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EL2450CN Datasheet(PDF) 11 Page - Elantec Semiconductor |
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EL2450CN Datasheet(HTML) 11 Page - Elantec Semiconductor |
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11 / 17 page  11 EL2250C, EL2450C 125MHz Single Supply Dual/Quad Op Amps some maximum value which should not be exceeded for optimum performance. If a large value of RF must be used, a small capacitor in the few picofarad range in par- allel with RF can help to reduce this ringing and peaking at the expense of reducing the bandwidth. As far as the output stage of the amplifier is concerned, RF + RG appear in parallel with RL for gains other than +1. As this combination gets smaller, the bandwidth falls off. Consequently, RF has a minimum value that should not be exceeded for optimum performance. For AV = +1, RF = 0Ω is optimum. For AV = -1 or +2 (noise gain of 2), optimum response is obtained with RF between 500 Ω and 1kΩ. For Av = -4 or +5 (noise gain of 5), keep RF between 2kΩ and 10kΩ. Video Performance For good video performance, an amplifier is required to maintain the same output impedance and the same fre- quency response as DC levels are changed at the output. This can be difficult when driving a standard video load of 150 Ω, because of the change in output current with DC level. Differential Gain and Differential Phase for the EL2250C/EL2450C are specified with the black level of the output video signal set to +1.2V. This allows ample room for the sync pulse even in a gain of +2 con- figuration. This results in dG and dP specifications of 0.05% and 0.05° while driving 150 Ω at a gain of +2. Setting the black level to other values, although accept- able, will compromise peak performance. For example, looking at the single supply dG and dP curves for RL=150Ω, if the output black level clamp is reduced from 1.2V to 0.6V dG/dP will increase from 0.05%/0.05° to 0.08%/0.25° Note that in a gain of +2 configuration, this is the lowest black level allowed such that the sync tip doesn’t go below 0V. If your application requires that the output goes to ground, then the output stage of the EL2250C/EL2450C, like all other single supply op amps, requires an external pull down resistor tied to ground. As mentioned above, the current flowing through this resistor becomes the DC bias current for the output stage NPN transistor. As this current approaches zero, the NPN turns off, and dG and dP will increase. This becomes more critical as the load resistor is increased in value. While driving a light load, such as 1k Ω, if the input black level is kept above 1.25V, dG and dP are a respectable 0.03% and 0.03°. For other biasing conditions see the Differential Gain and Differential Phase vs. Input Voltage curves. Output Drive Capability In spite of their moderately low 5mA of supply current, the EL2250C/EL2450C are capable of providing ±100mA of output current into a 10 Ω load, or ±60mA into 50 Ω. With this large output current capability, a 50 Ω load can be driven to ±3V with VS = ±5V, making it an excellent choice for driving isolation transformers in telecommunications applications. Driving Cables and Capacitive Loads When used as a cable driver, double termination is always recommended for reflection-free performance. For those applications, the back-termination series resis- tor will de-couple the EL2250C/EL2450C from the cable and allow extensive capacitive drive. However, other applications may have high capacitive loads with- out a back-termination resistor. In these applications, a small series resistor (usually between 5 Ω and 50 Ω) can be placed in series with the output to eliminate most peaking. The gain resistor (RG) can then be chosen to make up for any gain loss which may be created by this additional resistor at the output. Video Sync Pulse Remover Application All CMOS Analog to Digital Converters (A/Ds) have a parasitic latch-up problem when subjected to negative input voltage levels. Since the sync tip contains no use- ful video information and it is a negative going pulse, we can chop it off. Figure 1 shows a unity gain connected amplifier A of an EL2250C. Figure 2 shows the complete input video sig- nal applied at the input, as well as the output signal with the negative going sync pulse removed. |
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