Friday, May 01, 2009

“Rules of the Road” for High-Speed Differential ADC Drivers

Most modern high-performance ADCs use differential inputs to reject common-mode noise and interference, increase dynamic range by a factor of two, and improve overall performance. ADC drivers—circuits often specifically designed to provide differential signals—perform many important functions including amplitude scaling, single-ended-to-differential conversion, buffering, common-mode offset adjustment, and filtering.


At 2:24 PM, Anonymous josyb said...

Very thorough article!
I have one question though (already for a long time now): why the double termination in the SE to Diff circuit? I have a particular setup with 128 high-speed outputs (2048*2048*1000 fps image sensor). The drive of this outputs is limited and adding a 128 op-amps is space- and power- prohibitive, so I left out the parallel termination and added a resistor of equal value to the source impedance in the lower arm of the circuit and increased both feedback resistors with the same amount (50 ohms).
I'd value your comments.

At 11:33 AM, Anonymous Jonathan Pearson said...

Hello josyb,

Thank you for your comments. The parallel termination is inserted to match the impedance of the transmission line that is feeding the amplifier. A well-matched termination minimizes reflections from the load. If you have very short connections between your sources and loads, you can get away without the termination.

In your case, pixel edge speed compared with the round-trip transit time of the edge as it passes along the line is what's important. If the transit time is considerably less than the signal rise time, you may be okay without a termination. For reference, the propagation rates for 50 ohm traces on FR-4 are approximately 140 ps/in for outer traces and 180 ps/in for inner traces.

Regarding the amplifier design, the most important thing is keeping the feedback factors matched. Starting with equal Rf and Rg values, adding a resistor equal in value to your source resistance to the lower Rg keeps the feedback factors matched. You can always independently adjust the feedback resistor values (keeping them matched) to get your desired overall gain.

Please contact me directly if you have further questions.

Best regards,

Jonathan Pearson
Senior Applications Engineer
Analog Devices, Inc.


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