k – 4 resistor is 5.1Ω, and various loop compensation capaci- – 5V tors cause the output to exhibit an underdamped, critically 1% RESISTOR WORST-CASE CMRR = 22dB and overdamped response. TYPICALLY = 38dB LT1194 • TA11 9
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