![]() ![]() Therefore, the instrument output is actually: The resulting impedance resistance for the stage will therefore be equal to the termination resistor in parallel to Rg, in this case 750 ohm. ![]() The non-inverting input is connected to ground, and therefore the inverting input will also be at ground. This is because the ideal op amp model forces both inputs to the same voltage potential. In the inverting configuration, however, the inverting input presents a ground potential to the gain resistor Rg. The function generator anticipates this, and scales its output accordingly. The EVM contains a 49.9 ohm termination resistor, and therefore the output of the function generator will see (approximately) a 2:1 voltage divider. The function generator contains a 50 source resistor. There are two ways of looking at this problem:įrom the function generator side. The user might wonder, “what happened – where did the rest of the voltage (plus-minus 0.016 V) go”? The function generator indicates a level of plus-minus 0.5 V (1 VPP) – while only plus-minus 0.484 V (0.968 VPP) is applied to the board. Both Rg and Rt affect the level of voltage applied to the circuit. Most designers assume that the 49.9 ohm termination resistor, combined with resistors Rg and Rf will guarantee a gain of 1 – but wait! The real story is a little more complicated.Īssume that the signal source is a laboratory source set to 1 VPP. It assumes a laboratory source as described above. Inverting Stage AttenuatorsĬonsider the circuit of represented in Figure 1. The inverting case, however, is subtle and needs explanation. Single-ended op amps are a mature technology – therefore some of what is presented here may be a review. The closest standard value is 49.9 ohm, and this value has been used in the examples below.įour cases will be covered in this article: Inverting Stage attenuators Non inverting stage attenuators Single-ended to fully differential stages Differential to fully differential stages It is worth a brief mention that 50 ohm is not a standard one percent resistor value. Laboratory sources assume a 50 ohm termination in the circuit that is driven, and take it into account when generating their display. This resistor is shown as Rs in the schematics below. Laboratory sources can be thought of as an ideal voltage source (zero ohm output impedance) in series with a 50 ohm resistor. A laboratory signal source will be assumed. The techniques established here are applicable to any value of termination resistor, because the electrical laws governing them are universally applicable to other resistance values.įor every input termination resistor, there needs to be a companion series-matching resistor in the source. This article will focus on 50-ohm terminations – but this has more to do with the test equipment available in the author's lab than a preference for RF applications. All of these termination resistor values have a common purpose – they match impedances in the circuit and therefore attenuate reflections that would otherwise cause problems in the system. RF designers think about 50 ohm, video designers 75 ohm, audio and telecommunication designers 600 ohm – there are many possibilities. Termination – the term brings to mind different values of resistance to different specialties in electronics. ![]()
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