# Impedance matching from PCB to cable

I am designing a PCB board connecting to a 2 meter long cable with 75 Ω impedance. I have an output buffer amplifier at the output of the PCB.

For the buffer op amp with 0 output impedance, should I put a 75 Ω to impedance match or I can simply design the PCB trace to be 75 Ω?

If I put a 75 Ω resistor, should I put it near the op amp or near the output connector?

Edit: This is my circuit and the AC path is the one on the bottom. Frequency of interest is 1-30 MHz. But I am also very curious about what I should do for higher frequencies:

Is it advisable to change the whole board's trace impedance to be 75 Ω from 50?

• What is your highest signal frequency of interest? Please post a schematic of the op-amp and link the data sheet. Commented Aug 1, 2023 at 20:14
• Is the load at the far end of the cable terminated? Commented Aug 1, 2023 at 22:15
• It's not real easy to get 75 ohm traces on a typical PCB. You either need very thin traces (potential manufacturing problems), or atypical spacings between the signal and reference (GND) layer. Commented Aug 2, 2023 at 20:03
• I have attached my schematics. OPA2822's data sheet: ti.com/product/…. The load is the front end of an ADC, which is a TGC ( time gain compensation) AC coupled with a 10nF capacitance. Commented Aug 2, 2023 at 20:05

If the buffer is stable with a cable attached (including any worst-case load, short or open), then it could be done without series resistor, yes.

Note that this assumes the buffer is "0 ohms" at all frequencies of interest. Which is rarely the case, and perhaps you will find equalization or termination necessary after all.

An unterminated source requires a terminated load, and this is called a singly-terminated transmission line. Without a termination, signals reflect back and forth along the line, drawing large currents from the buffer (or developing large voltages at a CC buffer), and perhaps causing unexpected damage, or emissions.

The most common alternative is a doubly terminated line, where a CV source has added series resistance, or a CC source has shunt resistance, or the source (by nature, or through clever design) happens to have some internal resistance inbetween, in combination with external resistors as needed. In any case, the source resistance is made to match the line's characteristic impedance, and this terminates the line regardless if it's open or short, or anywhere inbetween, at the far end. The major downside is, the voltage or current gain is halved.

On another topic, a series resistor also provides a tiny amount of protection against direct ESD strike or cable discharge events (CDEs). Bandwidth allowing, this resistor can be used in combination with ESD clamping diodes or other protective devices to greatly enhance the buffer's robustness.

(As example: the most dramatic case I've seen in equipment was probably gas discharge tubes (GDTs) used with 22Ω carbon composition resistors, to protect three channels of a ~100MHz video amplifier from potential arc discharge of a cathode-ray tube (CRT). Being that the tube is powered by ~30kV, and has some ~nF behind that (2nd anode capacitance to aquadag), a direct strike can be incredibly destructive. It's rare enough you see 30kV ESD (HBM (human body model) or similar) ratings, but this is more like 30kV MM (machine model), far more severe. Carbon comp resistors are noted for high pulse tolerance, hence their choice here.)

Regarding trace impedance: note that it only needs to be matched, within some tolerance, over some length given by the maximum frequency of interest, and maximum tolerable impedance error or SWR. If frequencies are low relative to the trace or pin lengths, it doesn't much matter. If frequencies are very high, even pin and via impedances can matter (e.g., the latter particularly in the 10s of ps range).

Thus, if you place the buffer and connector adjacent, trace length is minimized, and the sensitivity to its impedance is minimized, for a given bandwidth and tolerance. Conversely, if they must be some distance apart, more attention will need to be paid to the impedance (and other aspects, like losses, or dispersion, if applicable).

I'd say the impedance matching is an issue only if you are concerned about reflection. So if your frequency isn't that high, reflection won't be significant. The 2 meters of cable makes it that even a few tens of MHz will start to have reflections. (note if you pass any signal that isn't sinusoidal, the frequency is dictated by harmonic content).

If you are in the area where your source impedance needs to be 75ohm, you need to make a matching circuit. Placing a resistor in series will work in DC, but that's not the point! You'll need a proper RF matching circuit. Their is a lot of theory, but without knowing more on your circuit, it's impossible to comment.

Finally, if you create a transmission line, ensure you don't change the characteristic impedance for fun! If you have a 75ohm source, a 75ohm cable and a 75ohm termination, you should also have a 75ohm trace.