Datasheet LT1204 (Analog Devices) - 10
| Manufacturer | Analog Devices |
| Description | 4-Input Video Multiplexer with 75MHz Current Feedback Amplifier |
| Pages / Page | 20 / 10 — APPLICATI. S I FOR ATIO. Capacitance on the Inverting Input. Large-Signal … |
| File Format / Size | PDF / 272 Kb |
| Document Language | English |
APPLICATI. S I FOR ATIO. Capacitance on the Inverting Input. Large-Signal Transient Response. Capacitive Loads. Slew Rate
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LT1204
O U U W U APPLICATI S I FOR ATIO Capacitance on the Inverting Input Large-Signal Transient Response
Current feedback amplifiers require resistive feedback from the output to the inverting input for stable operation. Take care to minimize the stray capacitance between the output and the inverting input. Capacitance on the invert- ing input to ground will cause peaking in the frequency response and overshoot in the transient response.
Capacitive Loads
The LT1204 can drive capacitive loads directly when the proper value of feedback resistor is used. The graph of Maximum Capacitive Load vs Feedback Resistor should 1204 AI02 VS = ±15V RF = 1k A R be used to select the appropriate value. The value shown V = 2 G = 1k RL = 400Ω is for 5dB peaking when driving a 1k load at a gain of 2. This is a worst-case condition. The amplifier is more
Large-Signal Transient Response
stable at higher gains and driving heavier loads. Alterna- tively, a small resistor (10Ω to 20Ω) can be put in series with the output to isolate the capacitive load from the amplifier output. This has the advantage that the ampli- fier bandwidth is only reduced when the capacitive load is present. The disadvantage is that the gain is a function of load resistance.
Slew Rate
The slew rate of the current feedback amplifier on the LT1204 is not independent of the amplifier gain the way 1204 AI03 VS = ±15V RF = 910Ω slew rate is in a traditional op amp. This is because both the AV = 10 RG = 100Ω RL = 400Ω input and the output stage have slew rate limitations. In high gain settings the signal amplitude between the nega-
Switching Characteristics and Pin 8
tive input and any driven positive input is small and the overall slew rate is that of the output stage. For gains less Switching between channels is a “make-before-break” than 10, the overall slew rate is limited by the input stage. condition where both inputs are on momentarily. The buffers isolate the inputs when the “make-before-break” The input slew rate of the LT1204 is approximately 135V/µs switching occurs. The input with the largest positive and is set by internal currents and capacitances. The voltage determines the output level. If both inputs are output slew rate is set by the value of the feedback equal, there is only a 40mV error at the input of the CFA resistors and the internal capacitances. At a gain of 10 with during the transition. The reference adjust (Pin 8) allows a 1k feedback resistor and ±15 supplies, the output slew the user to trade off positive input voltage range for rate is typically 1000V/µs. Larger feedback resistors will switching time. For example, on ±15V supplies, setting reduce the slew rate as will lower supply voltages, similar the voltage on Pin 8 to – 6.8V reduces the switching to the way the bandwidth is reduced. transient to a 50ns duration, and reduces the positive input The graph, Maximum Undistorted Output vs Frequency, range from 6V to 2.35V. The negative input range remains relates the slew rate limitations to sinusoidal inputs for unchanged at – 6V. When switching video “in picture,” this various gain configurations. short transient is imperceptible even on high quality 10
O U U W U APPLICATI S I FOR ATIO Capacitance on the Inverting Input Large-Signal Transient Response
Current feedback amplifiers require resistive feedback from the output to the inverting input for stable operation. Take care to minimize the stray capacitance between the output and the inverting input. Capacitance on the invert- ing input to ground will cause peaking in the frequency response and overshoot in the transient response.
Capacitive Loads
The LT1204 can drive capacitive loads directly when the proper value of feedback resistor is used. The graph of Maximum Capacitive Load vs Feedback Resistor should 1204 AI02 VS = ±15V RF = 1k A R be used to select the appropriate value. The value shown V = 2 G = 1k RL = 400Ω is for 5dB peaking when driving a 1k load at a gain of 2. This is a worst-case condition. The amplifier is more
Large-Signal Transient Response
stable at higher gains and driving heavier loads. Alterna- tively, a small resistor (10Ω to 20Ω) can be put in series with the output to isolate the capacitive load from the amplifier output. This has the advantage that the ampli- fier bandwidth is only reduced when the capacitive load is present. The disadvantage is that the gain is a function of load resistance.
Slew Rate
The slew rate of the current feedback amplifier on the LT1204 is not independent of the amplifier gain the way 1204 AI03 VS = ±15V RF = 910Ω slew rate is in a traditional op amp. This is because both the AV = 10 RG = 100Ω RL = 400Ω input and the output stage have slew rate limitations. In high gain settings the signal amplitude between the nega-
Switching Characteristics and Pin 8
tive input and any driven positive input is small and the overall slew rate is that of the output stage. For gains less Switching between channels is a “make-before-break” than 10, the overall slew rate is limited by the input stage. condition where both inputs are on momentarily. The buffers isolate the inputs when the “make-before-break” The input slew rate of the LT1204 is approximately 135V/µs switching occurs. The input with the largest positive and is set by internal currents and capacitances. The voltage determines the output level. If both inputs are output slew rate is set by the value of the feedback equal, there is only a 40mV error at the input of the CFA resistors and the internal capacitances. At a gain of 10 with during the transition. The reference adjust (Pin 8) allows a 1k feedback resistor and ±15 supplies, the output slew the user to trade off positive input voltage range for rate is typically 1000V/µs. Larger feedback resistors will switching time. For example, on ±15V supplies, setting reduce the slew rate as will lower supply voltages, similar the voltage on Pin 8 to – 6.8V reduces the switching to the way the bandwidth is reduced. transient to a 50ns duration, and reduces the positive input The graph, Maximum Undistorted Output vs Frequency, range from 6V to 2.35V. The negative input range remains relates the slew rate limitations to sinusoidal inputs for unchanged at – 6V. When switching video “in picture,” this various gain configurations. short transient is imperceptible even on high quality 10
