[volt-nuts] Matched resistors

[Bob Smither]

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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