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High Speed Amplifiers for Video Applications Pt 3 Slide 4

Here’s an example where four LMH6572 outputs are used to create an 8:1 RGB MUX. One of the three channels, R, G, or B, is shown here. The buffer, which is in the off state, will look like a very high impedance and will have no direct bearing on the output. However, as you can see, the gain setting resistors within the device responsible for achieving an overall gain of +2 will look like resistive loads to the top channel which is on. There is a slight loading effect to this on channel due to off ones as worked out in the calculation to the right of the schematic. In this case, the video signal at the load will experience a loss of about .6 db. This imposes an upper limit on the number of devices which can be tied together to expand the number of inputs. If the individual outputs are routed to the load through a common series resistance of 75 ohms, instead of the arrangement shown here, this expansion loss can be eliminated at the expense of added off-state output capacitance loading on the on channel.

PTM Published on: 2011-11-21