# Tag Info

26

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 ...

24

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

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, ...

17

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 = ...

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 ...

16

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 ...

15

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

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 ...

15

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 (...

14

how would I do a simple RC low pass filter on this differential signal? The simplest method: - Source. This circuit only filters differential signals; if there is an in-phase signal (or interference) on both lines (called a common-mode signal/noise) it provides no filtering to it. A more complex but effective method: - Modified from this Source. This ...

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

There are several ways to send a signal from one place to another over wires. There is no one best method for all cases. Everything is a trade-off. Voltages are always relative within a circuit, so to send any signal by wire inherently requires [at least] two wires. Often we talk about a "single-wire" signal or a "single-ended" signal ...

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 ...

10

The designer probably did that to reduce the capacitance of the probe. If you filled that area with ground, when you contacted the probe points to your circuit under test, you'd be connecting a capacitance between the probed points, and that could change the behavior of the circuit. Even without the ground, you're adding some capacitance, but removing the ...

10

What would happen if i pour ground copper at that area as well. The PCB legend implies it's good for 1 GHz. At 1 GHz a 1 pF capacitor has an impedance of 159 ohms. Do you really want to spoil the lack of capacitance at the probing end of the circuit with extra capacitance to ground circa +100 ohms on each input line (because of extending the ground plane)? ...

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 ...

9

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:

9

why does a differential signal need an inverse input They don't. What makes "differential" signals work is that the impedance of both wires is equal (balanced) which is necessary for noise to be picked up as common mode which can then be rejected. If the impedance of both wires is different, then some of the noise will be picked up as differential ...

9

For my application, the only relevant data is the difference in luminosity between these photodiodes. If both photodiodes are the same, wire them antiparallel to a single TIA. Wiring them antiparallel means that the only current feeding the TIA will be the difference current. Now, the problem of noise is halved. Use a balanced transmission system like this: ...

9

simulate this circuit – Schematic created using CircuitLab Andy's anti-parallel diode idea works nicely in a transimpedance amplifier. The dynamic range is maximized so long as the opamp doesn't run up against its DC supply rail with too much gain. Choose a feedback resistor value (R1, or R4) to stay within the opamp linear range. The photodiodes and ...

9

The probe itself is an active device which has input and output, each one with its characteristics (given in the manual): Impedance matching, like with a standard signal generator with 50 Ohm output, minimizes signal degradation caused by reflection. Edit: also check the answer provided by JRE on why 50 Ohm is commonly chosen.

8

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 ...

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

They are same if you operate the Differential amplifier in Saturation.. then it performs the function of Comparator. But basically, the Differential Amplifier is an Amplifier which takes it to ANALOG DOMAIN A comparator gives the comparison of Input Voltage against a Reference Voltage and gives either of the supply Vdd or Vss, this binary behavior takes it ...

7

TI Application Report Op Amp and Comparators -- Don't Confuse Them!1 provides an informative comparison of op amps vs. comparators. Summarizing the points highlighted in the report: Comparators' outputs are sometimes an open collector/drain for digital operation. Multiple comparators' outputs can be tied together in order to form a NAND gate: However, an ...

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