When choosing a device for an application, the output current capability is also very important and should be evaluated, especially when the load resistance is less than around 500 ohms or so. In the case of the video signal, the total load seen by the Op Amp is normally around 150 ohms as shown in the top right-hand side. The LMH6645 is a 55 MHz Op Amp which only consumes 650 microamps of supply current per channel. At these low power levels, the output current capability needs to be examined for the given amount of load. The LMH6645 output characteristics are shown here. Only output sourcing is considered since in a DC coupled arrangement as shown there is only current out of the amplifier into the load and not the opposite direction. This is not the case with an AC coupled output load where both sourcing and sinking currents are present. Represented this way, the amplifier output characteristics is relatively independent of the supply voltages used. Note that the vertical axis is labeled Vout from V+ in volts. For three volts supply operation, as in this case, the area of interest would be all values less than three volts on the vertical axis. To make sure the output is capable of driving the load, the plot shown should be examined. As you can see, with two volt maximum output voltage, we found – that this is the number that we found in the previous example – we would be at one volt from V+, or the positive supply voltage. The plot shows that with one volt on the vertical axis, amplifier ISOURCE, which is the X axis value is around 30 milliamps. Since a single 150 ohm video standard load needs less than 14 milliamps of current, this device is appropriate for this load. However, if the device is used to drive multiple 75 ohm loads, as would be the case in a multiple display application or distribution amplifier application without buffering, the output current could run into a limit. So the 30 milliamp output sourcing capability should be kept in mind. So it is important to always check the output current capability when in doubt, of course, especially when using low power devices such as the LMH6645. In cases where a plot like this is not available in the data sheet, you can consult the data sheet tables listing which list the device’s output short circuit current. The actual output current will always be less than the short circuit current number in the table as the maximum current flows when the output voltage is subjected to the short circuit condition.