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Is this a good value? A 10 kΩ resistor in series with AVCC won't cut the mustard at all. AVCC does draw current (PLL and ADC for example) and, this can be several mA so, an inductor is needed ...
#7: Post edited
by
Andy aka
·
2026-02-21T17:35:15Z (7 months ago)
- > _Is this a good value?_
- A 10 kΩ resistor in series with AVCC won't cut the mustard at all. AVCC does draw current (PLL and ADC for example) and, this can be several mA so, an inductor is needed (along with capacitor C2 in your diagram). Typically something like 10 μH appears to be recommended.
<sub>An inductor will barely drop any DC voltage at all; it should have a resistance of maybe less than 1 Ω to a few ohms so, if 10 mA were drawn by AVCC, the DC volt drop would be a few tens of mV maximum. Choose the inductor so that it remains inductive into the MHz region. All inductors become capacitive at their self resonant frequency so, it's important to choose a high value for SRF.</sub>- > _what determines the cutoff frequency for the lowpass filter in this situation?_
For an LC filter (the type recommended) the cut-off frequency formula is this: -- $$F_C = \dfrac{1}{2\pi\sqrt{LC}}$$
- So, for 10 μH and 100 nF, the cut-off frequency is 159 kHz. You can use [this calculator](https://www.omnicalculator.com/physics/resonant-frequency-lc) to help you with other value combinations. You should also factor-in what power supply spectral noise you have and maybe choose a higher value of inductor and capacitor.
- Because an LC network like this can "overshoot" the output voltage when the incoming supply rises quickly, you might consider adding a zener diode across the AVCC rail and GND/0 volts.
- Personally, I would test this by using a simulator.
- > _Is this a good value?_
- A 10 kΩ resistor in series with AVCC won't cut the mustard at all. AVCC does draw current (PLL and ADC for example) and, this can be several mA so, an inductor is needed (along with capacitor C2 in your diagram). Typically something like 10 μH appears to be recommended.
- <sub>An inductor will barely drop any DC voltage at all; it should have a resistance of maybe less than 1 Ω to a few ohms so, if 10 mA were drawn by AVCC, the DC volt drop would be a few tens of mV maximum. Choose the inductor so that it remains inductive into the MHz region. All inductors become capacitive above their self resonant frequency so, it's important to choose a high value for SRF.</sub>
- > _what determines the cutoff frequency for the lowpass filter in this situation?_
- For an LC filter (the type recommended), the cut-off frequency formula is this: -
- $$F_C = \dfrac{1}{2\pi\sqrt{LC}}$$
- So, for 10 μH and 100 nF, the cut-off frequency is 159 kHz. You can use [this calculator](https://www.omnicalculator.com/physics/resonant-frequency-lc) to help you with other value combinations. You should also factor-in what power supply spectral noise you have and maybe choose a higher value of inductor and capacitor.
- Because an LC network like this can "overshoot" the output voltage when the incoming supply rises quickly, you might consider adding a zener diode across the AVCC rail and GND/0 volts.
- Personally, I would test this by using a simulator.
#6: Post edited
by
Andy aka
·
2026-02-21T14:42:04Z (7 months ago)
- > _Is this a good value?_
- A 10 kΩ resistor in series with AVCC won't cut the mustard at all. AVCC does draw current (PLL and ADC for example) and, this can be several mA so, an inductor is needed (along with capacitor C2 in your diagram). Typically something like 10 μH appears to be recommended.
<sub>An inductor will barely drop any DC voltage at all; it should have a resistance of maybe less than 1 Ω to a few ohms so, if 10 mA were drawn by AVCC, the DC volt drop would be a few tens of mV maximum. Choose the inductor so that it remains inductive into the MHz region. All inductors become capacitive at their self resonant frequency so, it's important to choose a high value for SRF.</sub>
- > _Is this a good value?_
- A 10 kΩ resistor in series with AVCC won't cut the mustard at all. AVCC does draw current (PLL and ADC for example) and, this can be several mA so, an inductor is needed (along with capacitor C2 in your diagram). Typically something like 10 μH appears to be recommended.
- <sub>An inductor will barely drop any DC voltage at all; it should have a resistance of maybe less than 1 Ω to a few ohms so, if 10 mA were drawn by AVCC, the DC volt drop would be a few tens of mV maximum. Choose the inductor so that it remains inductive into the MHz region. All inductors become capacitive at their self resonant frequency so, it's important to choose a high value for SRF.</sub>
- > _what determines the cutoff frequency for the lowpass filter in this situation?_
- For an LC filter (the type recommended) the cut-off frequency formula is this: -
- $$F_C = \dfrac{1}{2\pi\sqrt{LC}}$$
- So, for 10 μH and 100 nF, the cut-off frequency is 159 kHz. You can use [this calculator](https://www.omnicalculator.com/physics/resonant-frequency-lc) to help you with other value combinations. You should also factor-in what power supply spectral noise you have and maybe choose a higher value of inductor and capacitor.
