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(AD9508-EP) Correct Single CMOS Output Design

Category: Hardware
Product Number: AD9508

I want to design a 1.8 [V] single CMOS output for AD9508-EP, but I see different recommendations:

  • AD9508 Datasheet (Rev. G) mentions that "Series termination at the source is generally required to provide transmission line matching and/or to reduce current transients at the driver.",
    with the following figure:


             This design recommends adding only series resistor (typically 10 to 100 [ohm]).

  • AD9508 Evaluation Board (EVAL-AD9508) designed for LVDS / HSTL differential outputs, with a note how to modify it to S.E CMOS output:


        This design recommends on adding 10 [pF] coupling cap, with 100 [nF] AC-coupling cap in series to 0 [ohm] resistor.

(As I saw on previous questions, the complementary output should remain floating)

1. What is the reason for this difference?

2. Which of the options above is the recommended way to design S.E CMOS output?

Please advise.

Best regards,


  • ADI North America will be on winter shutdown starting December 25, 2023; perhaps another community member can assist you until our return on January 8, 2024.
  • I would use the first method. CMOS inputs expect a direct (DC) coupling to the driver, so having a series capacitor is a very bad idea. Adding additional load capacitance (to ground) is often detrimental, although occasionally it helps in certain cases. But as a general rule, I don't recommend it. You want to place the 10 ohm series resistor as close to the AD9508 output pin as is practical, certainly less than 0.5 inch trace length. The 1 inch stripline length isn't critical, except that it should be at least 3x the length from the 9508 pin to the 10 ohm resistor. If it's much longer than an inch (i.e., 4 inches or more), you might need to increase the 10 ohm resistor up to maybe 22 or 30 ohms to prevent excessive overshoot/undershoot at the CMOS receiver. This is something you can simulate or measure on your first prototype boards. The stripline impedance isn't critical either, anything in the 45 to 60 ohm range is fine.