| Electronics > Projects, Designs, and Technical Stuff |
| ultra low noise power supply design |
| (1/6) > >> |
| OM222O:
If you've read my previous posts, you'll know that I'm designing a miliohm meter. I am considering using 16 bit or 24 bit ADCs which require very accurate voltage references to operate accurately (resolution is there, but the numbers would jump all over the place if the reference voltage is not accurate). As far as I know, every ripple on the supply line carries through, so even when you have a linear regulator, it will give you the 5v that you need, but all the noise and ripple follow through. same would be true for the chosen voltage reference and so on. I saw dave's video about capacitance multipliers before the input of the LDO and thought it would be really nice if I use one, so I made this quick and dirty simulation: http://tinyurl.com/y9z58vyj The supply has 0.2V peak to peak 50Hz mains ripple and 0.2v peak to peak high frequency noise. I chose a really small shunt on the output to draw as much current as possible (worst case scenario for noise) as well as 1k resistors as to not choke the base current of the darlington pair. The simulation shows about 10uV peak to peak noise, but I'm not sure if this is a realistic value. I tried switching out the darlington pair for a N channel fet but the current draw drops to 1.1A and the fet dissipates about 13 watts :o Can someone please help me understand why this happens and how much dropout voltage there would be across a fet vs a darlington pair setup. I also don't know if this method is good enough as the MCU also shares the same 5V line and it can create switching noise. I'm using the 8MHz internal oscillator as opposed to an external 16MHz which should help a lot with the noise problem, but it would be nice of someone knows a way of eliminating that all together without using another LDO just for the MCU. |
| T3sl4co1l:
Why is supply noise a problem? What does a milliohmmeter measure? Resistance. What is resistance? Resistance is defined by the ratio of voltage across a component, to the current through it: V = I*R If we have noise in V, we have an exactly* proportionate noise in I. If we are measuring both, we have lost no signal (information about R) except when V and I go to zero**, which happens only rarely. *Minus Johnson noise, which adds a noise voltage in series with the resistor, corresponding to its resistance and temperature. This voltage is unknown, so introduces the same error in Vmeasured. Or the Norton equivalent, and the proportional error in Imeasured, same thing. **Close enough to zero for the purposes of the circuit, anyway. Example: the least LSBs of your ADC, around whatever the zero or reference value is. DC is itself a vulnerable method: there are many sources of small DC voltages, like thermocouples. These can be swamped to some extent by using a larger excitation signal, or they can be nulled by using AC and filtering DC out of the measured signal entirely. AC likewise has some drawbacks, because even at low frequencies, reactance is nonzero, and may be nonzero enough to matter. We could address this with a properly phased detector (so we only measure the in-phase component: the resistance), or say by measuring at very low frequencies to begin with, or a selection of frequencies so we can analyze if the response is stable or not. In a sense, noise is the perfect stimulus, because it has all frequencies at once, and by measuring the source and the response, we can solve for everything at once (I mean, given unlimited computing power and cleverness -- in this case, a Fourier transform would serve nicely). You probably wouldn't want to bother going to such length, as there are easier signals that will do very nearly just as well -- just that you could. The other thing: it would be nice to measure only voltage (or only current, but almost nothing measures current directly). Which means we need another resistor, to convert current back into voltage. Thus, we would have a resistance bridge, or at least a half bridge (a divider). Best part about using an ADC in this way: the VREF doesn't matter either, it can be relatively noisy and indeterminate. As long as it gives an acceptable result, you're fine. All you are doing, is comparing the ratio of resistors, to the ratio of ADC steps. The exact voltage is unimportant! Tim |
| ogden:
--- Quote from: T3sl4co1l on October 15, 2018, 12:08:57 pm ---The other thing: it would be nice to measure only voltage (or only current, but almost nothing measures current directly). Which means we need another resistor, to convert current back into voltage. Thus, we would have a resistance bridge, or at least a half bridge (a divider). Best part about using an ADC in this way: the VREF doesn't matter either, it can be relatively noisy and indeterminate. As long as it gives an acceptable result, you're fine. All you are doing, is comparing the ratio of resistors, to the ratio of ADC steps. The exact voltage is unimportant! --- End quote --- Right. Only precision component needed is low TC reference resistor. Further reading: https://www.maximintegrated.com/en/app-notes/index.mvp/id/1753 and here: https://e2e.ti.com/blogs_/archives/b/precisionhub/archive/2016/04/29/it-39-s-in-the-math-how-to-convert-an-adc-code-to-a-voltage-part-2 |
| OM222O:
the best ones I can find have a 50 PPM TC which is about 0.1% for a temperature delta of 20C. https://www.mouser.co.uk/ProductDetail/Vishay-Dale/WSC25151R000FEA?qs=sGAEpiMZZMtlubZbdhIBIM3Zc6FLjf4lFsfcXDgsnpk%3D so you think the noise is gonna be the same everywhere regardless, but that's a bad assumption to make in my opinion (the noise will go through all the circuit, but to what degree? decoupling caps and other factors can change the amount of noise for each device on the board individually). firstly it requires use of star ground rather than a plane which results in high impedance traces which on their own can cause more error. also here we are working in uV sort of range for the resolution of even a 16bit ADC, let alone a 24 bit one. I think using a battery instead of a wall adapter is a far better option here (a 3S lipo would do just fine) but then battery life becomes a huge issue ... I think I'll use 2 LDOs with a fet based capacitance multiplier for the analog section |
| RandallMcRee:
--- Quote from: OM222O on October 15, 2018, 08:45:00 pm ---the best ones I can find have a 50 PPM TC which is about 0.1% for a temperature delta of 20C. https://www.mouser.co.uk/ProductDetail/Vishay-Dale/WSC25151R000FEA?qs=sGAEpiMZZMtlubZbdhIBIM3Zc6FLjf4lFsfcXDgsnpk= ... --- End quote --- 50ppm? You can do so much better. PTF56 series has 5ppm. Susumu RG series is available in 2ppm. Edwin Pettis sells 3ppm wirewounds. Etc. If you actually care! |
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