Datasheet LT1206 (Analog Devices) - 9

LT1206
applicaTions inForMaTion
12 capacitor and the supply current is typically 100µA. The VS = ±15V 10 shutdown pin is referenced to the positive supply through 8 RF = 1.2k an internal bias circuit (see the simplified schematic). An COMPENSATION 6 easy way to force shutdown is to use open drain (collec- 4 tor) logic. The circuit shown in Figure 2 uses a 74C904 2 RF = 2k NO COMPENSATION buffer to interface between 5V logic and the LT1206. The 0 switching time between the active and shutdown states VOLTAGE GAIN (dB) –2 RF = 2k COMPENSATION is less than 1µs. A 24k pull-up resistor speeds up the –4 turn-off time and insures that the LT1206 is completely –6 turned off. Because the pin is referenced to the positive –8 1 10 100 supply, the logic used should have a breakdown voltage FREQUENCY (MHz) 1206 F01 of greater than the positive supply voltage. No other
Figure 1
circuitry is necessary as the internal circuit limits the shutdown pin current to about 500µA. Figure 3 shows is flat to 0.35dB to 30MHz. The network has the greatest the resulting waveforms. effect for CL in the range of 0pF to 1000pF. The graph of Maximum Capacitive Load vs Feedback Resistor can be 15V used to select the appropriate value of feedback resistor. VIN + The values shown are for 0.5dB and 5dB peaking at a gain LT1206 VOUT of 2 with no resistive load. This is a worst case condition, –S/D as the amplifier is more stable at higher gains and with –15V R some resistive load in parallel with the capacitance. Also F shown is the –3dB bandwidth with the suggested feedback 15V resistor vs the load capacitance. RG 5V 24k Although the optional compensation works well with ca- ENABLE pacitive loads, it simply reduces the bandwidth when it is 1206 F02 74C906 connected with resistive loads. For instance, with a 30Ω load, the bandwidth drops from 55MHz to 35MHz when
Figure 2. Shutdown Interface
the compensation is connected. Hence, the compensation was made optional. To disconnect the optional compensa- tion, leave the COMP pin open.
Shutdown/Current Set
VOUT
If the shutdown feature is not used, the SHUTDOWN pin must be connected to ground or V–.
The shutdown pin can be used to either turn off the bias- ing for the amplifier, reducing the quiescent current to ENABLE less than 200µA, or to control the quiescent current in normal operation. A 1µs/DIV 1206 F03 The total bias current in the LT1206 is controlled by the cur- V = 1 RF = 825Ω rent flowing out of the shutdown pin. When the shutdown RL = 50Ω RPU = 24k pin is open or driven to the positive supply, the part is shut VIN = 1VP-P down. In the shutdown mode, the output looks like a 40pF
Figure 3. Shutdown Operation
1206fb 9 Document Outline Features Applications Description Typical Application Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Small-Signal Bandwidth Typical Performance Characteristics Simplified Schematic Applications Information Package Description Revision History Related Parts