Tag Info

23

As an addition to Armandas' answer: If you want to use vias, there is a simple trick to swap the lines: Rotate the vias by 90 degrees, i.e. put them "above" each other. If you enter both vias from the left in the top layer and leave them to the left in the bottom layer, both lines are swapped at no expense: (Just a quick drawing as my schematics computer ...

22

I'll try to answer this briefly, but a great resource for this type of question is Eric Bogatin's Signal and Power Integrity -- Simpified. You've listed and described several very high speed protocols that have signal edge rates in the hundreds of picosecond range. What this means is that even traces of just a couple of inches can be considered as ...

21

While the length and impedance are both important, 1mm of length differential will not affect your system's performance in any way, even for usb-2.0 high-speed. From the USB spec: 7.1.3 Cable Skew The maximum skew introduced by the cable between the differential signaling pair (i.e., D+ and D- (TSKEW)) must be less than 100 ps and is measured as ...

21

As a first approximation, there isn't a difference. A similar question might be "what's the difference between a DC motor and a generator?" Either will function in either capacity, but each is optimized to maximize certain qualities at the expense of others, according to the intended application. Let's compare the internal schematic for a common op-amp, ...

21

USB is not always differential. There are signalling states that require both D+ and D- to be low. Additionally D+/D- are used for other purposes, such as performing reset and speed detection which require sense resistor configurations to be changed. The pin isn't "wasted", it's "required". I suppose you should think of it less like a differential pair, ...

20

Assuming you're only using USB-low-speed or full-speed, you should be fine. Generally, layout considerations only really have to be taken if you're going long distances (many inches), or using USB-2.0. Even then, USB is surprisingly tolerant. USB 1.1 or USB2.0 low/full speed You really don't need to worry. There are (possibly apocraphyal) stories of ...

16

The reason you can't measure differential signals quite as easily with an oscilloscope has to do with the fact that oscilloscopes are (generally) not floating. The ground lead on the probes are connected to the oscilloscope chassis, which in turn is earth grounded. Because of this, anything you connect the ground lead to will also be connected to earth ...

16

The zener with series resistor R3 has about 10V on the anode wrt ground. It is seeing 50mA so the actual voltage will be a bit higher than the 10V nominal, maybe one percent on average. That voltage is buffered with Q7 and used to create a ~17mA current source for the current mirror composed of Q6 and Q5, which feeds the differential amplifier composed of ...

15

Think about a simple mechanical system like an elastic bar or a block attached to a spring against gravity, in real world. Whenever you give the system a pulse (to the block or to the bar), they will begin an oscillation and soon they will stop moving. There are ways that you can analyze a system like this. The two most common ways are: Complete solution = ...

15

The same impedance is good because capacitively coupled noise is inversely proportional to the impedance. Usually differential signals are arranged so that both are subjected to the same noise. Consider twisted pair cable, for example. The noise then has the same magnitude and polarity, while the intended signal has opposite polarity. By taking the ...

14

USB cables do require some precision engineering. There are stringent requirements on value of differential impedance, quality of interconnects, and amount of losses per cable. The high-speed part of USB cable, even at USB 2.0 480 Mbps data rate, is made of a twisted pair of wires, all wrapped into a shield. This makes it a "bi-axial" cable. The cable is ...

14

After actually building the thing I can finally answer my own question in hindsight. I've built the circuit as featured in the question, with a 1:10 attenuator. Could I get away without impedance matching both ends of the coax... Yes, but signal integrity does suffer from doing so. The blue trace is a ~6 ns rise and fall time square wave (...

13

If I were to suggest how to route this I would propose something more like this:

12

The positive and negative connectors in the cabling mentioned, form a balanced pair for signals. They typically carry identical signals of opposite polarity, i.e. a HIGH bit may be +5 Volts on the + conductor, and -5 Volts on the - conductor. The corresponding LOW bit would then be -5 Volts on the + conductor, and +5 Volts on the - conductor. Having such ...

