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EL8401ISZ-T7 Datasheet(PDF) 10 Page - Intersil Corporation |
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EL8401ISZ-T7 Datasheet(HTML) 10 Page - Intersil Corporation |
10 / 15 page 10 FN7105.3 August 29, 2007 Description of Operation and Application Information Product Description The EL8200, EL8201 and EL8401 are wide bandwidth, single supply, low power and rail-to-rail output voltage feedback operational amplifiers. The amplifiers are internally compensated for closed loop gain of +1 of greater. Connected in voltage follower mode and driving a 1k Ω load, they have a -3dB bandwidth of 200MHz. Driving a 150 Ω load, the bandwidth is about 130MHz while maintaining a 200V/us slew rate. The EL8200 is available with a power down pin to reduce power to 30µA typically while the amplifier is disabled. Input, Output and Supply Voltage Range The EL8200, EL8201 and EL8401 have been designed to operate with a single supply voltage from 3V to 5.0V. Split supplies can also be used as long as their total voltage is within 3V to 5.0V. The amplifiers have an input common mode voltage range from 0.15V below the negative supply (VS- pin) to within 1.5V of the positive supply (VS+ pin). If the input signal is outside the above specified range, it will cause the output signal to be distorted. The output of the EL8200, EL8201 and EL8401 can swing rail to rail. As the load resistance becomes lower, the ability to drive close to each rail is reduced. For the load resistor 1k Ω, the output swing is about 4.9V at a 5V supply. For the load resistor 150 Ω, the output swing is about 4.6V. Choice of Feedback Resistor and Gain Bandwidth Product For applications that require a gain of +1, no feedback resistor is required. Just short the output pin to the inverting input pin. For gains greater than +1, the feedback resistor forms a pole with the parasitic capacitance at the inverting input. As this pole becomes smaller, the amplifier’s phase margin is reduced. This causes ringing in the time domain and peaking in the frequency domain. Therefore, RF has some maximum value that should not be exceeded for optimum performance. If a large value of RF must be used, a small capacitor in the few Pico farad range in parallel with RF can help to reduce the ringing and peaking at the expense of reducing the bandwidth. As far as the output stage of the amplifier is concerned, the output stage is also a gain stage with the load. RF and RG appear in parallel with RL for gains other than +1. As this combination gets smaller, the bandwidth falls off. Consequently, RF also has a minimum value that should not be exceeded for optimum performance. For gain of +1, RF=0 is optimum. For the gains other than +1, optimum response is obtained with RF between 300Ω to 1kΩ. The EL8200, EL8201 and EL8401 have a gain bandwidth product of 100MHz. For gains ≥5, its bandwidth can be predicted by the following equation: Video Performance For good video performance, an amplifier is required to maintain the same output impedance and the same frequency response as DC levels are changed at the output. This is especially difficult when driving a standard video load of 150 Ω, because the change in output current with DC level. Special circuitry has been incorporated in the EL8200, EL8201 and EL8401 to reduce the variation of the output impedance with the current output. This results in dG and dP specifications of 0.03% and 0.05 °, while driving 150Ω at a gain of 2. Driving high impedance loads would give a similar or better dG and dP performance. Driving Capacitive Loads and Cables The EL8200, EL8201 and EL8401 can drive 10pF loads in parallel with 1k Ω with less than 5dB of peaking at gain of +1. If less peaking is desired in applications, a small series resistor (usually between 5 Ω to 50Ω) can be placed in series with the output to eliminate most peaking. However, this will reduce the gain slightly. If the gain setting is greater than 1, the gain resistor RG can then be chosen to make up for any gain loss which may be created by the additional series resistor at the output. When used as a cable driver, double termination is always recommended for reflection-free performance. For those applications, a back-termination series resistor at the amplifier’s output will isolate the amplifier from the cable and allow extensive capacitive drive. However, other applications may have high capacitive loads without a back-termination resistor. Again, a small series resistor at the output can help to reduce peaking. Disable/Power-Down The EL8200 can be disabled and placed its output in a high impedance state. The turn off time for each channel is about 25ns and the turn on time is about 200ns. When disabled, the amplifier’s supply current is reduced to 30µA typically, thereby effectively eliminating the power consumption. The amplifier’s power down can be controlled by standard TTL or CMOS signal levels at the ENABLE pin. The applied logic signal is relative to VS- pin. Letting the ENABLE pin float or applying a signal that is less than 0.8V above VS- will enable the amplifier. The amplifier will be disabled when the signal at ENABLE pin is 2V above VS-. Output Drive Capability The EL8200, EL8201 and EL8401 do not have internal short circuit protection circuitry. They have a typical short circuit current of 70mA sourcing and 140mA sinking for the output is connected to half way between the rails with a 10 Ω resistor. If the output is shorted indefinitely, the power Gain BW × 100MHz = EL8200, EL8201, EL8401 |
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