# High Speed PCB impedance question

I am going to design a high speed digital design including an FPGA and GSPS A/D interfaced with multi-gigabit/s serial ports. But I have found the impedance matching problem a little confusing, My questions are:

1-In a serial port (assume gbps or LVDS), is the impedance matched by using series termination resistors or by tuning the traces themselves to suitable width/distance?

2-I have found tools that calculate the impedance of a trace on a PCB. As I know the impedance of a trace should depend on its length. Why impedance is irrelevant to the length of a trace in these tools?

3-Could you please led light on what situation in general we should use resistors and on what situations the traces make the desired impedance?

Thanks

• Resistors are used to match source and destination impedances. Traces are used to match parallel signal impedances. Jul 6 '16 at 13:10
• The impedance of a trace is $Z\ = \sqrt {\frac {L} {C}}$ where L and C are the inductance and capacitance per unit length. This is for a lossless line which is a decent first order approach. Jul 6 '16 at 13:19
• Key point: the characteristic impedance of a transmission line is a slightly different concept than the impedance of a one-port device. Jul 6 '16 at 14:46

## 2 Answers

1. Impedance matching of PCB traces is controlled by the traces, not by the termination resistors. That being said, you still need the termination resistors because 1) the receivers measure the voltage across it to determine the logic level of the signal, and 2) the termination resistors match the driver impedance and receiver impedance. You must "tune" the traces by adjusting their widths (and the PCB layer stackup) to keep the desired impedance.

2. The length of the trace is pretty much irrelevant to the characteristic impedance. The length only affects the resistance and the propagation delay (that is, the amount of time it takes for the signal to travel from point A to point B). You do not need to take length into consideration when trying to match impedance, but high-speed signals should have length-matching as well to ensure the signals all arrive at the same time. This is very critical in a high-speed circuit -- If the circuit tries using the data when a signal hasn't reached the destination yet, then you can lose that data.

3. You need both the resistors AND the controlled impedance traces. However, many LVDS drivers and receivers have the termination resistors built-in. If this is the case you do not need external termination resistors. Check the driver/receiver datasheets to determine whether or not you need to include them externally.

1) You choose a suitable impedance that the transmitter can drive. The transmitter's current output must be able to get enough voltage across the receiver to meet the link specification for signal levels.

Almost always this is 100ohm balanced, or two complemently 50ohm lines. This is usually specified by the supplier or the SERDES chips. It is this impedance because a) it is 'about right' for sensible voltage and current levels on the interface b) it matches standard test gear connections c) it uses 'reasonable' dimensions on a PCB.

Knowing the impedance, you then choose line dimensions (width, spacing, thickness) for the PCB that will present that impedance. All good PCB manufacturers will be able to tell you dimensions for their process that will end up giving you the right impedance.

You then look at the SERDES specification and see whether the receiver already has line termination in it, or whether you have to implement that externally, and leave the end of the line open or connected to a resistor as required.

2) The impedance of a trace does not depend on the its length.

3) As explained in (1), traces must be deisgned to present the right impedance. Depending on whether the line is single or double-ended, source or destination or doubly terminated, resistors must be used for termination, and must have a value matching the trace.

Generally for point to point SERDES, the line is simply connected between the transmitter and the receiver, with the manufacturer of the chips having designed the TX output to drive the line directly, and put a suitable termination inside the receiver.