12

This is a crosstalk question as I understand it. JTAG is single-ended typically 3.3V signal swing. This can couple to your differential signals (which are most likely LVDS with much lower voltage swing) and create logic errors. The other way around is also possible (the LVDS signal coupling to the single-ended JTAG signal). As I understand it this is ...

12

Yes you can. There are some applications notes using the differential pairs inside an FPGA as a low cost ADC. There is a very good document describing this that you can use for your design: Analysis on Digital Implementation of Sigma-Delta ADC with Passive Analog Components

12

Pseudo-Differential means that the AN+ and AN- are not being sampled simultaneously. Therefore dynamic common mode rejection is not improved, but DC common mode rejection is. Also, the different sampling times will affect the apparent phase relationship between AN+ and AN-. For these reasons, Pseudo-Differential is only useful for DC signals. To give you an ...

10

1.Transformer If your signal is a single, or narrow band, frequency this method is fine. However, anything else and you introduce the transformers frequency response into your signal. 3.simply ground the negative part with matching a load This method is ok, IF the signal source is close to the receiver. Since you are are throwing away the common mode ...

9

There are various ways to send a signal from one place to another over wires. There is no one best method for all cases. Everything is a tradeoff. Voltages are always relative within a circuit, so to send any signal over wires requires at least two wires. Often we talk about a "single wire" or a single ended signal because the second wire is implied to ...

9

Your amplifier has average noise characteristics, the problem is that your signal is very, very weak. The amplifier is responsible only of the noise part of the SNR, so it has not a "poor SNR", but a "poor input referred noise". To obtain a better SNR you can either amplify your signal, without adding noise, or reduce the noise. Since amplifying without ...

8

Since it's hard to describe it in the comments, I'll put it as an answer. If space permits, you can route your signals like so:

8

The maximum slope for a sine wave that has no offset and an ampliutde $A_0$ occurs exactly during the zero crossings. Its value is simply $A_0 \omega$ The derivation is $$\frac{d}{dt} A_0 \sin (\omega t) = A_0 \omega \cos(\omega t)$$ which gives the slope of the sine wave. The maximum of the cosine is 1. Therefore the maximum is $A_0 \omega$.  \...

8

Is it because "environmental" noise affects both signals the same way? It is entirely for that very reason. Impedance balance to earth (or earth impedance balance) it is called and, in the presence of some interfering signal, both lines (wires) will be largely affected equally thus, when using a differential receiving amplifier, the common-mode signal due ...

7

"Ground" as a concept needs some clarification. If you have just one signal line and one ground, then yes, it's hard to tell the difference. But if there's anything else going on, it matters. All AC signalling involves current flow, even if it's being measured as a voltage on the receiver. At a minimum, you have to charge/discharge the parasitic capacitance ...

7

Good questions. I have been researching this topic myself recently, and will try to provide some brief answers here. what exactly is TMDS vs LDVS? LVDS (low-voltage differential signaling) is simply an electrical specification for a differential signaling interface, while TMDS (transition-minimized differential signaling) is both an electrical ...

7

I'm going to state this up front - a differential amplifier made from an op-amp and four resistors is perfectly fine when (and only when) the input sources are much, much lower impedance than the resistors used. You will not get this when a bridge is attached because, as the bridge is off-balanced by whatever it is you are measuring, its output resistance ...

7

Pick one of the inputs and follow the signal to the output. This is easy and the result obvious if you understand the basics of how transistors work in a circuit. If not, then that's what you should be asking about before trying to learn complete circuits. When Va goes up, T1 takes more of the fixed Ibias current from T2. In other words the current thru ...

7

Its a NL\switchable part that is used for identification only (almost definitely). It appears that they use a capacitor to indicate what kind of board it is visually. On other boards, if the grounds were different on either side I'd say it was a ground connection but the ground is the same so it serves no purpose electrically.

7

LM358 has very low gain-bandwidth product (1MHz) therefore the Sallen-Key filter topology you used might not work well as a lowpass above a few tens of kHz. What happens is the HF jumps over the opamp through C5, and at HF it doesn't have enough feedback to keep its output pin low impedance, so the HF just goes on to the output. Also as Scott says in the ...

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