Since the emergence of the wideband, fully differential amplifier (FDA) topology with the introduction of the AD8138 in 1999, a popular application of this type of device has been to convert a single-ended input to a differential signal around some common-mode voltage. This is a typical requirement to drive all modern, high-speed ADCs.

At higher speeds, where a doubly terminated transmission line-type environment is desirable, a common application would be to present a defined, single-ended input impedance to match some source impedance. Then, the FDA converts that single-ended input into a differential signal with some gain centered on the common-mode voltage V_{cm} of the ADC.

Thus far, a closed-form solution for the resistor values in this application of the FDA has proven remarkably elusive, with iterative techniques the current norm. While effective, these hunt-and-peck solutions miss some of the more useful insights a closed-form solution can provide. Building on previous work, an exact solution for the required resistors will be presented here along with a design example using the 4-GHz ISL55210 FDA and the low-power, 10-bit, 500-MSPS ISLA110P50 ADC.

This lengthy, detailed article, which also includes extensive references, is presented in two parts. To read **Part 1**, click **here**; **Part 2** will be posted shortly.

**About the author**

**Michael Steffes** is Senior Applications Manager, Intersil Corp. with more than 25 years of experience in high-speed amplifier design, applications, and marketing. Previously, he was the Market Development Manager for High-Speed Signal Conditioning, and a Distinguished Member of the Technical Staff, at Texas Instruments Inc. He currently provides product definition and customer design-in support.