Datasheet LT1187 (Analog Devices) - 10
| Manufacturer | Analog Devices |
| Description | Low Power Video Difference Amplifier |
| Pages / Page | 18 / 10 — APPLICATIONS INFORMATION. Operating with Low Closed-Loop Gains. Figure 5. … |
| File Format / Size | PDF / 596 Kb |
| Document Language | English |
APPLICATIONS INFORMATION. Operating with Low Closed-Loop Gains. Figure 5. Small-Signal Transient Response
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LT1187
APPLICATIONS INFORMATION 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%. 9 8 C 7 FB = 0pF AV = 2, WITH 8µF FEEDBACK CAPACITOR 1187 F05 6 5
Figure 5. Small-Signal Transient Response
CFB = 5pF 4 3 CFB = 10pF
Extending the Input Range
2 VS = 5V T 1 A = 25°C A Figure 2 shows a simplified schematic of the LT1187. In CLOSED-LOOP VOLTAGE GAIN (dB) V = 2 0 RFB = 900Ω normal operation REF, Pin 1, is grounded or taken to a DC R –1 G = 900Ω offset control voltage and differential signals are applied 100k 1M 10M 100M FREQUENCY (Hz) between Pins 2 and 3. The input responds linearly until 1187 F03 all of the 345µA current flows through the 1.1k resistor
Figure 3. Closed-Loop Voltage Gain vs Frequency
and Q1 (or Q2) turns off. Therefore the maximum input swing is 380mVP or 760mVP-P. The second differential 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 Pins 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 AV = 2, OVERSHOOT = 25%, RFB = RG = 1k 1187 F04 in the following scope photo.
Figure 4. Small-Signal Transient Response
1187fb 10 For more information www.linear.com/LT1187 Document Outline Features Applications Typical Application Description Absolute Maximum Ratings Pin Configuration Order Information ±5V Electrical Characteristics 5V Electrical Characteristics Typical Performance Characteristics Applications Information Simplified Schematic Package Description Revision History Related Parts
APPLICATIONS INFORMATION 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%. 9 8 C 7 FB = 0pF AV = 2, WITH 8µF FEEDBACK CAPACITOR 1187 F05 6 5
Figure 5. Small-Signal Transient Response
CFB = 5pF 4 3 CFB = 10pF
Extending the Input Range
2 VS = 5V T 1 A = 25°C A Figure 2 shows a simplified schematic of the LT1187. In CLOSED-LOOP VOLTAGE GAIN (dB) V = 2 0 RFB = 900Ω normal operation REF, Pin 1, is grounded or taken to a DC R –1 G = 900Ω offset control voltage and differential signals are applied 100k 1M 10M 100M FREQUENCY (Hz) between Pins 2 and 3. The input responds linearly until 1187 F03 all of the 345µA current flows through the 1.1k resistor
Figure 3. Closed-Loop Voltage Gain vs Frequency
and Q1 (or Q2) turns off. Therefore the maximum input swing is 380mVP or 760mVP-P. The second differential 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 Pins 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 AV = 2, OVERSHOOT = 25%, RFB = RG = 1k 1187 F04 in the following scope photo.
Figure 4. Small-Signal Transient Response
1187fb 10 For more information www.linear.com/LT1187 Document Outline Features Applications Typical Application Description Absolute Maximum Ratings Pin Configuration Order Information ±5V Electrical Characteristics 5V Electrical Characteristics Typical Performance Characteristics Applications Information Simplified Schematic Package Description Revision History Related Parts
