Datasheet LT1207 (Analog Devices) - 9
| Hersteller | Analog Devices |
| Beschreibung | Dual 250mA/60MHz Current Feedback Amplifier |
| Seiten / Seite | 16 / 9 — APPLICATI. S I FOR ATIO. Figure 1. Figure 2. Shutdown Interface. … |
| Dateiformat / Größe | PDF / 326 Kb |
| Dokumentensprache | Englisch |
APPLICATI. S I FOR ATIO. Figure 1. Figure 2. Shutdown Interface. Shutdown/Current Set
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LT1207
O U U W U APPLICATI S I FOR ATIO
12 V typically 100µA. Each Shutdown pin is referenced to the S = ±15V 10 positive supply through an internal bias circuit (see the 8 RF = 1.2k COMPENSATION Simplified Schematic). An easy way to force shutdown is 6 to use open drain (collector) logic. The circuit shown in 4 Figure 2 uses a 74C904 buffer to interface between 5V 2 RF = 2k NO COMPENSATION logic and the LT1207. The switching time between the 0 active and shutdown states is less than 1µs. A 24k pull-up VOLTAGE GAIN (dB) –2 RF = 2k COMPENSATION –4 resistor speeds up the turn-off time and insures that the –6 amplifier is completely turned off. Because the pin is –8 referenced to the positive supply, the logic used should 1 10 100 have a breakdown voltage of greater than the positive FREQUENCY (MHz) LT1207 • F01 supply voltage. No other circuitry is necessary as the internal circuit limits the Shutdown pin current to about
Figure 1.
500µA. Figure 3 shows the resulting waveforms. The network has the greatest effect for CL in the range of 15V 0pF to 1000pF. The graph of Maximum Capacitive Load vs VIN + Feedback Resistor can be used to select the appropriate 1/2 LT1207 VOUT value of the feedback resistor. The values shown are for –SHDN 0.5dB and 5dB peaking at a gain of 2 with no resistive load. This is a worst-case condition, as the amplifier is more stable –15V RF at higher gains and with some resistive load in parallel with the capacitance. Also shown is the –3dB bandwidth with the 15V RG suggested feedback resistor vs the load capacitance. 5V 24k ENABLE Although the optional compensation works well with LT1207 • F02 74C906 capacitive loads, it simply reduces the bandwidth when it is connected with resistive loads. For instance, with a 30Ω
Figure 2. Shutdown Interface
load, the bandwidth drops from 55MHz to 35MHz when the compensation is connected. Hence, the compensation was made optional. To disconnect the optional compensa- tion, leave the COMP pin open.
Shutdown/Current Set
V OUT
If the shutdown feature is not used, the Shutdown pins must be connected to ground or V –.
Each amplifier has a separate Shutdown pin which can be used to either turn off the amplifier, which reduces the ENABLE amplifier supply current to less than 200µA, or to control the supply current in normal operation. LT1207 • F3 AV = 1 RPU = 24k RF = 825Ω VIN = 1VP-P R The supply current in each amplifier is controlled by the L = 50Ω current flowing out of the Shutdown pin. When the Shut-
Figure 3. Shutdown Operation
down pin is open or driven to the positive supply, the For applications where the full bandwidth of the amplifier amplifier is shut down. In the shutdown mode, the output is not required, the quiescent current may be reduced by looks like a 40pF capacitor and the supply current is connecting a resistor from the Shutdown pin to ground. 9
O U U W U APPLICATI S I FOR ATIO
12 V typically 100µA. Each Shutdown pin is referenced to the S = ±15V 10 positive supply through an internal bias circuit (see the 8 RF = 1.2k COMPENSATION Simplified Schematic). An easy way to force shutdown is 6 to use open drain (collector) logic. The circuit shown in 4 Figure 2 uses a 74C904 buffer to interface between 5V 2 RF = 2k NO COMPENSATION logic and the LT1207. The switching time between the 0 active and shutdown states is less than 1µs. A 24k pull-up VOLTAGE GAIN (dB) –2 RF = 2k COMPENSATION –4 resistor speeds up the turn-off time and insures that the –6 amplifier is completely turned off. Because the pin is –8 referenced to the positive supply, the logic used should 1 10 100 have a breakdown voltage of greater than the positive FREQUENCY (MHz) LT1207 • F01 supply voltage. No other circuitry is necessary as the internal circuit limits the Shutdown pin current to about
Figure 1.
500µA. Figure 3 shows the resulting waveforms. The network has the greatest effect for CL in the range of 15V 0pF to 1000pF. The graph of Maximum Capacitive Load vs VIN + Feedback Resistor can be used to select the appropriate 1/2 LT1207 VOUT value of the feedback resistor. The values shown are for –SHDN 0.5dB and 5dB peaking at a gain of 2 with no resistive load. This is a worst-case condition, as the amplifier is more stable –15V RF at higher gains and with some resistive load in parallel with the capacitance. Also shown is the –3dB bandwidth with the 15V RG suggested feedback resistor vs the load capacitance. 5V 24k ENABLE Although the optional compensation works well with LT1207 • F02 74C906 capacitive loads, it simply reduces the bandwidth when it is connected with resistive loads. For instance, with a 30Ω
Figure 2. Shutdown Interface
load, the bandwidth drops from 55MHz to 35MHz when the compensation is connected. Hence, the compensation was made optional. To disconnect the optional compensa- tion, leave the COMP pin open.
Shutdown/Current Set
V OUT
If the shutdown feature is not used, the Shutdown pins must be connected to ground or V –.
Each amplifier has a separate Shutdown pin which can be used to either turn off the amplifier, which reduces the ENABLE amplifier supply current to less than 200µA, or to control the supply current in normal operation. LT1207 • F3 AV = 1 RPU = 24k RF = 825Ω VIN = 1VP-P R The supply current in each amplifier is controlled by the L = 50Ω current flowing out of the Shutdown pin. When the Shut-
Figure 3. Shutdown Operation
down pin is open or driven to the positive supply, the For applications where the full bandwidth of the amplifier amplifier is shut down. In the shutdown mode, the output is not required, the quiescent current may be reduced by looks like a 40pF capacitor and the supply current is connecting a resistor from the Shutdown pin to ground. 9