29 votes
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What and Why's of termination?

May be, a more mechanic explanation helps to understand: Imagine you have a long rope, one end fixed to a wall, the other end held by you. By a short stroke upwards, you can create a wave travelling ...
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  • 8,430
29 votes

SPI bus termination and pull-up resistors

At 1 MHz and 50 mm (2 inches) you don't need terminations. You have ordinary on-board digital signals, and not even very fast ones. You don't need pullups or pulldowns on SPI lines. When used, SPI ...
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21 votes

What and Why's of termination?

Here's what finally helped me understand termination and reflections: Suppose you have a really, really long coaxial cable, with the far end shorted together. If you put current through it, what will ...
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  • 7,639
15 votes
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Can we reduce power consumption of digital interfaces by using high impedance transmission lines?

While, in theory, high impedances would reduce power dissipation for the same voltage swing, there are several important issues in practice. 1) It's the power, not the voltage, of a signal that ...
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  • 142k
13 votes
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Are special terminals required for connecting thermocouple wires to a PCB?

Using a terminal block made with ordinary materials is quite sufficient for a relatively modest accuracy system as you're aiming for. The cold junction compensation depends on the cold junction ...
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13 votes
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Why is the signal amplitude low on the oscilloscope even though the impedance is matched?

Do not confuse power transfer with voltage transfer. If the source is indeed 50 ohms, and your scope is set to 50 ohms, it becomes a voltage divider and what you see is half the voltage that you will ...
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12 votes
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Are termination resistors needed for UART, I2C and SPI?

Take 50 MHz clock SPI. 50 MHz has a fundamental wavelength of 6 metres but you could argue that due to the fast rising edges everything up to the 5 th harmonic is potentially capable of disrupting ...
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  • 384k
12 votes

Does it matter which 2 nodes on a CAN bus you terminate?

Each beginning has its end. This can't be answered, because the question is not correct. What you have is the transmission line - a twisted pair that has two ends, and that's the place where the ...
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11 votes
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Why is there such a huge price range for BNC terminators?

For BNC terminators there are usually the following factors that contribute to the price: Accuracy of the specified values (i.e. impedance is matching for the whole assembly) Wattage of the resistor ...
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  • 6,319
11 votes

Driver impedance of a microcontroller pin, for series termination?

It's not the frequency of the signal that's sent down a PCB trace, but the risetime that's important. A 21cm PCB trace is about 30cm 'air equivalent', and as the speed of light is 1nS/ft (in units ...
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  • 142k
11 votes

Does it matter which 2 nodes on a CAN bus you terminate?

It doesn't matter which nodes are at the ends, but it does matter that the terminators are at each end. The bus is a transmission line. To keep edges from reflecting at the ends of the cable, the ...
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11 votes
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The CAN bus in this vehicle has only one 120 ohm termination resistance. What problems would this provoke?

From a professional point of view, this is a critical problem. Missing termination will cause energy bouncing back at the end which isn't terminated. This could lead to strange random noise on the ...
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11 votes
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When do you need a complex termination?

So the characteristic impedance is a complex number. But why is never a complex termination used? When \$j\omega\$ is significant, the formula reduces to this: - $$Z_0 = \sqrt{\dfrac{j\omega L}{j\...
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10 votes
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SPI bus connections - to loop or not to loop?

Loop back? NO. If the line is long enough to need termination (longer than risetime*c/10 or so), then drive it strongly enough to end terminate it properly, and match the line and termination ...
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10 votes
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How Does an Adjustable Transistor Size CMOS Driver Work?

The approach to do this is alluded to in the datasheet snippet you linked, which mentions "fingers", or separate parallel gates in a wide MOSFET that forms the output driver. These fingers ...
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8 votes

Why does the CAN bus use a 120 ohm resistor as the terminating resistor and not any other value?

CAN Bus is a differential bus. Each differential pair of wire is a transmission line.Basically, the terminating resistor should match with the Characteristic Impedance of the transmission line to ...
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8 votes
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Most ideal wire termination for round screw terminals

Note that if soldering wire ends and then clamping with a screw in a terminal block or connector post as seen in your photo then the majority of the wire end MUST NOT be soldered - even though this is ...
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  • 143k
8 votes
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Why is Bob Smith termination for Ethernet recommended if it's wrong?

A presentation by Royce Bohnert clearly shows that termination has no effect. Conditions of the experiment: How much of an impact does this termination actually have on radiated emissions? Marine VHF ...
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  • 96
8 votes
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How and why is impedence matching done for a CAN bus?

The "terminators" of a CAN bus serve two purposes: They terminate the transmission line. The type of CAN you are referring to is intended to be implemented as a twisted pair. 120 Ω is roughly ...
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7 votes

Why is Bob Smith termination for Ethernet recommended if it's wrong?

I "Bob Smith" with 75 ohms and I believe you should too. You can find a few corner cases where this termination doesn't work as well as something else, but overall it is the most robust if you do ...
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  • 71
7 votes

Most ideal wire termination for round screw terminals

You absolutely, positively do NOT want to solder anything in this environment. When you put a solder-tinned wire under a screw terminal, the solder will "creep", causing the connection to loosen over ...
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  • 165k
7 votes
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How to choose the impedance of a BNC adapter?

Most normal BNC terminated lab equipment uses 50 Ohm. 75 Ohm is mainly used for cable tv and related equipment. Can you read the number printed on the side of the cables? For example, RG58 is 50 Ohm ...
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  • 29.6k
7 votes
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Will this selectable CAN BUS termination circuit work?

It looks like it should work if you're not pushing the CAN bus to the limit. You can definitely turn on the LED with 3.3 V since the maximum LED voltage is 1.5 V. It needs about 5 mA, so you can ...
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7 votes

Can we reduce power consumption of digital interfaces by using high impedance transmission lines?

This termination resistance closes the current loop between two differential signal lines and creates a decent amount of power loss so that low voltage amplitudes are required to reduce power ...
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  • 45k
6 votes
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Bob Smith Termination with POE

I found the following examples of POE termination from around the internet: Example 1 Figure 5 Example 2 Figure 4 Example 3 Figure 5 After posting the same question on TI's E2E forum, I got the ...
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  • 425
6 votes
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Causes of ringing in a PCB

For short traces, ringing is caused by parasitic inductance / capacitance effects. For long traces, it can be caused by impedance mismatch / reflection or parasitic inductance / capacitance effects, ...
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  • 24.4k
6 votes

What and Why's of termination?

Termination is needed when you are working with transmission lines and (relatively) high frequency signals. Signals travelling down transmission lines actually travel as an electromagnetic wave, and ...
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6 votes
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What should I do with the unused Op-amp inputs?

It depends a bit on the op-amp. If it's a bipolar op-amp with high input bias current in a non-precision circuit (referring to the other op-amp) it may be safe to leave the inputs open. Think LM324 in ...
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6 votes
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USB hub and slave on same pcb - are serial resistors and capacitors needed?

The 15kOhm pull-down resistors are most definitely required. The slave device has internal pull-up resistors on one or both of the DM/DP lines which are used by the host to detect that a slave has ...
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  • 4,491
6 votes

USB trace impedance calculations, with termination resistors

question 1: This doesn't seem right The default Er is 4, but usually the ER for standard FR4 is around 4.6. Although for this calculation you will need the effective Er because the top of the trace ...
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