The EL8200 , EL8201 and EL8401 are high bandwidth, single supply, low power, rail-to-rail output voltage feedback operational amplifiers. The amplifier is internally compensated for a larger closed-loop gain of 1. Connected in voltage follower mode and driving a 1kΩ load, they have a -3dB bandwidth of 200MHz. Driving a 150Ω load, the bandwidth is approximately 130MHz while maintaining a slew rate of 200V/us. The EL8200 can be connected with a power-down pin to reduce the power supply by 30µA typical while the amplifier is disabled.

The input, output and supply voltage ranges of the EL8200, EL8201 and EL8401 have been designed to operate from a single supply voltage of 3V to 5.0V. Split supplies can also be used as long as their total voltage is within 3V to 5.0V. The amplifier's inputs have a common mode voltage range below the negative supply from 0.15V (VS- pin) to 1.5V at the positive supply (VS+ pin). If the input signal is outside the range specified above, the output signal will be distorted. The outputs of the EL8200, EL8201 and EL8401 can swing rail-to-rail. As the load resistance becomes lower, the ability to drive closer to each track is reduced. For a load resistance of 1kΩ, the output swing is approximately 4.9V at a 5V supply. For a load resistance of 150Ω, the output swing is approximately 4.6V.

pin

Selection of Feedback Resistor and Gain Bandwidth Product For applications requiring +1 gain, 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 parasitic capacitance at the inverting input. As this pole becomes smaller, the phase margin of the amplifier is reduced. This results in ringing in the time domain and peaks in the frequency domain. Therefore, RF has some maximum optimum performance that should not be exceeded. When large values of R and F must be used, a small capacitor in the range of several picofarads using R in parallel with F can help reduce ringing and peaking at the expense of reduced bandwidth. As far as the output stage of the amplifier is concerned, the output stage is also related to the load gain stage. ?F and RG appear in parallel with R and L for gains other than 1. As this combination gets smaller, the bandwidth falls off. Therefore, RF also has a minimum value that should not be exceeded for optimal performance. For 1, R gain F=0 is optimal. For other gains than 1, the best response is obtained using R between 300Ω and 1kΩ.

The EL8200, EL8201 and EL8401 have a gain bandwidth of 100MHz. For gain ≥ 5, its bandwidth can be predicted by the following formula: gain × BW = 100MHz

Pin Description

Video performance

For good video performance, the amplifier needs to maintain the same output impedance and at the same frequency response as the DC level is changed at the output. This is especially difficult when finding a standard video load of 150Ω because of the variation in output current with the DC level. Special circuits have been incorporated in the EL8200, EL8201 and EL8401 to reduce the change in output impedance with current output. This results in 0.03% and 0.05° specifications of dG and dp while driving 150Ω at a gain of 2. Driving a high impedance load will give a similar or better performance for DG and DP.

Driving capacitive loads and cables

The EL8200, EL8201 and EL8401 can drive a 10pF load in parallel with 1kΩ with less than 5dB peak at a gain of 1. If less peak shaving is required in the application, a small resistor (usually 5Ω to 50Ω) can be placed in series with the output to eliminate most of the spikes. However, this will reduce the gain slightly. If the gain is set to greater than 1, the gain resistor RG can then be selected to compensate for any gain loss which can be created by adding a series of resistors to the output. When used as a cable driver, dual termination is always recommended for non-reflective performance. For those applications, a reverse-terminated series resistor at the output of the amplifier will isolate the amplifier from the cable and allow for a wide range of capacitive drives. However, other applications can have high capacitive loads without back-terminating resistors. Also, a small resistor in series with the output can help to reduce peaking.

Disable/Power-Down The EL8200 can be disabled and place its outputs in a high impedance state. The off-time is approximately 25ns for each channel and the on-time is approximately 200ns. When disabled, the amplifier's supply current is reduced to a typical 30µA, effectively eliminating power consumption. The amplifier's power drop can be controlled by standard TTL or CMOS signal levels on the ENABLE pin. The application logic signal is phased to the VS- pin. Leaving the ENABLE pin floating or applying a signal below 0.8V above VS- will enable this amplifier. The amplifier will be disabled by the signal when the ENABLE pin is 2V above VS-.

Output Drive Capability The EL8200, EL8201 and EL8401 have no internal short circuit protection circuit. They have a typical short circuit sourcing of 70mA and a sinking current of 140mA with a 10Ω resistor between the output connected to the halfway rail. If the output is shorted indefinitely, the power dissipation can easily increase so that the part will be destroyed. Maximum reliability is maintained if the output current does not exceed ±40mA. This limitation is set by the design of the internal metal interconnects.