Datasheet LT1210 (Analog Devices) - 9

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APPLICATIONS INFORMATION
The LT1210 is a current feedback amplifier with high out- 14 put current drive capability. The device is stable with large 12 VS = ±15V CL = 200pF R capacitive loads and can easily supply the high currents F = 3.4kΩ 10 NO COMPENSATION required by capacitive loads. The amplifier will drive low 8 RF = 1.5kΩ impedance loads such as cables with excellent linearity COMPENSATION 6 at high frequencies. 4 2
Feedback Resistor Selection
VOLTAGE GAIN (dB) 0 –2 The optimum value for the feedback resistors is a function –4 RF = 3.4kΩ of the operating conditions of the device, the load imped- COMPENSATION –6 ance and the desired flatness of response. The Typical AC 1 10 100 FREQUENCY (MHz) Performance tables give the values which result in less 1210 F01 than 1dB of peaking for various resistive loads and oper-
Figure 1.
ating conditions. If this level of flatness is not required, a higher bandwidth can be obtained by use of a lower Also shown is the –3dB bandwidth with the suggested feedback resistor. The characteristic curves of Bandwidth feedback resistor vs the load capacitance. vs Supply Voltage indicate feedback resistors for peak- ing up to 5dB. These curves use a solid line when the Although the optional compensation works well with response has less than 1dB of peaking and a dashed line capacitive loads, it simply reduces the bandwidth when when the response has 1dB to 5dB of peaking. The curves it is connected with resistive loads. For instance, with a stop where the response has more than 5dB of peaking. 10Ω load, the bandwidth drops from 35MHz to 26MHz when the compensation is connected. Hence, the com- For resistive loads, the COMP pin should be left open (see pensation was made optional. To disconnect the optional Capacitive Loads section). compensation, leave the COMP pin open.
Capacitive Loads Shutdown/Current Set
The LT1210 includes an optional compensation network
If the shutdown feature is not used, the SHUTDOWN pin
for driving capacitive loads. This network eliminates most
must be connected to ground or V–.
of the output stage peaking associated with capacitive loads, allowing the frequency response to be flattened. The Shutdown pin can be used to either turn off the bias- Figure 1 shows the effect of the network on a 200pF load. ing for the amplifier, reducing the quiescent current to Without the optional compensation, there is a 6dB peak less than 200µA, or to control the quiescent current in at 40MHz caused by the effect of the capacitance on the normal operation. output stage. Adding a 0.01µF bypass capacitor between The total bias current in the LT1210 is controlled by the output and the COMP pins connects the compensation the current flowing out of the Shutdown pin. When the and greatly reduces the peaking. A lower value feedback Shutdown pin is open or driven to the positive supply, resistor can now be used, resulting in a response which the part is shut down. In the shutdown mode, the output is flat to ±1dB to 40MHz. The network has the greatest looks like a 70pF capacitor and the supply current is typi- effect for CL in the range of 0pF to 1000pF. The graphs of cally less than 100µA. The Shutdown pin is referenced to Bandwidth and Feedback Resistance vs Capacitive Load the positive supply through an internal bias circuit (see can be used to select the appropriate value of feedback the Simplified Schematic). An easy way to force shutdown resistor. The values shown are for 1dB and 5dB peaking at is to use open-drain (collector) logic. The circuit shown a gain of 2 with no resistive load. This is a worst-case con- in Figure 2 uses a 74C906 buffer to interface between 5V dition, as the amplifier is more stable at higher gains and logic and the LT1210. The switching time between the with some resistive load in parallel with the capacitance. active and shutdown states is about 1µs. A 24kΩ pull-up Rev C For more information www.analog.com 9 Document Outline Features Applications Typical Application Description Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Small-Signal Bandwidth Typical Performance Characteristics Applications Information Typical Applications Simplified Schematic Package Description Revision History Typical Application Related Parts