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| A High-Performance Open Source Oscilloscope: development log & future ideas |
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| tom66:
I'm aware KiCad can do that, so it makes sense to use that at some point. Actually, CircuitMaker doesn't even do length matching or tuning, that is reserved for the premium Altium product. You do at least get length lists, but if you want to length match to the longest group, you have to do it manually, both the calculation and the tuning. Control/address bus needs to be the longest of all of the groups, then the data buses need to be equally matched to each other, including strobe and mask (though data groups can be independent lengths as they are each tuned on power up by Zynq DDR3 controller.) So lots of Excel spreadsheets and hand calculations. I also noticed a recent mistake when reviewing this design as I'm working on a commercial project now that uses a Zynq too. I didn't terminate ODT ball to Vtt, as I read somewhere that it was a CMOS signal. Wrong, it is high speed. Somehow, the scope board still works OK, but I guess the ODT signal has a lot of reflections on it, which might affect when the termination on the data bus side is enabled. I'll make sure to terminate ODT in the new design! |
| nctnico:
--- Quote from: nctnico on November 17, 2020, 07:11:33 pm --- --- Quote from: tom66 on November 17, 2020, 11:21:36 am --- --- Quote from: nctnico on November 17, 2020, 10:56:11 am ---Just wondering... has any work been done on an analog front-end? I have done some work on this in the past; I can dig it up if there is interest. Looking at the Analog devices DSO fronted parts it seems that these make life a lot easier. --- End quote --- I've got a concept and LTSpice simulation of the attenuator and pre-amp side, but nothing has been tested for real or laid out. It would be useful to have an experienced analog engineer look at this - I know enough to be dangerous but that's about it. --- End quote --- I have attached a design I created based on earlier circuits. IIRC it is intended to offer a 100MHz bandwidth and should survive being connected to mains (note the date!). Left to right, top to bottom: - Input section with attenuators. Note sure whether the capacitance towards the probe is constant. - Frequency compensation using varicaps. This works but requires an (digitally) adjustable voltage of up to 50V and I'm not sure how well a calibration holds over time. Using trim capacitors might be a better idea for a first version. - over voltage protection - high impedance buffer - anti-aliasing filter. Looking at it I'm not sure whether a 7th order filter is a good idea due to phase shifts. - single ended to differential amplifier and analog offset - gain control block and ADC. Nowadays I'd skip the external gain control and use the internal gain control of the HMCAD1511/20 devices. It could be a nice Christmas project to see how it behaves. --- End quote --- Meanwhile I have been slowly moving forward with this and had a PCB made which is based on the schematic I posted earlier. I'm not going to post the schematic of this new circuit yet because some parts are experimental and I don't want a flurry of schematics floating around. This first version has the following design targets: - sensitivity from 500uV/div to 20V/div by using 1:2.5, 1:10 and 1:200 attenuators - constant input capacitance - DAC driven offset adjust - target 200MHz bandwidth - 2 different anti-aliasing filters - DAC driven voltage adjustable capacitors to make the attenuators have a flat frequency response; however the adjustable capacitors I have found turned out to have a too low resistance path and thus are useless. I have found some low voltage varicaps though which seems suitable as well; these are on the way. - adjustable, high precission compensator output which can be used for self-calibration of the front-end - 20x or 2x gain consisting of fixed 2x gain stage and switchable 10x gain stage. I want to see if the 10x can work by switching a feedback resistor (probably not) or an extra amplifier stage is needed. The prototype supports both. - differential output compatible with tom66's prototype board which uses a SATA connector and the HMCAD15xx ADCs It will probably take a while before I get it fully tested & tweaked. I just wanted to give an update. |
| gf:
