-------- In message <[email protected]>, Attila Kinali writes:
I just want to say up front that all the stuff I'm dragging up here is in no way meant to discourage you from trying, but rather to give you ideas what to look out for. For reasons of computer-archeology I spent a lot of time with the HP3458A firmware and anybody who thinks the hardware is 50% of the magic in the HP3458A is badly underestimating the software. >> 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'm not entirely convinced the LTC2380 will either, unless your input signal has just the right amount of thermal noise. >> 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. After calibration, they cancel out as perfect as you can calibrate them. This is one of the reasons why HP "reserve" some bits of their theoretical resolution: They expend them in autocal. >The behaviour >of the switches is also the reason why I use current sources instead of >resistors and reference voltages. Current sources contain feedback loops and therefore react (nonlinearly?) to the switch being operated. I don't think calibrating that out is feasible (enough.) You will have to autocal the full "ADSR" (see: "Moog") of the the switches no matter what you do, so trying to use temporally nonlinear current sources to compensate for the 'S' part seems pointless. Resistors and reference voltages are much simpler to model and calibrate. >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). They also calibrate that in software, and I seem to recall that they have some screwy math involving temperature in that code. You will need to do something like that too, but you can probably approximate a polynomia over time, given enough data. >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. That's certainly feasible, but far from enough. You will need to periodically do most of the steps in the 10V section of the HP3458A's "ACAL DCV" which takes 16 seconds for good reasons. How often you will need to do that "big autocal" is probably largely a matter of components and pratical construction. It may be well worth your time to pick through 1000 random integration capacitors to find the best one, but don't bother buying a batch of 1000 sequentially produced capacitors, modern quality control is far too good for that to work. And you probably have to mount all of it dead-bug style to keep the parasitics down and carefully balance the number of thermocouples. Poul-Henning -- Poul-Henning Kamp | UNIX since Zilog Zeus 3.20 [email protected] | TCP/IP since RFC 956 FreeBSD committer | BSD since 4.3-tahoe Never attribute to malice what can adequately be explained by incompetence. _______________________________________________ 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.
