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Questions tagged [impedance]

Impedance defines the voltage/current characteristic of any dipole circuit.

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67 votes
8 answers
185k views

Why is high input impedance good?

Naive perhaps, but Why is high input impedance a good thing? Is high input impedance always a good thing?
Everyone's user avatar
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65 votes
5 answers
15k views

What is impedance?

This is presented as both a resource for the community and a learning experience for myself. I have just enough knowledge of the subject to get myself into trouble, but I don't have the best grasp of ...
57 votes
5 answers
48k views

How can PCB trace have 50 ohm impedance regardless of length and signal frequency?

Hmm, this seems to be just another question on line impedances. I understand that when we say "transmission line" effects we talk about things like cross talk, reflections and ringing (I guess that ...
quantum231's user avatar
48 votes
2 answers
167k views

What is input and output impedance of an opamp?

I have never understood input and output impedances of an op-amp. If anyone can explain what these two terms mean in an op-amp I'd highly appreciate it. Thank you! http://www.eecs.tufts.edu/~dsculley/...
user734861's user avatar
36 votes
4 answers
6k views

Why do characteristic impedances matter only when traces are longer than half a wavelength?

Why are characteristic impedances of traces not considered when the traces are shorter than half a wavelength? I've had the same issue with light diffraction, which happens when pinholes are smaller ...
user42875's user avatar
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29 votes
6 answers
18k views

Frequency dependence of electrolytic capacitors

It's said that electrolytic capacitors behave as inductors at high frequencies, which is why we put small ceramic caps in parallel with them: Electrolytic, paper, or plastic film capacitors are a ...
endolith's user avatar
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28 votes
2 answers
36k views

Resistance Vs Impedance?

What is the difference between the Resistance and Impedance? When we will say it is an Impedance and when we will say it as an Resistance? Can you explain it with diagram (If possible) and real time ...
Gouse Shaik's user avatar
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26 votes
3 answers
18k views

Why are high impedance circuits more sensitive to noise?

Why are high impedance circuits being more sensitive to the noise? They have less current flowing through them, but how is that related to noise, since external noise becomes voltage on the wires, and ...
1p2r3k4t's user avatar
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23 votes
4 answers
12k views

How is xΩ impedance cable defined?

This is probably a really simple question, but I can't seem to find a definite answer anywhere. I'm guessing 50Ω cable means 50Ω per unit length. What unit length is this? If this is not how its ...
midnightBlue's user avatar
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23 votes
2 answers
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Why does ST recommend 100 nF decoupling capacitors for a 72 MHz MCU? (And not 10 nF.)

I've been reading about decoupling capacitors, and I can't seem to understand why ST recommends 100 nF decoupling capacitors on a 72 MHz ARM microcontroller. Usually 100 nF decoupling ...
Mike's user avatar
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21 votes
10 answers
10k views

Why is impedance represented as a complex number rather than a vector?

I am trying to understand why impedance is not represented using vectors. I assume it is due to complex numbers having the property that $$j = \sqrt {-1}$$ but with my limited knowledge I can't see ...
JShorthouse's user avatar
19 votes
2 answers
46k views

Input impedance of Arduino Uno analog pins?

I'm using the ACS712 hall-effect based current sensing chip on some small robotic arm motors, and reading in the analog voltage with an Arduino Uno. I've had reasonably good results, but only after ...
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18 votes
8 answers
17k views

Op-amp input resistor?

I'm reading through the datasheet for the TL064, which contains this figure on page 16: This is an instrumentation amplifier that apparently uses an inverting amplifier's output instead of a ground ...
In silico's user avatar
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18 votes
4 answers
30k views

Electronic aspects of iPhone 3.5mm audio output

I am working for a while now on a small project of building an external mic for the iPhone to be hooked in parallel to headphones (via the 3.5 mm jack). Basically, I want to replace the iPhone's ...
user11678's user avatar
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18 votes
2 answers
43k views

Constant current, constant power and constant impedance loads

Ive been looking for information on constant power, constant current and constant impedance/resistance loads, there's some information about the constant current loads, but very little that really ...
S.s.'s user avatar
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17 votes
4 answers
3k views

When laying out circuit board traces, what impedances do I need to consider?

I do low-speed circuit design for microcontrollers and such (usually at less than 20 MHz), and now I'm getting started on some more high-speed circuits. What I want to know is: What considerations ...
machinaut's user avatar
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15 votes
5 answers
3k views

Why do cables specify 75 Ω impedance when impedance is a two-dimensional quantity?

