yes, charge injection is an issue with all these switches and these also vary, in other words are somewhat unpredictable. now some of this may be compensated by a bigger c, but there are natural limits too. so for a production unit to sell, this would probably be a killer, but also for home-use, how do you predict the behavior over time?
another option to use if you want to stay away from resistors is the pwm-solution as implemented by datron in e.g. their 4910. the pwm signal can today of course be done by uCs. > Gesendet: Montag, 21. Juli 2014 um 22:28 Uhr > Von: "Bob Smither" <smit...@c-c-i.com> > An: "Discussion of precise voltage measurement" <volt-nuts@febo.com> > Betreff: Re: [volt-nuts] Matched resistors > > On 07/17/2014 10:26 AM, Randy Evans wrote: > > Frank, > > > > The high cost is my concern, although high performance demands high price > > typically. I am trying to double the voltage reference from either an > > LM399 or LTZ1000, hence the need for precision matched resistors for a x2 > > non-inverting amplifier (using a LT1151 precision op amp). An alternative > > I am investigating is using the LTC1043 in a voltage doubling circuit as > > shown in Linear Technology app note AN 42, page 6, Figure 16. It states > > that Vout = 2xVin +/- 5 ppm. I am less concerned about the absolute > > accuracy than I am about the long term stability. I assume that a high > > quality capacitor is required (low leakage, low ESR, low dielectric > > absorbtion, etc.) but the circuit does not appear to be dependent on the > > absolute value of the capacitors. I'm not sure if the two 1uF caps need > > to be matched. If they do then that would be a show stopper. > > > > Does anyone have any experience using the LTC1043 in such a circuit? > > Hi Randy, > > There are some other error sources that might need to be considered when using > the LTC1043. > > I have not used the LTC1043, but note that on the data sheet there is a small > charge injection at each of the switch pins. In the multiply by 2 circuit > shown > on the data sheet they are using 1 ufd caps. Typical charge injection > (depends > on voltage level) is 8 pC. With the 1 ufd caps this is 8 uV. I assume there > is > some offsetting effect - but this might be a significant contributor to the 5 > ppm error that is mentioned. > > There is also a 6 nA (typical) leakage mentioned. During the "hold" time ( > about 1 msec) of the output 1 ufd cap this comes to 6 uV. > > Regards, > Bob Smither > > _______________________________________________ > 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.