[volt-nuts] Matched resistors

acbern at gmx.de acbern at gmx.de
Tue Jul 22 07:34:47 EDT 2014

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" <smither at c-c-i.com>
> An: "Discussion of precise voltage measurement" <volt-nuts at 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
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