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TOPIC: About input impedance, I have some question

About input impedance, I have some question 6 months 2 weeks ago #1314

  • woshiren
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in MicroAmp,the impedance is Zin=(R1//R2)//(R3+Zinopamp)=(22M//10M)//(1K+106M)=6.8M
so in pedalshield ,the impedance is Zin=(R1//R2)//(R3+Zinopamp)≈ R1//R2=1M//1M=500K
but I saw that in page,I don't know which is true....
Specifications.

Based on Arduino MEGA 2560 / ADK (16MHz, 8KB RAM).
Analog stages using TL972 rail-to-rail operational amplifier.
ADC: 10bits.
Output Stage: 16 bits (2x8bits PWMs running in parallel)
OLED Screen: 128x64 resolution, 1.3 inches (also compatible with 0.96"), I2C.
Interface:
2 Configurable push buttons.
1 Configurable switch.
1 programmable blue led.
True Bypass Foot-switch
OLED Display.
Connectors:
Input Jack, 1/4 inch unbalanced, Zin=1MΩ.
Output Jack, 1/4 inch unbalanced, Zout=100Ω.
Power supply: power taken from the Arduino MEGA board (12V DC).
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About input impedance, I have some question 6 months 2 weeks ago #1315

  • woshiren
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And also in pedalSHIELD DUE
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About input impedance, I have some question 6 months 2 weeks ago #1317

  • Ray
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Hi woshiren, thanks for spotting the errors:

In pedalSHIELD MEGA:
The input impedance could be calculated as:
Zin = R1 // (R2//(R3+Zinopamp))
The Zin of the TL972 op-amp is not mentioned in the datasheet (www.ti.com/lit/ds/symlink/tl971.pdf), they just mention "The inputs of an operational amplifier have a very high resistance which puts a negligible current load on the voltage source". So assuming that this value is much bigger than R2, the formula could be reduced to:
Zin = R1//R2 = 1MΩ//1MΩ = 500K as you mentioned.

500K is a good input impedance for a guitar pedal, but if some reason you need a higher one (1M), you could use a higher value for R1 (i.e R1=10MΩ would make the Zin = 0.9MΩ ) OR just remove the R1 (because it is a pull-down resistor to avoid popping sounds when you engage the pedal, it is good to have it, but not mandatory). Removing R1 would make Zin = 1MΩ

The good thing is that I found in the datasheet that the output impedance is lower than it was stated, Zout < 0.1 ohms (Figure 15 in the datasheet).

in pedalSHIELD DUE

You are in the same situation, as you mentioned the input impedance is 5MΩ (not 10).

I have updated both articles, thanks for your help !!! ;)
Last Edit: 6 months 2 weeks ago by Ray.
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