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LT1187CJ8 Datasheet(PDF) 9 Page - Linear Technology |
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LT1187CJ8 Datasheet(HTML) 9 Page - Linear Technology |
9 / 12 page 9 LT1187 APPLICATIO S I FOR ATIO Operating with Low Closed-Loop Gains The LT1187 has been optimized for closed-loop gains of 2 or greater. For a closed-loop gain of 2 the response peaks about 2dB. Peaking can be eliminated by placing a capacitor across the feedback resistor, (feedback zero). This peaking shows up as time domain overshoot of about 25%. Closed-Loop Voltage Gain vs Frequency Small-Signal Transient Response Small-Signal Transient Response Extending the Input Range Figure 1 shows a simplified schematic of the LT1187. In normal operation the REF pin 1 is grounded or taken to a DC offset control voltage and differential signals are ap- plied between pins 2 and 3. The input responds linearly until all of the 345 µA current flows through the 1.1k resistor and Q1 (or Q2) turns off. Therefore the maximum input swing is 380mVP or 760mVP-P. The second differen- tial pair, Q3 and Q4, is running at slightly larger current so that when the first input stage limits, the second stage remains biased to maintain the feedback. Occasionally it is necessary to handle signals larger than 760mVP-P at the input. The LT1187 input stage can be tricked to handle up to 1.5VP-P. To do this, it is necessary to ground pin 3 and apply the differential input signal between pin 1 and 2. The input signal is now applied across two 1.1k resistors in series. Since the input signal is applied to both input pairs, the first pair will run out of bias current before the second pair, causing the amplifier to go open-loop. The results of this technique are shown in the following scope photo. FREQUENCY (Hz) 9 LT1187 • AI03 100M 10M 1M 100k 7 5 –1 1 3 VS = ±5V TA = 25°C AV = 2 RFB = 900Ω RG = 900Ω CFB = 0pF 0 2 4 6 8 CFB = 10pF CFB = 5pF AV = 2, OVERSHOOT = 25%, RFB = RG = 1k LT1187 • AI04 AV = 2, WITH 8pF FEEDBACK CAPACITOR LT1187 • AI05 |
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