Selecting differential filter and common-mode cutoff frequencies for diff. ADC - Page 1
Forum section: electronics / beginners

author: pwalther
author_rank: Contributor
timestamp: April 14, 2020, 12:03:39 pm
content: I'm currently working on a design to interface with two PT100 RTDs and i want to connect them in a radiometric 3-wire RTD measurement system.What i already have would probably work perfectly fine in practice as it is really close to the TI reference design but i don't properly understand all of it so i still have a few questions.ADS1248 DatasheetTI reference designAs described from page 76 to 81 in the above linked datasheet.My designThe way i connected the current outputs and the adc inputs may seem a bit weird, but that's because it will be easier to layout that way and with the internal mux i can use every pin as every function anyway.I use a similar circuit but instead of a 3.3V analog supply voltage i use 5V. Because of that, i will use a 1.5mA current for the current source instead of the 1mA they use to still get 94% of the input range with a PGA gain of 4.Question 1In the Datasheet on page 80 they say that the 20 samples per second result in a -3 dB bandwith of 14.8Hz. That's the first thing i don't understand, how do you arrive at 14.8Hz from a 20SPS sampling rate?Question 2Also on page 80 they mention this:QuoteQuoteDo they mean 10 times 14.8Hz? Because 250Hz is closer to 20 times that than 10 times.Question 3Then they go on to calculate the input filter differential capacitor to have a cutoff frequency of the roughly the 250Hz they mentioned above. I'm fine with that as soon as i understand where the 250Hz come from.After that they calculate the common-mode cutoff frequencies and i'm not quiet sure what that is.They calculate it in a way that it's a 4.13kHz to 4.24kHz bandpass on the adc input and a 4.87kHz to 5.31kHz bandpass on the reference voltage input.Why these numbers? How do you know that it should be in that region?I would be glade if someone could help me out. I tried to google my way to the answers but if you are not really in the analog design world it's really hard to find the proper technical terms to google for information.

author: David Hess
author_rank: Super Contributor
timestamp: April 14, 2020, 03:48:17 pm
content: Quote from: pwalther on April 14, 2020, 12:03:39 pmThe delta-sigma converter has a modified sin(x)/x frequency response which depends on the integration time. Frequencies above the Nyquist frequency produce aliasing products which follow this frequency response just like any other ADC.QuoteThe value is not that critical as long as it is high enough to avoid altering the ADC's frequency response if that is important. This filter is mostly to keep RF out of the ADC and reduce noise above the modulator frequency but see below.QuoteThe common mode filtering removes RF and noise above the modulator frequency.What is really going on here is that mismatch between the common mode filter time constants to each input of the differential pair will convert common mode noise into differential noise which would then be measured by the ADC. The common solution is to add a third capacitor, the larger differential capacitor, to swamp this out.

author: pwalther
author_rank: Contributor
timestamp: April 14, 2020, 06:52:49 pm
content: Ok, that already helped me to understand a lot more.I now found this illustration:Am i correct in my assumption, that this graph shows that -3dB 14.8Hz Bandwith from 0Hz - 14.8Hz:Also i think i completly missunderstood the common mode input filter. It's not a bandpass, these are two lowpasses one on the positiv analog input and one on the negativ analog input.Now this starts to make a lot more sense because i really didn't get what a bandpass would be used for there.I now found that nice post on the TI blog:Three guidelines for designing anti-aliasing filtersThanks for answering my stupid questions

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 14, 2020, 07:31:10 pm
content: Note that antialiasing filters don't really apply so much to sigma-delta ADCs. They filter naturally. So the filter itself is pretty lax.Last time I used that, I was told by the customer that a power supply switching option would be nice so as to power-cycle reset the device. Apparently it's an MCU internally so the internal state, and ADC or SPI state, can get... weird.There's a similar ADC by ADI that made the incredible mistake of not resetting the bus state when nCS goes high; its SPI register is always listening to SCK. If it ever gets off by one cycle, lord help you.Apparently it's traditional for 24b+ ADCs to have really shitty errata. Beware.Tim

author: pwalther
author_rank: Contributor
timestamp: April 15, 2020, 05:16:12 pm
content: Yep, had a similar thing happen in another design with a similar adc as the one i use now from TI.The datasheet said that you could leave the nCS pin low if you don't use the SPI bus for any other device, so i did. But from time to time i had to reset the adc because the ADC would miss one bit and then it was all a mess from there.Fortunatly it wasn't that big of a deal because only scales where connected to that adc that where read once a second. So i just reset the ADC after i read out all 8 values. Bad practice but worked fine.Can you reccomend any good ressources to learn more about analog design? I know the basics from my apprenticeship but i never really applyed much of it as i did mostly digital designs and wrote software. When it comes to filters everything that is more complex than two passiv elements already uses 3 of my 4 braincells.

author: David Hess
author_rank: Super Contributor
timestamp: April 16, 2020, 04:26:44 am
content: Quote from: pwalther on April 14, 2020, 06:52:49 pmThat is right, but the frequency response is also non-linear so the usual rules relating cutoff frequency to transition time and noise do not apply. The same applies to sampling instruments which usually have a sin(x)/x frequency response.As T3sl4co1l points out, the delta-sigma converter does not need an anti-aliasing filter based on its sample rate because it handles that itself by oversampling. But it does still need anti-alias filtering based on its modulator frequency which is usually an easy task because the modulator frequency is so high.