Re: [volt-nuts] Switches in integrating ADC
On 14/04/2014 10:03, John Devereux wrote: Jan Fredriksson writes: What kind of switches are used in integrating ADC, ie to switch between voltage sources (ref and external) and to switch in multisloping resistors? FETs? Yes, but I believe they are integrated ones usually. Either ye olde 4066 style or custom integrated circuits in the case of the HP 3458A. As Jan says, the 3458A switches around the ADC are integrated according to the April 1989 HP journal which describes the 3458A design: *"Because the switches are in series with the resistors, they can add to the temperature coefficient of the ADC. A custom chip design was chosen so that each switch could be scaled to the size of the resistor to which it is connected. This allows the ADC to be sensitive to the ratio-tracking temperature coefficient of the switches and not to the absolute temperature coefficient.**"* I expect that optimising and balancing charge injection would have been an important design objective too. It would be interesting to know how modern off-the-shelf analogue switches compare - ie. with low enough on resistance so that absolute temp coefficient doesn't matter, without introducing excessive charge injection. I expect that's a bit of a tall order. The 8 digit Solartron 7081 uses discrete Fets, but it uses a voltage to time converter for its ADC. The HP 6 digit 34401A uses a 74HC4053D 2:1 Mux to switch the ADC integrator. For interest, the signal switching in the input path of the 3458A, for selecting high voltage divider / low voltage input, current sources and DC amplifier gains etc. all use Siliconix J2472 J-FETs (N channel depletion mode). I guess there were no packaged switches up to the job at the time. Vishay bought Siliconix since and shut down production some while ago so good luck finding any parts or even a datasheet. I expect they are very low leakage types; no doubt there are suitable alternatives available - perhaps ones recommended for electrometer applications? Tony H ___ volt-nuts mailing list -- volt-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the instructions there.
Re: [volt-nuts] Switches in integrating ADC
Many DVMs from the '70s or '80s (34740A/34750A, or the 3465x series) used discrete JFETS, for example from the 2N4392 or 2N4117 series. The 4066 and equivalent CMOS switches were far too leaky for this use. Many of these JFETS were selected for low leakage currents, such as the 1855-0412, selected from the 2N439x series, or the 2N4117A which is an off-the-shelf part. As usual, HP's manuals are your perfect source of information! Joel Setton - Mail original - De: "John Devereux" À: volt-nuts@febo.com Envoyé: Lundi 14 Avril 2014 11:03:56 Objet: Re: [volt-nuts] Switches in integrating ADC Jan Fredriksson writes: > What kind of switches are used in integrating ADC, ie to switch > between voltage sources (ref and external) and to switch in multisloping > resistors? FETs? Yes, but I believe they are integrated ones usually. Either ye olde 4066 style or custom integrated circuits in the case of the HP 3458A. -- John Devereux ___ volt-nuts mailing list -- volt-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the instructions there. ___ volt-nuts mailing list -- volt-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the instructions there.
Re: [volt-nuts] Switches in integrating ADC
Jan Fredriksson writes: > What kind of switches are used in integrating ADC, ie to switch > between voltage sources (ref and external) and to switch in multisloping > resistors? FETs? Yes, but I believe they are integrated ones usually. Either ye olde 4066 style or custom integrated circuits in the case of the HP 3458A. -- John Devereux ___ volt-nuts mailing list -- volt-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the instructions there.
[volt-nuts] Switches in integrating ADC
What kind of switches are used in integrating ADC, ie to switch between voltage sources (ref and external) and to switch in multisloping resistors? FETs? ___ volt-nuts mailing list -- volt-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the instructions there.
