Is this DC-DC supply ripple plausible in a harsh environment? - Page 1
Forum section: electronics / beginners

author: Captain_Blackbeard
author_rank: Newbie
timestamp: July 17, 2023, 02:27:01 pm
content: I have been working on a DIY engine management computer for a diesel engine for some time now and I am struggling with the power supply. I am looking for input as to how realistic my expectations are.As my project sits now, I have built a Linear DC-DC supply with some filtering, I believe the terminology is correct, I am 100% self taught though. I believe I am currently achieving about 17mV p-p ripple and about 3mV p-p noise on my output and have anywhere from 120mV to 250mV of ripple on my input @ various point from 0-540kHz based on my measurements with a DSO5000 scope. My design requirements for power input is 8V min to 32V max, with a nominal input voltage of 12V on relatively dirty automotive power. I am trying to output 5V right now but I am not stuck on that number. I believe 5V and 3.3V are plausible, the choice of 5V is based on a manufacturer reference design to excite an LVDT coil that I must use.I am trying to achieve about 3mV p-p ripple and as long as the noise is that or less it does not matter to me. The intent is to use this source as a reference and/or supply for several ADCs and a sine wave generator(wienbridge) that will be amplified to drive a LVDT at apx 200mA max(based on resistance and DC, I don't know how to calculate the current for driving it with AC yet, I'll get there...), I may also run a micro off of it but I do not think it requires this clean of power for my application. The 3mV p-p number is coming from some basic napkin math I have done based on the usable resolution I would like from the ADCs. I believe the total expected draw will be about 2A at peak and about 350mA at minimum.It it feasible to achieve the 3mV p-p @ 5V with a relatively dirty input of 8V-32V from an automotive power system(1980-1995 era)? $$ isn't really much of a constraint on the design as it is more of a passion project.I think this is my first forum post ever so If I am doing something incorrect please let me know the correct etiquette.Thank you.

author: m k
author_rank: Super Contributor
timestamp: July 20, 2023, 04:50:33 pm
content: Maybe you should have left the self thought part out first, it may have diminished replies, like from those LVDT designers usually hang around.Input voltage difference just means that you regulate accordingly.Similarly, sort of, with output ripple, it will compensate itself away by averaging if the actual measurement operation is slow enough.What kind of filters you currently use?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 20, 2023, 07:35:29 pm
content: Why use a system with seemingly intentionally low PSRR?The big trick to metrology is setting up ratiometric measurements wherever possible. It's the few cases where you can't, that you have your work cut out for you.An LVDT application has many such opportunities:- Don't use an analog oscillator, use a DDS + DAC- Don't use the supply as a reference, use an especially clean reference voltage, and op-amps and DSP with reasonable PSRR- Don't use sine waves at all, use a pulse sequence or square waves*- Measure the supply voltage and divide by it, effectively calibrating the oscillator in real time- Measure the oscillator output (reference AC voltage) and divide by it, calibrating even faster (effectively, many times per cycle)- Correlate samples to an ideal sine wave, i.e. measure the fundamental directly- Correlate samples to sin/cos, i.e. measure the full analytical / complex-valued voltage*Not actually different, because a DDS + DAC is just a sequence of steps. More to say: a sine wave with relatively low sample rate, or little or no filtering, so the steppy waveform is evident. The harmonic/noise content may make the system more sensitive to device characteristics, but harmonics can be filtered out so long as the device is linear = obeys superposition = response at the fundamental is independent of harmonics.Several of these can be combined at once, for example to measure the I + jQ voltage sent to the transformer to calibrate the oscillator,andmeasure the same of the transformer's outputs. Divide (complex number arithmetic) and you have the total effect of the transformer at the fundamental frequency, isolated from any other effects (again, given assumptions which can be checked). What to do with the complex value? Eeh--unno. Discard it? Take magnitude? Use it as a system health indicator / error assert?Can also correlate to harmonics, or do the whole ass FFT for that matter, if you have the power to spare of course, though there's probably not much use to that here. Low even and odd harmonics (mainly say 2nd, 3rd), due to core saturation, or like contact rectification or something, might possibly give subtle indications of position, or health. (On a separate note -- not relevant to LVDTs, but there are current sensors which operate by asymmetrical core saturation and can therefore sense all the way down to DC. A harmonic balance method returns current measured. See also: MPPT.)Correlation and sampling may sound like a lot of work, but it's really not very complicated these days. Even simple MCUs offer reasonable DSP performance; an 8-bit AVR can do sampling up to ~100kS/s and all of the DSP arithmetic can be accomplished in a few dozen lines of C.By measuring and calibrating the AC transfer function, including of phase, we can also account for output and input filters (to further improve immunity to both nonlinearity of the transformer and to external noise), without being very concerned about how precise they are.Assuming a high sample rate and low operating frequency, bit depth also isn't a big deal. For example, 256 samples per cycle at 12 bits (DAC and ADC) grants an extra 4 bits or so, or 16-bit equivalent. I did a low frequency impedance analyzer of this sort, some time ago, and got, well, basically as much resolution in the measurement as ENOB in the acquisition (and internal arithmetic) -- which was exactly this example, 12-bit converters and 16-bit arithmetic. Which was quite nice to see, being able to resolve minute capacitances or resistances of real components.Sample rate vs fundamental isn't even the end of it, it's just the easiest example to see; as long as the bandwidth (time resolution of the transfer function being measured) is low, acquisitions (complex values) can be averaged down, with exactly the same reduction in noise (including quantization noise). This is because quantization noise is additive and the whole signal chain is linear.Things get more difficult if the frequency is high; you can't low- or under-sample without incurring aliasing. (Aliasing can work for you, like when equivalent-time sampling; but the samples contain everything, uncorrelated noiseandthe repetitive waveform of interest. Which really shouldn't be a surprise, as the sample rate in general is low, so the SNR isn't improved by digital filtering.) And, obviously you can't get synchronous IQ demodulation with only two samples per cycle (well, unless you rotate the phase and average between cycles, but that's doing EQ, isn't it?). This may push the digital solution into higher power MCUs, or an FPGA, or an analog solution plus switches (analog switches or relays) to measure and calibrate the system from time to time (to make up for the analog system being crudely made, i.e. somewhat less than a Hewlett-Packard network analyzer).I'm guessing, doing it at up to say 10s of kHz, will be easy enough for low power MCUs to handle. Which given my rudimentary familiarity with LVDTs, probably covers a lot of situations.Tim