You are likely to need a more complex integrator than one using a single opamp to achieve adequate linearity. The 3458A used 3 opamps, the 34401a used 2 opamps in their integrators. To minimise the effect of dielectric absorption an integrator control loop that maintains low average, ideally zero, (not just within low upper and lower bounds) output voltage is likely necessary. Also the the Ron of the switches can be compensated by ensuring that the output Resistance of the signal "current source" matches that of the ramp current sources. Simplest solution is to set the output resistance of all to 20k.
Bruce > On 08 July 2018 at 08:35 Attila Kinali <[email protected]> wrote: > > > Hoi Poul-Henning! > > On Sat, 07 Jul 2018 19:51:36 +0000 > "Poul-Henning Kamp" <[email protected]> wrote: > > > I am not sure why you call your circuit a "multi-slope ADC", I can > > only see one pair of current sources ? > > Ah..sorry.. because of the multi-slope run-up. In the setup I have > there is no need for the run-down due to the high resolution ADC > following the integrator. Sorry, I should have been more clear > about this. > > > The trick to multislope is that it can cut down the convergence > > time, and that in turn reduces the effect of all the drift/leakage > > error sources proportionally. > > The run-down is cut away completely. The LTC2380-24 takes about 1024 > samples to get down to <2LSB rms error, which takes approximately 0.5ms. > I do not think that any multi-slope run-down would get to this precision > this quickly. > > > I suspect you also significantly underestimate the "non-ideal > > component" source of errors. > > Very likely. Though the main one that I could not properly quantify > yet are the charge injections from the switches, because.... > > > The trick in Fig 7/p.12 is much more important than most people > > realize and HP's implementation is significantly more interesting > > than Fig 7 lets on, because it involves 8 switches instead of two. > > ... I don't know how well the keep-the-number-of-switch-operations-constant > trick cancels things out. For the moment I assume that they do perfectly > cancel out to the point that the system is noise limited. The behaviour > of the switches is also the reason why I use current sources instead of > resistors and reference voltages. This way, the current sources eliminate > the error due to change of resistance through the switches. Unfortunately, > they add another error due to the jump of the current during the switch > operation. I have not yet spend enough time to quantify this error to > say anything about its magnitude. > > Another source of non-ideality that I was unable to quantify so far > is the dielectric absorption of the integration capacitor. I hope that > the idea of keeping its voltage below 1V should help keeping this problem > at bay. Especially considering that the HP3458 gets away with a simple > MLCC ceramic capacitor (SA10 series 330pF from AVX) and a small compensation > circuit with a tau of 24µs (100pF+243R). > > > If you want to have any dream of getting performance like HP3458, > > you will have to write a lot of code to do the same "auto"-calibrations > > as the HP3458, and you will have to run it on a regular basis to > > cope with component drift. > > Yes. The idea is that the ADC runs a cycle offset+gain calibration, > 7 measurements, calibration, 7 measurements,... etc pp which should > result in a cycle time of slighly less than 2s. My assumption is that > the drift of components is low enough that they change insignificantly > within those 2s. To keep the calibration measurments short is the reason > why ther are only a single positive and negative current source, and not > multiple as the HP3458 does. > > > There is no free lunches after bit 20... > > Oh yes... That design is the results of many weeks of reading up on > how ADCs work and how to get them accurate and precise. > > > Attila Kinali > -- > <JaberWorky> The bad part of Zurich is where the degenerates > throw DARK chocolate at you. > > _______________________________________________ > volt-nuts mailing list -- [email protected] > To unsubscribe, go to > http://lists.febo.com/mailman/listinfo/volt-nuts_lists.febo.com > and follow the instructions there. _______________________________________________ volt-nuts mailing list -- [email protected] To unsubscribe, go to http://lists.febo.com/mailman/listinfo/volt-nuts_lists.febo.com and follow the instructions there.
