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LT1210CS Datasheet(PDF) 8 Page - Linear Technology |
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LT1210CS Datasheet(HTML) 8 Page - Linear Technology |
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8 / 16 page  8 LT1210 S APPLICATI I FOR ATIO The LT1210 is a current feedback amplifier with high output current drive capability. The device is stable with large capacitive loads and can easily supply the high currents required by capacitive loads. The amplifier will drive low impedance loads such as cables with excellent linearity at high frequencies. Feedback Resistor Selection The optimum value for the feedback resistors is a function of the operating conditions of the device, the load imped- ance and the desired flatness of response. The Typical AC Performance tables give the values which result in less than 1dB of peaking for various resistive loads and oper- ating conditions. If this level of flatness is not required, a higher bandwidth can be obtained by use of a lower feedback resistor. The characteristic curves of Bandwidth vs Supply Voltage indicate feedback resistors for peaking up to 5dB. These curves use a solid line when the response has less than 1dB of peaking and a dashed line when the response has 1dB to 5dB of peaking. The curves stop where the response has more than 5dB of peaking. For resistive loads, the COMP pin should be left open (see Capacitive Loads section). Capacitive Loads The LT1210 includes an optional compensation network for driving capacitive loads. This network eliminates most of the output stage peaking associated with capacitive loads, allowing the frequency response to be flattened. Figure 1 shows the effect of the network on a 200pF load. Without the optional compensation, there is a 6dB peak at 40MHz caused by the effect of the capacitance on the output stage. Adding a 0.01 µF bypass capacitor between the output and the COMP pins connects the compensation and greatly reduces the peaking. A lower value feedback resistor can now be used, resulting in a response which is flat to ±1dB to 40MHz. The network has the greatest effect for CL in the range of 0pF to 1000pF. The graphs of Bandwidth and Feedback Resistance vs Capacitive Load can be used to select the appropriate value of feedback resistor. The values shown are for 1dB and 5dB peaking at a gain of 2 with no resistive load. This is a worst-case condition, as the amplifier is more stable at higher gains and with some resistive load in parallel with the capaci- FREQUENCY (MHz) 1 –6 –2 2 6 10 10 100 1210 F01 –4 0 4 8 12 14 VS = ±15V CL = 200pF RF = 1.5k COMPENSATION RF = 3.4k NO COMPENSATION RF = 3.4k COMPENSATION Figure 1 tance. Also shown is the – 3dB bandwidth with the sug- gested feedback resistor vs the load capacitance. Although the optional compensation works well with capacitive loads, it simply reduces the bandwidth when it is connected with resistive loads. For instance, with a 10 Ω load, the bandwidth drops from 35MHz to 26MHz 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 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 biasing for the amplifier, reducing the quiescent current to less than 200 µA, or to control the quiescent current in normal operation. The total bias current in the LT1210 is controlled by the current flowing out of the Shutdown pin. When the Shut- down pin is open or driven to the positive supply, the part is shut down. In the shutdown mode, the output looks like a 70pF capacitor and the supply current is typically less than 100 µA. The Shutdown pin is referenced to the posi- tive supply through an internal bias circuit (see the Simpli- fied Schematic). An easy way to force shutdown is to use open-drain (collector) logic. The circuit shown in Figure 2 uses a 74C904 buffer to interface between 5V logic and the LT1210. The switching time between the active and shut- down states is about 1 µs. A 24k pull-up resistor speeds |
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