# Questions tagged [impedance]

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

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### 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 ...
39k 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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### 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 ...
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### 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 ...
9k 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 ...
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### 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 ...
16k 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 ...
241 views

### Arriving at a wrong output impedance for a BJT Emitter Follower Configuration Circuit

We've to calculate output impedance of the circuit (figure 1). Although i understood the textbook solution (figure 2, figure 3), I took a different approach and arrived at a wrong answer. step 1: I ...
13k views

### Op-Amp Input Resistor?

I'm reading through the datasheet for the TL064, which contains this figure on page 16: This is of course an instrumentation amplifier that apparently uses an inverting amplifier's output instead of ...
4k views

### Do I really need the open loop output impedance of an op amp?

There are several answers on the internet and here on how to calculate the output impedance of the opamp with a given feedback. My intention, though, is to find this value with an open loop ...
36k 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 ...
27k 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 ...
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### 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 ...
13k views

### line-level audio into an iPhone headset jack?

What level and/or impedance matching (or pre-amp) circuits should be used to send a line-level audio signal into the mic input ring of an iPhone headset jack with minimal distortion and/or loss?
22k 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, ...
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### How can we have high voltage and low current (in transformers), if V = I * Impedance?

I have researched a lot on this topic (including this site) and found this link somewhat helpful : How is it possible to have high voltage and low current? ...     But still it seems ...
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### Standard 2.54/1.27mm pinheader impedance

Note I turned my answer into something of a collection of free impedance calculators.. I was thinking of using one of the most ubiquitous connectors in the world - 2.54mm pin header. The snag here is ...
3k views

### What does 100Ohm @ 100MHz exactly mean in ferrite beads?

I am trying to understand the parameters of ferrite beads and having trouble in understanding the impedance vs frequency parameter. What does 100Ohm at 100MHz exactly mean? Does it mean that the ...
737 views

### Bus interface for short distances, many device data exchange - CAN, RS485? + characteristic impedance and media capacitance

I ran into some troubles trying to design a wired bus which will connect in about 64 testing devices. My first choice of mind was CAN bus, known of it's reliability and low EMC interference. However, ...
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### How many turns do I need for the primary coil of a transformer?

I need to know the number of turns that should be connected to 220 volts AC. I can measure resistance only, I don't have a device that measures inductance. If I need the primary coil to be 220V / 1A (...
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### Output impedance of STM32H743 MCU?

How do I find the output impedance of the STM32H743 (and potentially other similar devices)? I've looked through the datasheet (DS12110 Rev 5) and the reference manual (RM0433 Rev 5) however haven't ...
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### measurement of output impedance of a CMOS inverter

I am measuring output impedance of CMOS inverter using ngspice. No matter how I measure the output impedance, the result can never come any close to the following theoretical calculation if I reduce ...
2k views

### MOSFET common source amplifier output impedance calculation

What I've read in microelectronic texts like Prof's Razavi's Fundamentals of Microelectronics is that for calculating output impedance, one has to make all independent sources 0, i.e., short all ...
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### Confusion with output impedance of non-inverting and inverting opamp circuit

I'm trying to understand why the output impedance of non-inverting and inverting opamp circuit is almost zero. I found the following text on the derivation: β is the closed loop gain above. But I ...
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### Common emitter, o.p. set by feedback resistor

I am analyzing common emitter amplifier where operating point is set by feedback resistor between b-c. Voltage gain showed by symulator is much lower than (-)R2/R1 and input resistance is lower than ...
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### Output impedance seen by the load in this emitter follower

I'm trying to understand what the concept of "output impedance" relates in a simple buffer such as an emitter follower conceptually without digging in formulas. Imagine the Rload is about to be ...
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### 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?
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### 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?
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### How is transmission line impedance selected?

I understand (roughly) why transmission line impedance has to be matched to the source and the load. What I don't understand is how different technologies have chosen to use different impedance. (...
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### Why does a real world capacitor behave like an inductor at frequencies above its self-resonant frequency?

I've come across some graphs comparing the impedance of a capacitor over frequency and it understandably declines as frequency increases -- up until a certain point. Afterwhich, the impedance begins ...
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### 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 ...
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### How to specify 100 ohm impedance on a pcb gerber

I've been using Altium for several years when I've needed to designed boards. However it's all been for simple analog designs and never anything too technical. I'm now trying to take a jump into a ...
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### When does controlled impedance on a PCB matter?

Suppose I have a PCB that contains a microprocessor, or an FPGA (a "controller"). This "controller" drives a single-ended signal to an LVDS driver IC and a single-ended signal to a connector that goes ...
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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 ...
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### Reflecting impedance in a transformer

Please can anyone tell me what exactly is meant by reflecting of impedance in a transformer? I do not know much about transformers, but I want to know about its uses and phenomena behind it. I will ...
621 views

### Output impedance of Op Amp with Voltage Divider?

I'm trying to figure out the output impedance of this op Amp circuit and I keep getting infinity. I'm not sure if the steps I'm doing are wrong. The reason I ignore everything besides the voltage ...