--- Quote from: nctnico on February 07, 2021, 11:52:23 pm ---- sensitivity from 500uV/div to 20V/div by using 1:2.5, 1:10 and 1:200 attenuators --- End quote --- Do you mean 2.5x attenuation minimum, even for 500µV/div? (that were only 200µV/div after the attenuator -- aren't that possibly ~8dB of renounced SNR?) --- Quote ---- 20x or 2x gain consisting of fixed 2x gain stage and switchable 10x gain stage. I want to see if the 10x can work by switching a feedback resistor (probably not) or an extra amplifier stage is needed. The prototype supports both. --- End quote --- Hmm, how is 20x gain supposed to suffice for 500µV/div (i.e. 5mV full scale on a 10 div display)? After 1:2.5 attenuation, a gain of rather 1000x were required to obtain the 2V full scale input voltage for the HMCAD1511 (and without prior attenuation it were still 400x gain). In 8-bit mode I see the possibility to augment the analog gain by some amount of HMCAD1511's digital gain, with only small SNR degradation, but according to [1] the useful range is still limited to <= 10x. OTOH, for the 12/14-bit modes of HMCAD1520 (which were discussed in this thread, too), which already utilize (almost) the full DR of the ADC, I don't see digital (coarse) gain as an option, so that the whole amplification to 2V full scale needs to be done in the analog domain. [Digital fine gain can possibly still be used for small calibration adjustments of +/- a few percent, but I'm not sure if "no missing codes" ist still guaranteed in 14-bit mode then.] [1] https://www.analog.com/media/en/technical-documentation/application-notes/using_digital%20gain_feature_of_hmcad1511.pdf --- Quote ---Lab testing has shown that gain settings up to 8x (corresponding to 0.25Vpp-diff. input full-scale) show minimal loss in SNR (as evident in Figure 5). SNR in dBc starts to degrade rapidly beyond digital gain of 10x, so the user is advised to keep the digital gain setting at 10x or less. --- End quote --- |
| nctnico:
--- Quote from: gf on February 08, 2021, 11:44:36 pm --- --- Quote from: nctnico on February 07, 2021, 11:52:23 pm ---- sensitivity from 500uV/div to 20V/div by using 1:2.5, 1:10 and 1:200 attenuators --- End quote --- Do you mean 2.5x attenuation minimum, even for 500µV/div? (that were only 200µV/div after the attenuator -- aren't that possibly ~8dB of renounced SNR?) --- End quote --- Yes. This is to have some protection of the input and provide a constant input capacitance. --- Quote --- --- Quote ---- 20x or 2x gain consisting of fixed 2x gain stage and switchable 10x gain stage. I want to see if the 10x can work by switching a feedback resistor (probably not) or an extra amplifier stage is needed. The prototype supports both. --- End quote --- Hmm, how is 20x gain supposed to suffice for 500µV/div (i.e. 5mV full scale on a 10 div display)? After 1:2.5 attenuation, a gain of rather 1000x were required to obtain the 2V full scale input voltage for the HMCAD1511 (and without prior attenuation it were still 400x gain). In 8-bit mode I see the possibility to augment the analog gain by some amount of HMCAD1511's digital gain, with only small SNR degradation, but according to [1] the useful range is still limited to <= 10x. OTOH, for the 12/14-bit modes of HMCAD1520 (which were discussed in this thread, too), which already utilize (almost) the full DR of the ADC, I don't see digital (coarse) gain as an option, so that the whole amplification to 2V full scale needs to be done in the analog domain. [Digital fine gain can possibly still be used for small calibration adjustments of +/- a few percent, but I'm not sure if "no missing codes" ist still guaranteed in 14-bit mode then.] [1] https://www.analog.com/media/en/technical-documentation/application-notes/using_digital%20gain_feature_of_hmcad1511.pdf --- Quote ---Lab testing has shown that gain settings up to 8x (corresponding to 0.25Vpp-diff. input full-scale) show minimal loss in SNR (as evident in Figure 5). SNR in dBc starts to degrade rapidly beyond digital gain of 10x, so the user is advised to keep the digital gain setting at 10x or less. --- End quote --- --- End quote --- For the 500uV/div (which translates to 4mVpp with 8 divisions) the ADC gain will need to be set to maximum indeed and then you still won't get the full range. This is a limitation. Still there is plenty of room to increase the amplification later on. I just had to start somewhere and concentrated on the 8 bit ADC. I would like to avoid using a VGA because that adds extra noise to the signal path. If you translate the SNR to what is being displayed; setting the gain to maximum results in degradation of about 12dB / 2 bits /4 LSB. So basically you get 1/8th of a vertical division of noise on screen (assuming 8 divisions is full range) with the ADC gain set to max. I want to see how the analog front end behaves where it comes to noise first before worrying about the ADC. |
| tom66:
In my original sketching of ideas I had always planned for an IC like LMH2832. https://www.ti.com/lit/ds/symlink/lmh2832.pdf This would be combined with a single -39dB attenuation relay to get you a 78dB attenuation range in total. If you want the precision modes of the ADC, you can't use the gain stages. For 8-bit mode they may be sufficient. Not sure if the gain stages vary with the '1511, if it uses just an 8-bit core internally, or if the true difference between the parts is only the ability to export that precision data out. The HMCAD1511 also requires all of its inputs to be centred around VCOM, about 1 volt. Shouldn't be a problem, just be aware it has a limited common mode range. |
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