- Posts: 4
- Thank you received: 3
About input impedance, I have some question
6 years 3 weeks ago - 6 years 3 weeks ago #1317
by Ray
Replied by Ray on topic About input impedance, I have some question
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 !!!
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 !!!
Please Log in to join the conversation.
Time to create page: 0.059 seconds