# Terminating a Single ended source driving a differential driver for an ADC

How do I terminate a single ended source that is driving a diff amp (a.k.a. ADC driver)? I'm using a single ended source to generate a differential output signal used to drive a differential ADC input. My question is the ADC driver isnt a high input impednace like an op amp, so how do I terminate it properly?

• That is a great question, to terminate a single ended source, you first need to know what the input impedance is of the differential amplifier is. As you already pointed out the input impedance is not very high, so you need to take into account the amplifier input impedance as this will appear across the termination resistor. This impedance in parallel with the termination resistor will provide the proper system termination. The input impedance is largely determined by the input resistor RG. There is an equation (EQ1) that you can use as shown below for single ended inputs, its different for a differential input, for that case it is simply 2xRG.

Rin= RG/1-RF/2*(RG+RF) EQ1 (Single Ended input impedance)

To calculate the input impedance for a single ended input application. RF is the feedback resistor. Analog Devices has a very good application note AN-1026 that goes over this and many other facts about how to use differential amplifiers.

There's an even better way to calculate the termination resistor that is to use our Differential Amplifier Calculator, we call it DiffAmpCalc and let it do the work for you. It’s free and downloadable and you can find it at www.analog.com/diffampcalc. It’s a very versatile tool and will perform all the differential amplifier calculations that you need. Noise, SNR, gain, power dissipation and much more, check it out and let me know what you think.

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