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How to select cutoff frequency of LPF
Nikos A.:
Hi everyone,
I am new to this and I try to design a mixed signaling circuit and I need to find the desired cutoff frequency in order to design the LPF.
I have a buck converter as shown below. This particular convert has two outputs (see Vout and Load pins). I will split the LOAD pin into AVdd and DVdd and will a LPF at each leg.
On Vout I will connect my MCU (nRF52840) and the IMU (BMX160)
On AVdd I will connect an opAmp, Voltage Reference and a Multiplexer
On DVdd I will connect a flash memory (MX25R6435F)
Any advice of how could I find the cutoff frequency for each of the LPF? Should I take into consideration the components that are connected on Vout (MCU and IMU)?
Converter's datasheet
https://www.ti.com/lit/ds/symlink/tps62740.pdf?ts=1592300565512&ref_url=https%253A%252F%252Fwww.ti.com%252Fproduct%252FTPS62740
My design
T3sl4co1l:
FYI, don't use multiple net names on a single net (VSEL, VSEL1-4). Use one net name. Multiple is confusing and will lead to erroneous and missing connections.
Anyway, how much filtering do you need?
What is the PSRR of the various components? Or do they have a supply ripple requirement? Or supply impedance requirement?
Adding arbitrary filters can actually make things worse, as the AC impedance is raised, particularly around the resonant frequency 1 / (2 pi sqrt(L C)). If you don't know you need them, you probably don't, and may be better off that way as well.
Tim
Nikos A.:
Hi Tim,
Op-Amp (MCP6V61/1U/2/4) = min. 117db , typ. 134db
Also there is this information in the datasheet
I have included the supply bypassing but I do not know how to handle the noise reduction
Voltage Reference (LT6650): The datasheet sais that "PSRR can be significantly enhanced by adding a low-pass RC filter on the input, with a time-constant of 1ms or higher"
I have included the filter at the input of Voltage Reference as shown below
Multiplexer (ISL84781) : I couldn't find any information
Thanks
T3sl4co1l:
What frequencies are you measuring? What is the amp's PSRR at frequency? (There's a plot of this.)
Tim
Nikos A.:
I am into the design process, I cannot measure any frequency..
This is the PSRR vs frequency plot
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