Electrical Engineering Stack Exchange
Electrical Engineering Stack Exchange

Questions tagged [input-impedance]

227 questions
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6 answers

Why are oscilloscope input impedances so low?

My question is two-fold: Where does the input impedance come from? I'm wondering where the input impedance of your average multimeter or oscilloscope comes from? Is it just the input impedance to the device's input stage (such as an amplifier or…
PersonWithName
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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 with an output impedance of approximately 10 kΩ or…
Pugz
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11
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2 answers

How to avoid Johnson noise in high input impedance amplifier

I don't have a circuit I'm working on, this is more of a theoretical question - I am trying to remedy a flaw in my understanding. Imagine I want to build a high input impedance amplifier to work in the low mV range, with a few nV/√Hz noise. I want…
Jack B
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11
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4 answers

Is the input impedance of an operational amplifier (op amp) infinite or zero?

Ideally, the input impedance is infinite. But, in calculating the input resistance (Rin) of a difference amplifier, the author took the concept that the two input terminals are short circuited, which is also true, since the open loop gain is…
electronics
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2 answers

What is this circuit called, and what is it used for?

I analyzed this circuit and found that it's an inverting amplifier and that the impedance of the circuit goes to zero when the frequency of the input voltage goes to infinity. Does anyone know the specific name of this circuit, if it has one?
umutcankalayci
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How are such low op-amp input currents possible?

I understand that op-amps have low input currents; that's one of their defining characteristics. But looking at the datasheet for the LMC6001 (amusingly called an "Ultra, Ultra-Low Input Current Amplifier" because one ultra just wasn't enough), I…
Hearth
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8
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2 answers

Why do we not care about matching the input impedance of non-RF amplifiers?

Talking about BJT topologies: A common base amplifier is always given as an example of an amplifier with very similar characteristics to the CE amplifier but with the low input impedance. simulate this circuit – Schematic created using…
midnightBlue
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8
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1 answer

Calculate differential amplifier's input impedance

How does one calculate the input impedance of a differential amplifier? Since the input impedance of an op-amp is very high, the input impedance for V2 should be R2 + Rg. But what would be the input impedance for V1?
zud
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1 answer

How to measure input/output impedance of amplifiers in a simulation?

What is the most effective way of measuring the input/output impedance of an amplifier in a simulation? When I know the impedance I want to measure is purely resistive, I usually set up an input signal Vin and a test resistor as a resistive divider…
Costagero
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2 answers

Understanding resistors on inputs/outputs of op amps

I was looking at the schematic for Dave Jones' uCurrent, and I can't seem to make sense of why there are 270 ohm resistors on the inputs and outputs of the op amps in his circuit. The resistance is low enough that it wouldn't affect the filter in…
ccpmark
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How much current is used with a μC GPIO input and pullup resistor?

I'm trying to understand power budgets a bit better and I have a question regarding pullups. I have a input to a microcontroller that needs to be logic high at all times. The micro goes to sleep now and again, so the internal pullup isn't an option.…
BabaBooey
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3 answers

When talking about input and output impedances, what are we comparing them to?

Let's consider a theoretical highpass filter: simulate this circuit – Schematic created using CircuitLab I'm trying to show that the worst-case input and output impedance is R but the whole concept of input / output impedance isn't really clicking…
Aust
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Why do we care about matching the input impedance of receiving RF amps?

Lets say we have a receiving antenna that intercepts a certain amount of power \$P_r\$ = \$SA_r\$, where \$S\$ is the power flow density of the incoming EM wave, and \$A_r\$ the effective area of the antenna. This power, \$P_r\$, is also called the…
Troels Folke
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What is the relation between Op-Amp input current and input impedance?

How should I interpret these specs? They can't both be correct all the time, right? Looking at for example the CA3140 opamp. the Zin (input impedance) is 1.5T Ohm, yet the input current is 10pA. Solving this for voltage (Ohm's law, 1,5e12 * 10e-12)…
Plumpie
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Using single resistor to receive 5v signal on the 3.3v rated pin

I have been investigating ways to safely receive 5v signal on the input pin that can only support 3.3v input signal. I came across an article that states that it's possible to only use a single 10k Ohm resistor to achieve that. I can understand how…
EGDima
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