[volt-nuts] Homebrew DVM

[Bruce Griffiths]

Andreas Jahn wrote:
> Hello Fred,
>
>> Got it stable up to 1 mV in a 1 V range.
>
> Did you check only noise and temperature or also linearity?
>
> My experience is that with a simple dual slope integrator the 
> capacitor soaking creates big linearity errors.
> I built my first dual slope with a 2,49V LM336 reference a 4051 and a 
> TLC271 and a simple 1uF mylar capacitor.
> Due to single supply I used the 2,5V of the reference as vitual 
> ground. So the input range of 0..5V was
> divided into two ranges from 0..2.5 (negative with respect to virtual 
> ground) and from 2.5 to 5V. (positive).
> With a simple mylar capacitor the non-linear gap between negative and 
> positive range was around 1 mV.
>
> So linearity is only around 12 bits with this design. To reach this 
> linearity I had to do some software
> trick to deal with the soakage. After each switching of the input 
> multiplexer to another channel I did
> some waste A/D conversions with the new channel to let the capacitor 
> stabilize.
>
> Years later I read the article of Bob Pease about capacitor soaking 
> and exchanged the mylar against
> polypropylene. The non-linear gap is now around 100uV wich is around 
> 15 bits linearity.
The HP3458A and the HP34401A ensure that the integrator output voltage 
only represents a small fraction of the integral of the signal input.
This significantly reduces the effect of dielectric absorption in the 
integrator feedback capacitor.
The integrator and control logic act somewhat like a sigma-delta 
integrator during the runup phase.
The effect of dielectric absorption can be further reduced by suitable 
choice of runup algorithm.
The on resistance modulation of analog switches in series with the 
resistor connected between the signal and the integrator summing 
junction can be a significant source of nonlinearity, however it is, in 
principle, possible to correct for this.

Bruce