Terminating a differentialinput signal
A previous column explains that terminating transmission lines at the driving and receiving ends minimizes signal reflections (Reference 1). You must calculate terminationresistor values for singleended and fully differential circuits. The calculations for singleended circuits are simple; the noninvertingcircuit configuration separates the termination and gainsetting resistors. The calculations in fully differential circuits are complicated, because you can't separate the termination and gainsetting resistors (Figure 1).
Assuming that you're dealing with an ideal op amp simplifies the calculations; doing so makes the amplifier gain infinite with no frequency degradation. Under these assumptions, V_{N}=V_{P}, causing a virtual short across the opamp inputs. Thus, R_{1} is effectively in series with R_{3}, and the series combination is in parallel with R_{T}. The equation for the terminatingresistor value follows:
where R_{S} is the driver/source termination resistor.

You must account for the source output impedance, R_{S}, and the cabletermination resistance, R_{T}, in the gain equation, because they are parts of gainsetting resistors R_{1} and R_{3} (R_{G} in the general case). Begin the gainresistor calculations by using Thevenin's theorem to obtain a seriesequivalent circuit to replace R_{T} and R_{S}. Theveninequivalent voltage and resistance equations for a series model follow:
Substituting these three equations into the ideal gain equation, V_{OUT}=V_{IN} (R_{F}/R_{G}), yields the gain equation for a circuit with significant source/termination impedance. This equation applies to fully differential inputs and outputs; hence, R_{G}=R_{1}=R_{3}, and R_{F}=R_{2}=R_{4}.
A design example calculates resistor values for a common circuit with a 50Ω differentialbalance source and an overall differential gain of one (Reference 2). First, choose a value for R_{G}, use that value to calculate R_{T}, and then use the values for R_{T} and R_{G} to calculate the feedbackresistor value. An appropriate feedbackresistor value for an op amp in this frequency range is 500Ω. R_{T} approximately equals R_{S}, so the gain through the termination resistors is about 0.5. The opamp gain must be two to achieve an overall gain of one, so begin the calculations with R_{G}=(R_{F}/2)=249Ω (closest 1% standard value). Now, use the first equation to calculate R_{T}=55.6Ω and select R_{T} as the closest 1% standard value, 56.2Ω. An algebraically manipulated equation follows, which solves for R_{F}=495.5Ω, and the closest 1% standard value is 499Ω.
Figure 2 shows the complete circuit with the calculated resistor values.
References 

System level design and integration challenges with multiple ADCs on single chip
Understanding the basics of setup and hold time
Product Howto: Digital isolators offer easytouse isolated USB option
Managing noise in the signal chain, Part 2: Noise and distortion in data converters
War of currents: Tesla vs Edison
Simple reversepolarityprotection circuit has no voltage drop
Control an LM317T with a PWM signal
Start with the right op amp when driving SAR ADCs
Currently no items