Datasheet LT1497 (Analog Devices) - 8

LT1497
U U W U APPLICATIONS INFORMATION
The LT1497 is a dual current feedback amplifier with high
Capacitance on the Inverting Input
output current drive capability. Bandwidth is maintained Current feedback amplifiers require resistive feedback over a wide range of voltage gains by the appropriate from the output to the inverting input for stable operation. choice of feedback resistor. These amplifiers will drive low Take care to minimize the stray capacitance between the impedance loads such as cables with excellent linearity at output and the inverting input. Capacitance on the invert- high frequencies. ing input to ground will cause peaking in the frequency response (and overshoot in the transient response), but it
Feedback Resistor Selection
does not degrade the stability of the amplifier. The optimum value for the feedback resistor is a function of the operating conditions of the device, the load imped-
Power Supplies
ance and the desired flatness of frequency response. The The LT1497 will operate on single or split supplies from Small-Signal Bandwidth table gives the values which ±2V (4V total) to ±15V (30V total). It is not necessary to result in the highest bandwidth with less than 1dB of use equal value split supplies, however, the offset voltage peaking for various gains, loads and supply voltages. If and inverting input bias current will change. The offset this level of flatness is not required, a higher bandwidth voltage changes about 1mV per volt of supply mismatch. can be obtained by use of a lower feedback resistor. The The inverting bias current can change as much as 10µA characteristic curves of Bandwidth vs Supply Voltage per volt of supply mismatch, though typically the change indicate feedback resistors for peaking up to 5dB. These is less than 2.5µA per volt. curves use a solid line when the response has less than 1dB of peaking and a dashed line when the response has
Thermal Considerations
1dB to 5dB of peaking. Note that in a gain of 10 peaking is always under 1dB for the resistor ranges shown. Reducing The LT1497 contains a thermal shutdown feature that the feedback resistor further than 270Ω in a gain of 10 will protects against excessive internal (junction) tempera- increase the bandwidth, but it also loads the amplifier and ture. If the junction temperature of the device exceeds the reduces the maximum current available to drive the load. protection threshold, the device will begin cycling between normal operation and an off state. The cycling is
Capacitive Loads
not harmful to the part. The thermal cycling occurs at a slow rate, typically 10ms to several seconds, depending The LT1497 can drive capacitive loads directly when the upon the power dissipation and the thermal time con- proper value of feedback resistor is used. The graph of stants of the package and the amount of copper on the Maximum Capacitive Load vs Feedback Resistor should board under the package. Raising the ambient tempera- be used to select the appropriate value. The graph shows ture until the device begins thermal shutdown gives a feedback resistor values for 5dB frequency peaking when good indication of how much margin there is in the driving a 1k load at a gain of 2. This is a worst-case thermal design. condition. The amplifier is more stable at higher gains and driving heavier loads (smaller load resistors). Alterna- For surface mount devices heat sinking is accomplished tively, a small resistor (10Ω to 20Ω) can be put in series by using the heat spreading capabilities of the PC board with the output to isolate the capacitive load from the and its copper traces. Experiments have shown that the amplifier output. This has the advantage in that the ampli- heat spreading copper layer does not need to be electri- fier bandwidth is only reduced when the capacitive load is cally connected to the leads of the device. The PCB present, and the disadvantage that the gain is a function of material can be very effective at transmitting heat between the load resistance. the pad area attached to V – pins of the device and a ground 8