As I understand it, impedance has an imaginary and a real part. To be able to perform any useful calculations, one needs to know both. Cables and speakers usually only say like "8 Ω" or &...
Stephen Hubschrauber's user avatar
15 votes
5 answers
48k views

How are speakers matched with audio amplifiers? (avoiding overloading either)

I know a similar question title has been asked, but I believe that does not answer my question (and I couldn't think of a better way to phrase the question). I'm a bit confused by how exactly an amp ...
midnightBlue's user avatar
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15 votes
2 answers
3k views

Is paying extra for "Controlled Impedance" boards necessary?

I'm doing a bit of RF PCB design, and one of the things that caught my eye is the "Controlled Impedance" option. Checking more boxes always costs more, so I want to know if this is worth the extra ...
W5VO's user avatar
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15 votes
2 answers
6k views

How do you calculate the noise of an op-amp circuit?

I think I know how to do this, but you can find a lot of different instructions and calculators online that contradict each other. I have yet to find a clear, concise procedure for calculating the ...
endolith's user avatar
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14 votes
4 answers
7k views

Why does most test equipment have 10 MΩ voltage input impedance?

I'm only speaking for equipment interesting in voltage measurements. Multimeters, oscilloscopes and conventional lock-in amplifiers seem to have mainly input impedances of 10 MΩ. I understand the need ...
BlackPenguin's user avatar
14 votes
7 answers
57k views

What is a load resistor?

I am not able to understand what a load resistor is and how does it relate with a load. Can anyone explain how the load resistor works and how is it different from the general resistor.
brewbot15's user avatar
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14 votes
5 answers
15k views

ADC input impedance on MCUs

What is the input impedance of a typical MCU ADC? In this case I'm working with a PIC24FJ64GA004. I don't need high speed sampling - a maximum of 100 samples per second. I wish to connect a resistive ...
Thomas O's user avatar
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13 votes
4 answers
2k views

Do 50 Ω signal generators have a series resistor or not?

I am wondering about something really simple and silly. If functions generators have an internal series 50 Ω termination, then they would produce only half the set amplitude over a matched load. ...
tobalt's user avatar
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13 votes
5 answers
14k views

Required output impedance for ADC input?

On the Atmel ATmega328P on the Arduino Uno, the ADC RAIN value is 100 MΩ - which I assume is the ADC input impedance. However, in the datasheet it says "The ADC is optimized for analog signals ...
Pugz's user avatar
  • 841
13 votes
2 answers
2k views

Differential Signals - Why is having an equal impedance an advantage?

I lately heard in a course that a main advantage of differential signals is that both signals show the same impedance, whereas a single-ended signal, which is measured against ground, has a high ...
Daiz's user avatar
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13 votes
3 answers
14k views

Is capacitor reactance [sometimes] defined with negative sign?

Wikipedia currently claims so but I've looked in 6 books via Google Books and it's not defined like that, i.e. it's just $$ X_c = \frac{1}{\omega C} = \frac{1}{2\pi f C} $$ Is Wikipedia full of ...
got trolled too much this week's user avatar
13 votes
5 answers
4k views

How is it possible to have a 5W transmitter driving 50 ohms with a 12V supply?

So let's say you have a circuit, which generates a carrier wave at some frequency (let's say 27MHz) and it's connected to a 50 ohm dummy load (which I gather is equivalent to an antenna for circuit ...
Frogging101's user avatar
13 votes
4 answers
43k views

What does it mean for reactive power to be delivered / consumed?

Real power makes sense since there is actual consumption, but regarding reactive power; what is consumed / delivered? And how does the circuit change once this happens?
Chris-Al's user avatar
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12 votes
5 answers
2k views

Trace impedance ohm meaning

I've been trying to understand the principle of trace impedance (single ended and differential) for a while, read several articles (all the best google has to offer), but I have still some serious ...
Ilya's user avatar
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12 votes
4 answers
4k views

Why do cables have multiple grounds?

Many cables have multiple grounds. Why? For example, according to Wikipedia: The SATA standard defines a data cable with seven conductors (3 grounds and 4 active data lines in two pairs) and 8 mm ...
End Antisemitic Hate's user avatar
12 votes
3 answers
29k views

Input impedance of inverting amplifier clarification

Take a standard inverting amplifier: I'm unclear on what the input impedance is. At first I thought it was the equivalent resistance from the inverting input to the ground, which would be Rin || Rf, ...
The Beruriah Incident's user avatar
12 votes
4 answers
33k views

Circuit to Convert Line Level to Mic Level

How would I design a circuit that converts a line level signal to be able to be fed into a microphone input? The input impedance of the microphone jack is 2 kohms.
kittachami's user avatar
12 votes
2 answers
15k views

Output impedance of voltage divider?

