[volt-nuts] Switches in integrating ADC

Tony Holt vnuts at toneh.demon.co.uk
Mon Apr 14 11:53:23 EDT 2014

On 14/04/2014 10:03, John Devereux wrote:
> Jan Fredriksson <jan at 41hz.com> writes:
>> What kind of switches are used in integrating ADC, ie to switch
>> between voltage sources (ref and external) and to switch in multisloping
>> resistors? FETs?
> Yes, but I believe they are integrated ones usually. Either ye olde 4066
> style or custom integrated circuits in the case of the HP 3458A.
As Jan says, the 3458A switches around the ADC are integrated according 
to the April 1989 HP journal which describes the 3458A design:

    *"Because the switches are in series with the resistors, they can
    add to the temperature coefficient of the ADC. A custom chip design
    was chosen so that each switch could be scaled to the size of the
    resistor to which it is connected. This allows the ADC to be
    sensitive to the ratio-tracking temperature coefficient of the
    switches and not to the absolute temperature coefficient.**"*

I expect that optimising and balancing charge injection would have been 
an important design objective too. It would be interesting to know how 
modern off-the-shelf analogue switches compare - ie. with low enough on 
resistance so that absolute temp coefficient doesn't matter, without 
introducing excessive charge injection. I expect that's a bit of a tall 

The 8 digit Solartron 7081 uses discrete Fets, but it uses a voltage to 
time converter for its ADC. The HP 6 digit 34401A uses a 74HC4053D 2:1 
Mux to switch the ADC integrator.

For interest, the signal switching in the input path of the 3458A, for 
selecting high voltage divider / low voltage input, current sources and 
DC amplifier gains etc. all use Siliconix J2472 J-FETs (N channel 
depletion mode). I guess there were no packaged switches up to the job 
at the time.

Vishay bought Siliconix since and shut down production some while ago so 
good luck finding any parts or even a datasheet. I expect they are very 
low leakage types; no doubt there are suitable alternatives available - 
perhaps ones recommended for electrometer applications?

Tony H

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