A U T O M A T I C Z E R O - D R I F T C O M P E N S A T I O N OF

W I T H V I B R A T I N G C A P A C I T O R S

A. V, P r o k u r o v a n d V. G. S h i r o k o p y t o v

E L E C T R O M E T E R S

UDC 621.317.723.088.6

In various fields of science and technology it is necessary to measure smal l direct currents (10"13-10-16 A) and

voltages (0.1-1.0 mV) supplied by transducers with a high internal resistance and to monitor continuously any var ia- tions in the measured current or voltage. Moreover, in many instances i t is desirable to provide prolonged continu- ous operat ion of e lectrometers without readjustment, to express measurement results in a d igi ta l form, and to record

them automat ica l ly .

One of the main factors which impede the production of au tomat ic meters for smal l currents and voltages con-

sists of the zero instabil i ty (zero drift) of dc amplifiers. The zero drift in amplif iers used for measuring very small direct currents is reduced mostly by converting the direct into an al ternating current by means of a vibrating capac i - tor. However, i t was found impossible by this means to reduce the slow drifting of zero below 100-200 #V per day for a constant ambient temperature. This drift is due mainly to changes in the contac t potent ia l differences between

the operating surfaces of the capaci tor plates.

Ambien t temperature changes may produce in vibrating capacitors with stainless s teel plates a zero drift rate

at taining 20-50 /2V/deg. However, taking into consideration that the rate of temperature variations inside a measur- ing head which carries the vibrating capaci tor is normally below 0.5-1.0 d e g / m i n , i t is possible to expect that the rate of the zero drift due to the potent ia l difference wil l not exceed 10-28 # V / m i n even for considerable variations

of the ambien t temperature.

Owing to the re la t ive ly slow varia t ion of the contact potent ia l i t becomes possible, without any apprec iable

effect on measurements, to make periodic automat ic corrections of the zero reading in amplif iers with vibrating c a - pacitors. The method of periodic corrections of ampl i f ie r zero readings was first suggested by D. G. Prinz [1] and

then developed for e lectrometers by D, E. P01onnikov [2J.

I t is suggested in [2] that zero readings can be corrected per iodica l ly by storing the contact potent ia l difference

in a capaci tor which is connected to the output of a dc ampl i f ie r with a 100% feedback (Fig. 1). When switches S 1 and S 2 are closed the vol tage across capaci tor Csr is equal to the contact potent ia l difference, since with a 100% feedback Uou t = - U i n . The sign of the vol tage across Csr is opposite to that of the contact potent ia l difference,and,

therefore, when switches S 1 and S 2 are disconnected the contact potent ia l difference becomes compensated.

However, i t is impossible to obtain comple te compensat ion owing to the stray signal generated during switch- ing by the contact relay and to the stray charge on the storage capaci tor which must also possess an except iona l ly large self-discharge t ime constant. Thus, if i t is assumed that the t ime intervals between the contact po ten t ia l - difference compensat ion instants amount to 1 rain, the maximum value of this difference is 10 mV and the permissi- ble self discharge is 20/~V, i t becomes necessary to have a storage capaci tor with a self-discharge t ime constant of at least 2 .6.104 sec, i .e . , a 0.1 gF capaci tor should have a leakage resistance of 2.6.1011 t?. Such a high leakage

resistance can be at tained only in capacitors with a teflon or polystyrene instal la t ion (types FT or FD). However, such capacitors have the tendency of accummula t ing a stray charge produced by stray insulator currents and other

factors, which lead to a charge across the capaci tor plates.

The physical processes which lead to the appearance of stray charges on the capaci tor plates in the absence of any obvious external effects have not as yet been studied in detai l . Experimental investigations have shown that, for instance, the rate at which the stray vol tage rises under normal conditions in 0.047-0.1 /2F capacitors type FT may at ta in 20 g V / m i n and i t increases considerably if the ambient temperature is changed.

Another disadvantage of the c i rcui t shown in Fig. 1 consists of the fact that prolonged transient processes are

inevi tab le when switch S 2 is opened or closed, since the t ime constant of the feedback c i rcui t is usually made large

in order to reduce the effect of thermal fluctuations on the leve l of the stored voltage.

Translated from Izmer i t e l ' naya Tekhnika, No. 2, pp. 41-43, February, 1967. Original ar t ic le submitted

April 23, 1966.

