How to select cutoff frequency of LPF - Page 1
Forum section: electronics / projects

author: Nikos A.
author_rank: Regular Contributor
timestamp: June 16, 2020, 09:55:48 am
content: 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 MultiplexerOn 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 datasheethttps://www.ti.com/lit/ds/symlink/tps62740.pdf?ts=1592300565512&ref_url=https%253A%252F%252Fwww.ti.com%252Fproduct%252FTPS62740My design

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 16, 2020, 10:46:49 am
content: 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

author: Nikos A.
author_rank: Regular Contributor
timestamp: June 16, 2020, 04:02:04 pm
content: Hi Tim,Op-Amp (MCP6V61/1U/2/4)= min. 117db , typ. 134dbAlso there is this information in the datasheetI have included the supply bypassing but I do not know how to handle the noise reductionVoltage 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 belowMultiplexer (ISL84781): I couldn't find any informationThanks

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 16, 2020, 04:04:08 pm
content: What frequencies are you measuring? What is the amp's PSRR at frequency? (There's a plot of this.)Tim

author: Nikos A.
author_rank: Regular Contributor
timestamp: June 16, 2020, 04:11:45 pm
content: I am into the design process, I cannot measure any frequency..This is the PSRR vs frequency plot

author: jkostb
author_rank: Regular Contributor
timestamp: June 18, 2020, 08:34:47 pm
content: As Tim already has written you must be carefull with LC filters, because the filter can oscillate depending on signal. Normally you can power digital circuits directly from switchers (if ripple is sufficient low). So your flash memory can be powered without extra LC filtering. Only for sensitive analog circuits you need to pay attention that the analog circuits are powered from a clean power supply. There are 2 ways to accomplish this. The first is by use of a LC filter. A LC filter has a roll off of 40 dB per decade. So you need to know magnitude of harmonics of power supply and the PSRR of the amplifier. Choose the corner frequency in such a way that the largest harmonics are sufficiently attenuated e.g. if you choose corner frequency at 1/10 of switching frequency then you know that first harmonic is attenuated with 40dB.Another way to power sensitive analog circuits is by use of an LDO. Problem with LDO is that it is not very efficient. For this reason the supply voltage is first converted down by use of a SMPS (buck converter) and then cascaded by a LDO. You can create a very clean analog output voltage by inserting a low pass filter consisting of a ferrite bead and capacitor just before the LDO. This will attenuate the high frequency noise very effectively! The LT3045 for example is a LDO with very high PSRR and ultralow noise, so a perfect LDO for this job.One last note I see that you pay a lot of attention to filtering the power supply and that you use in the end an AD converter integrated in the microcontroller. This does not make sense. If accuracy is really important then you also need to use an external AD converter. A 12 bit external AD converter has for example 10 effective number of bits while the same 12 bit AD converter integrated in microcontroller has < 10 effective number of bits. So effectively you could also omit the whole power supply filtering.....

author: Nikos A.
author_rank: Regular Contributor
timestamp: June 19, 2020, 09:57:47 am
content: Thank you for your suggestions jkostb!!Quote from: jkostb on June 18, 2020, 08:34:47 pmHow does the AD is related with the power supply filtering?

author: Siwastaja
author_rank: Super Contributor
timestamp: June 19, 2020, 11:36:24 am
content: Quote from: jkostb on June 18, 2020, 08:34:47 pmNote that, as a strictly general advice, this is misleading while widely seen.You need a very specific kind of linear regulator which specifies decent PSRR to very high frequencies, some to tens of MHz. Not just any linear regulator.And not necessarily an LDO. A normal drop out linear regulator is an option as well, depending on what power rails you have available.Just a bog-standard linear regulator you are likely to choose if you don't know better, passes high-frequency components almost straight through.Even with those high-frequency PSRR linear regs, you likely need some additional filtering for even higher frequencies (say, >10MHz), so it's not either-or. But this is easily done using ferrite bead as L, which is lossy to prevent ringing (oscillation), just make sure the ferrite bead does not saturate, so this applies to very low-power circuits only, but ADC power often is such.Also using lossy C (say, tantalum) in parallel with the much smaller ceramic C provides damping for the filter.Then, the tasks are shared so that the small (typically ferrite bead based) LC takes care of noise above around 10MHz or so, and the (correct type of) linear regulator below about 10MHz (which would require large passives).Also note that "LDO" is nowadays very widely being used synonymous to "linear regulator", but this is very wrong, it's a specific type of linear regulator with low dropout. Whenever you don't need low dropout, look at all linear regulators to get more choice and not to arbitrarily limit what is available.Things are getting easier and easier all the time; I'm thinking about a modern ADC which takes so little current that it has allowed the ADC designers to combine power and reference without causing power-related current draw drifts; also allowing powering up the whole ADC from a typical voltage reference rated at say 20mA; also making it possible to use very small ferrite beads to filter without saturating, and so on. As technology goes forward, integration goes up, and modern ICs are easier to use than ever.Finally, I'm sure you know this, but local bypass cap right at the amplifier power pins offers the final and maybe the most important filter.