- Because an LC network like this can "overshoot" the output voltage when the incoming supply rises quickly, you might consider adding a zener diode across the AVCC rail and GND/0 volts.
- Personally, I would test this by using a simulator.
#5: Post edited
by
Andy aka
·
2026-02-21T13:04:37Z (7 months ago)
- > _Is this a good value?_
- A 10 kΩ resistor in series with AVCC won't cut the mustard at all. AVCC does draw current (PLL and ADC for example) and, this can be several mA so, an inductor is needed (along with capacitor C2 in your diagram). Typically something like 10 μH appears to be recommended.
<sub>An inductor will barely drop any DC voltage at all; it should have a resistance of maybe less than 1 Ω to a few ohms so, if 10 mA were drawn by AVCC, the DC volt drop would be a few tens of mV maximum. Choose the inductor so that it remains inductive into the MHZ region. All inductors become capacitive at their self resonant frequency so, it's important to choose a high value for SRF.</sub>
- > _Is this a good value?_
- A 10 kΩ resistor in series with AVCC won't cut the mustard at all. AVCC does draw current (PLL and ADC for example) and, this can be several mA so, an inductor is needed (along with capacitor C2 in your diagram). Typically something like 10 μH appears to be recommended.
- <sub>An inductor will barely drop any DC voltage at all; it should have a resistance of maybe less than 1 Ω to a few ohms so, if 10 mA were drawn by AVCC, the DC volt drop would be a few tens of mV maximum. Choose the inductor so that it remains inductive into the MHz region. All inductors become capacitive at their self resonant frequency so, it's important to choose a high value for SRF.</sub>
#4: Post edited
by
Andy aka
·
2026-02-21T13:03:43Z (7 months ago)
- A 10 kΩ resistor in series with AVCC won't cut the mustard at all. AVCC does draw current (PLL and ADC for example) and, this can be several mA so, an inductor is needed (along with capacitor C2 in your diagram). Typically something like 10 μH appears to be recommended.
- <sub>An inductor will barely drop any DC voltage at all; it should have a resistance of maybe less than 1 Ω to a few ohms so, if 10 mA were drawn by AVCC, the DC volt drop would be a few tens of mV maximum. Choose the inductor so that it remains inductive into the MHZ region. All inductors become capacitive at their self resonant frequency so, it's important to choose a high value for SRF.</sub>
- > _Is this a good value?_
- A 10 kΩ resistor in series with AVCC won't cut the mustard at all. AVCC does draw current (PLL and ADC for example) and, this can be several mA so, an inductor is needed (along with capacitor C2 in your diagram). Typically something like 10 μH appears to be recommended.
- <sub>An inductor will barely drop any DC voltage at all; it should have a resistance of maybe less than 1 Ω to a few ohms so, if 10 mA were drawn by AVCC, the DC volt drop would be a few tens of mV maximum. Choose the inductor so that it remains inductive into the MHZ region. All inductors become capacitive at their self resonant frequency so, it's important to choose a high value for SRF.</sub>
#3: Post edited
by
Andy aka
·
2026-02-21T12:09:46Z (7 months ago)
A 10 kΩ resistor in series with AVCC won't cut the mustard at all. AVCC does draw current (PLL and ADC for example) and, this can be several mA so, an inductor is needed (along with capacitor C2 in your diagram). Typically something like 10 μH appears to be recommended.
- A 10 kΩ resistor in series with AVCC won't cut the mustard at all. AVCC does draw current (PLL and ADC for example) and, this can be several mA so, an inductor is needed (along with capacitor C2 in your diagram). Typically something like 10 μH appears to be recommended.
- <sub>An inductor will barely drop any DC voltage at all; it should have a resistance of maybe less than 1 Ω to a few ohms so, if 10 mA were drawn by AVCC, the DC volt drop would be a few tens of mV maximum. Choose the inductor so that it remains inductive into the MHZ region. All inductors become capacitive at their self resonant frequency so, it's important to choose a high value for SRF.</sub>
#2: Post edited
by
Andy aka
·
2026-02-21T11:52:50Z (7 months ago)
A 10 kΩ resistor in series with AVCC won't cut the mustard at all. AVCC does draw current (PLL for example) and, this can be several mA so, an inductor is needed (along with capacitor C2 in your diagram). Typically something like 10 μH appears to be recommended.
- A 10 kΩ resistor in series with AVCC won't cut the mustard at all. AVCC does draw current (PLL and ADC for example) and, this can be several mA so, an inductor is needed (along with capacitor C2 in your diagram). Typically something like 10 μH appears to be recommended.
#1: Initial revision
by
Andy aka
·
2026-02-21T11:50:29Z (7 months ago)
A 10 kΩ resistor in series with AVCC won't cut the mustard at all. AVCC does draw current (PLL for example) and, this can be several mA so, an inductor is needed (along with capacitor C2 in your diagram). Typically something like 10 μH appears to be recommended.