| Electronics > Beginners |
| Driving a buffered SAR ADC |
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| SethGI:
@kleinstein, before the ADC I'll be using a bandpass with 3db corners at 20 and 45khz. The reason for the filter is to make the digital processing a bit easier; we want to run fast and any steps which can be handled before the a/d conversion is a plus. The reason for the choise of 4th order butterworth (in 2 2nd order multiple feedback stages) is primarily for the good step response characteristics, quick enough rolloff, and no ripple in passband. The freuqency range that I really care about is 25-40khz. As suspected, I will be connecting to an FPGA. I haven't decided which yet, but probably one from Intel as I have the most experience with those. --- Quote from: Kleinstein on August 11, 2019, 08:58:42 am ---The ADAR7251 is a sigma delta ADC and thus does not need much anti aliasing filtering at the input. For the frequency in question here (some 10-50 MHz) the filter could be just simple RC, maybe LC filter, if at all. Just the limited BW of the amplifiers may be enough. Even with not so high sampling rate (e.g. 300 kSPS) the digital interface is quite fast. This requites a good board layout to keep interference away from the analog side. This also applies to all comparable ADCs - 4 fast 16 ADCs just produce a lot of data and thus need a fast interface. Chances are the ADC would be coupled to an FPGA or maybe DSP, that can also produce EMI. An important question is which frequency range is really of interest. --- End quote --- |
| SethGI:
Do I still need that anti-aliasing buffer with an ADC that has the LNA and PGA stages built in? If so, can I just use any old op-amp that's low noise and fast enough, or do I need something in particular? Or will my filter suffice? --- Quote from: ogden on August 11, 2019, 10:41:58 am ---I would put it this way: every ADC needs anti aliasing buffer at the input, not every ADC needs low impedance signal source. --- End quote --- |
| ogden:
--- Quote from: SethGI on August 11, 2019, 02:55:32 pm ---Do I still need that anti-aliasing buffer with an ADC that has the LNA and PGA stages built in? --- End quote --- For ADC with LNA you do not need anti-aliasing buffer, you do need anti-aliasing filter. Passive filter is enough, but if you use active filter which will provide low impedance output - it is OK as well. Anti-aliasing filter removes unneeded unwanted frequency components which can cause aliasing and add noise or even ruin ADC results. Buffer does buffering - for ADC's which does not have built-in buffer. |
| Kleinstein:
The active filter for 20-45 kHz also acts as an anti aliasing filter. For a SD ADC AA filtering is about really high frequencies, not the usual 1/2 the sampling frequency, but more like > 10 MHz. I have not checked the DS. It is well possible that ADAR7251 with internal amplifier may not need an external AA filter at all, as chances are the internal amplifier is slow enough to also act as AA filter. If filter response is critical one usually prefers digital filters. Even if the response is not ideal, the response is known and stable. |
| SethGI:
OK that makes way more sense to me, I was confused by the previous comments. I didn't know how a buffer could do any anti-aliasing. So we'll call that a typo. Just to clarify, it looks like my approach of: piezo -> fully differential charge amplifier -> active filter -> ADAR7251 is viable. As to the doing the filtering digitally, I don't know if that would work. It's a pretty noisy enviornment and I want to make sure I'm isolating the frequencies I care about before amplification. So I'd prefer to keep a bandpass before the amplification step to make sure I'm making best use of the resolution of the ADC. One final question. I heard someone mention that the output of a piezoelectric transducer is inherently differential. That doesn't make a ton of sense to me intuitively. So then the question becomes: Do I need to convert from single ended to differntial, or can I just amplify the transducer signal as if it is already fully differential. I've found examples of such conversion circuits online, but nothing which treats the sensor as producing a differential output, which makes me doubt even more that it's a real thing. Here's what I was basing it on: http://tesi.cab.unipd.it/40176/1/Thesis.pdf (notably pages 2 and 8 ) I attached an image with a sketch of the two ideas. Again, thanks for your patience with my noobish questions. |
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