[volt-nuts] Matched resistors

[Randy Evans]

I agree that there are potentially some serious unknown issues with drift
due to time and temperature due to changes in leakage current, charge
injection, etc.  I would think some serious characterization would be
needed before this approach could be used. One approach for the charge
injection is to try and have the input voltage near Vdd/2 so the charge
injection effects are nulled out.  I'm trying to figure out how to do that
for a X2 circuit.  Any ideas?

 I talked to Vishay and they, of course, could do the resistors and I am
awaiting a quote.  They stated they would have to "tune" the vhd200
resistor pair to have a very low (0.2ppm/C) ratio stability .  Doable but
potentially very expensive.  Unfortunately, the maximum value of the
resistors is 20Kohms which would greatly increase the power dissipation of
the resistors, not good for long term stability.  Still working the issue.

Randy


On Tue, Jul 22, 2014 at 8:12 AM, <acbern at gmx.de> wrote:

> and yes, I forgot: only down-dividing of course, so to reach 10V, two
> LTZ1000 would be needed in series. advantage is that noise statistically is
> reduced by factor of about 1.4. formally also applies to drift.
>
>
> > Gesendet: Montag, 21. Juli 2014 um 20:28 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
> >
> > _______________________________________________
> > volt-nuts mailing list -- volt-nuts at 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 at febo.com
> To unsubscribe, go to
> https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts
> and follow the instructions there.
>