I am looking at the VCA820 and needed some clarification on load handling capability. The description notes that this will "easily drive doubly-terminated 50W lines... over the ±3.9V output voltage range". Is this referring to a 50W to ground at both ends of the line (i.e. 25W net load), or is it referring to a 50W in series at the amplifier and a 50W to ground at the receiving end, i.e. a net 100W.. The electrical characteristics table gives the voltage into 100W, but it doesn't spec the minimum load that can be driven.
When discussing "doubly terminated 50-ohm lines", this follows conventional high frequency/RF design practice of a 50W source resistor (series R placed near the output of the VCA820), then into a 50W characteristic impedance trace/cable, and then a 50W resistor to ground at the receiving end. This causes a 6-dB attenuation to occur in the system, but ensures minimal reflections and proper SWR of 1:1.
As to what is the lowest resistance, it really depends on the signal swing. The specification section shows the maximum output current drive of ±160mA Typ. If you couple this with the graph showing Harmonic Distortion vs. Rload and you can see a relationship between load resistance, current drive, and performance.
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