Understand capacitor soakage to optimize analog systems

Fig 2  To model the soakage characteristic of a 1?F Mylar capacitor, consider a circuit that incorporates a 0.006?F capacitor to represent the dielectric's chargestorage characteristics. 

A capacitor exhibiting dielectric absorption acts as if during its long precharge time the dielectric material has soaked up some charge that remains in the dielectric during the brief discharge period. This charge then bleeds back out of the dielectric during the relaxation period and causes a voltage to appear at the capacitor terminals. Fig 2 depicts a simple model of this capacitor: When 10V is applied for 1 min, the 0.006?F capacitor gets almost completely charged, but during a 6sec discharge period it only partially discharges. Then, over the next minute, the charge flows back out of the 0.006?F and charges the 1?F capacitor to a couple of dozen millivolts. This example indicates that a longer discharging time reduces soakage error but that discharging for only a small fraction of that time results in a larger error. Illustrating this point, Fig 3 shows the results of conducting Fig 1's basic test sequence for 1, 6 and 12sec discharge times. Note that the capacitor tries to remember its old voltage, but the longer you hold it at its new voltage, the better it forgets  in the Fig 3 case, soakage errors equal 31 mV at t_{DISCHARGE}=l sec, 20 mV at t_{DISCHARGE}=6 see and 14 mV at t_{DISCHARGE}=12 sec.



Fig 5  Capable of automatically sequencing the dielectricabsorption tests, a circuit employing timers, a sample/hold and limiting stages allows you to make measurements for a wide range of T_{CHARGE}, T_{HOLD}, and t_{DISCHARGE} values. Fig 7 shows the results obtained using the circuit shown here. (View a larger version of the image.) 
Notes:

Such experiments illustrate that if you put a certain amount of charge into a lessthanideal capacitor, you will get out a different amount of charge, depending on how long you wait. Thus, using lowsoakage capacitors proves important in applications such as those involving highresolution dualslope integrating ADCs. And sure enough, many topoftheline digital voltmeters do use polypropylene (a lowsoakage dielectric) devices for their main integrating capacitors.

1 mSEC = T_{HOLD} 0.5 mSEC/cm 10 mSEC = T_{HOLD} 5 mSEC/cm 
WITH FIG 9 CIRCUIT 1 mSEC = T_{HOLD} 0.5 mSEC/cm 10 mSEC = T_{HOLD} 5 mSEC/cm 
Notes:
Originally published in EDN, RAP Update: FIFTEEN years ago, back in 1982, when I wrote this, I had never seen any study of capacitors and their "soakage"  nor of the kind of circuits you could use to shrug off the effects of soakage. Nobody ever talked about this, at high speed. To this day, I have not seen any other articles that covered either subject. So this is still about the prime source of info on how to evaluate capacitors for soakage, AND how to build good SampleandHold circuits, so as to NOT get hurt by that soakage. 