When connecting a 10S LiPo (42V) to my ADC via a voltage divider, I began wondering how to calculate the output impedance of the divider. simulate this circuit – Schematic created using ...
JimmyB's user avatar
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12 votes
3 answers
2k views

Impedance matching

What is impedance matching? Why it is used?
coldshine's user avatar
  • 515
12 votes
3 answers
1k views

Measuring impedance

I am designing and building circuits which use 100Mb/s on a Low Voltage Differential Signaling (LVDS) bus. Some of these signals need to travel between PCBs on hand made cables. The problem is that I ...
Rocketmagnet's user avatar
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11 votes
6 answers
2k views

What is the difference between impedance and reactance?

I mean how are they physically different? What does it mean to have imaginary numbers in the reactance?
FISqrt's user avatar
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11 votes
2 answers
3k views

Should each trace carrying RF be 50Ohm in characteristic impedance? How?

I have to translate a VHF (160MHz) receiver schematic into a PCB. After looking here and there, I am a bit confused. It seems like the main issues with RF are to avoid stray inductors and ...
user42875's user avatar
  • 1,344
11 votes
2 answers
5k views

Can the characteristic impedance Z0 of a conductor be measured?

We have a custom made data cable at my work, is there any good way to measure \$Z_0\$ of a cable, or must the formulas be used? Trying to work out what value termination resistor to use.
fred basset's user avatar
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11 votes
5 answers
6k views

Impedance Matching and Large Trace Widths

I am currently working on a design in which one of my ICs specifies the use of a 50 ohm trace. The answer to this question, Characteristic impedance of a trace, shows that a 120 mil trace is required ...
Kellenjb's user avatar
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11 votes
2 answers
1k views

What does degree in a schematic mean when drawing a PCB?

I am designing a PCB for LNA with an SPF5189z. I found a reference schematic design in the SPF5189z datasheet: (Image excerpted from the previously linked datasheet.) What do they mean by those ...
Asif Rahman's user avatar
11 votes
7 answers
6k views

Complex Impedances

What does it mean to have a complex impedance? For example, the impedance of a capacitor (in the Laplace domain?) is given by 1/sC (I believe) which equates to \$ \dfrac{1}{j \cdot 2 \pi \cdot f \...
JonaGik's user avatar
  • 251
11 votes
4 answers
2k views

Setting audio output impedance for output protection

I'm building a module for a modular synthesizer, according to the Eurorack standard. So this is a module intended to be connected to other modules via patch cables. While the above standard page does ...
Timo's user avatar
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11 votes
5 answers
2k views

Art of Electronics: Emitter-Follower Zout

I am a growing frustrated with the Art of Electronics. It is such an approachable book in Chapter 1, and then in Chapter 2 it seems like the authors wanted to make it more textbook-like and they ...
Dr. Watson's user avatar
10 votes
5 answers
9k views

How to compute input impedance?

I use a Sallen-Key filter for a project at university and I need to know its input impedance. Is there a way to compute it theoretically ? Here is my circuit:
snickers's user avatar
  • 357
10 votes
7 answers
2k views

If RF signals require matched impedance to remove reflections, why do approaches to shunt noise need only a “good path to ground?”

From my understanding, for signals to propagate without reflection the impedance of the path must be constant and can be shown via the telegraph equation. When it comes to noise on the line, for ...
George kirby's user avatar
10 votes
1 answer
2k views

Why does this circuit oscillate?

The circuit below is an oscillator. When I simulate it with ltspice, it does indeed generate a waveform (although it doesn't seem to be a very pure sine wave). What I fail to understand is why it ...
blondiepassesby's user avatar
10 votes
4 answers
12k views

Measuring output impedance

Given I have designed and produced an RF circuit with an output for connecting an antenna. I know this antenna has an input impedance of 50Ω. I now wish to measure the actual output impedance of my ...
bjarkef's user avatar
  • 738
10 votes
2 answers
16k views

USB trace impedance calculations, with termination resistors

I'm designing a PCB featuring a Renesas RZ-A1 microcontroller, and want to check that I'm doing my USB D+ and D- traces correctly. This application note from Fairchild (alternative link), which is one ...
beammy's user avatar
  • 331
10 votes
2 answers
18k views

Why Characteristic Impedance must be 50 ohms?

Where does this number come from? For single ended it must be 50 ohm and for differential pairs 100 ohm. Why? For PCB with controlled-impedance these are that common numbers. Outside of the PCB you ...
Jesus Castane's user avatar

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