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High Speed Amplifiers for Video Applications Part 2 Slide 15

Here is a way you can switch between two video sources, say the signal from two distinct cameras aimed at different targets. The LMH6732, which is a high-speed Op Amp with adjustable supply current, is used for this job in this case. The adjustable supply current feature is utilized in such a way to allow the multiplexing function. As you can see, the CD4049, CMOS logic, supplies the enable signal to U1, for example, and its compliment to U2. So, either U1 or U2 is on at a given time. The top right-hand picture shows the LMH6732’s supply current adjustment circuit block diagram. If the supply current adjust pin is driven high, the current IP is reduced or stopped. Then internal to the device, IP is used to set the supply current flowing throughout the functional signal path of the LMH6732. Driving pin eight’s – sorry, the supply current adjust pin’s voltage sufficiently high, will of course, cutoff all IP which is how in this application one device is completely turned off. As the LMH6732 is disabled, its output impedance increases to the point where both U1 and U2 can be tied together at the output without any problems. In the lower right-hand corner, you can see the scope photo showing the transition between a single cycle sine wave and a repetitive triangular waveform. For the LMH6732, switching times are a function of operating supply current. With the values shown, the switching times will be less than 50 nanoseconds. If the supply current is increased to about nine milliamps or so, the switching times are reduced to less than 20 nanoseconds.

PTM Published on: 2011-11-21