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In order to reduce the effect of a l l the a bove -men- t ioned det r imenta l factors on the leve l of the stored vol tage i t is desirable to amplify (amplif ier k 1 in Fig. 2) the signal together with the contact potent ia l difference up to a high enough level in comparison with which the spurious vol tage produced across the storage capaci tor by the above factors becomes insignificant.

A dc ampli f ier c i rcui t free from the above-ment ioned deficiences is shown in Fig. 2. In this c i rcui t the vol- tage corresponding the contact potent ia l difference across vibrating capaci tor Cvl is stored in capaci tor C z which is

connected to the output of the first amplif ier .

When switch S t is closed, the vol tage at the output of the first ampl i f ier is equal to

Uoutl ~-- kl Uc,

where k 1 is the gain of the first ampl i f ier with the feedback taken into consideration, Uc is the contact potent ia l dif-

ference across vibrating capaci tor Cvi.

When switch S z is closed (as wel l as switch Si) , capaci tor C2 is charged up to vol tage klU c. The measuring condit ion is prepared by a simultaneous opening of switches S 1 and $2. Moreover, i f C 2 >> Cvz, the vol tage at the in- put of the second ampl i f ier is equal m

Uin2 = kl (Uc + us)-- kl Uc = kl ~ ,

where U s is the vol tage of the useful signal at the input of the first amplif ier .

I t is obvious that this equali ty holds only in the case when gain k 1 does not vary between the compensation instants.

The u t i l iza t ion of a vibrating capaci tor at the input of the second dc ampli f ier serves to e l imina te comple te ly the zero drift which could arise if an ordinary dc ampli f ier were used and to work at re la t ively low vol tage levels of the first amplif ier .

The exislence of a second vibrating capaci tor compl ica tes somewhat the circuit , but simplifies considerably the design of the most important unit of an e lec t rometer , i .e . , of the input measuring head, since i t becomes unneces- sary to provide both plates of vibrating capaci tor Cvl with a high insulation and the special relay in the feedback c i r - cui t can be e l iminated.

A certain drawback of this method consists of the fact that i t is necessary to provide the first dc ampl i f ie r with a large enough dynamic range to ensure a l inear transmission of voltages ranging from the min imum useful signal to the sum of the maximum signal and the contact potent ia l difference of the vibrating capacitor . However, the in i t ia l value of the contact potent ia l difference can be easily compensated in adjusting the electrometer , and, therefore, the

ampl i f ie r must provide a l inear transmission of voltages extending from the minimum signal to a level of

Uin.max = Us.ma x + AUc,

where Uin.max is the maximum possible vol tage at the input of the first de amplif ier , Us.ma x is the maximum vol- tage of the useful signal, AUc is the maximum possible contact potent ia l difference variat ion with t ime and t em- perature,

/(!

, H2F

Fig, 2. SD) Synchronous detector.

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For instance, the total drift of the in i t i a l contac t potent ia l difference in a vibrat ing capac i tor DRK-3 with per- ma l loy N79M plates and a maximum contac t potent ia l difference rate of change of 0.5-0.7 mV per 10 ~ does not ex- ceed in one year the value of 0.5-1.0 inV.

Thus, for the above vibrating capac i tor the maximum possible change in the contact potent ia l difference amounts to 6-7 mV for temperature variations of - 4 0 ~ to +60~

A minimum useful signal input vol tage which corresponds to the fu l l - sca le deflect ion of the output measuring instrument is normal ly not less than 1 mV, and, therefore, the first dc ampl i f ie r ' s dynamic range must be 6-7 t imes greater than in the case when U c is compensated at the input. This l imits the m a x i m u m gain of the first amplif ier . However, by using a second ampl i f ie r with a vibrating capaci tor i t becomes possible to design the first ampl i f ie r with the objec t of raising the useful signal only up to a level which exceeds considerably the contact potent ia l difference

of the second vibrating capac i tor with its zero drift taken into consideration. Normally U c + Udr does not exceed 30 mV, and i t is sufficient to provide at the output of the first ampl i f ie r a min imum signal of 200-300 mV, which corresponds to the first reading point.

If the first reading point is assumed to equal 10% of the fu l l - sca le deflect ion, the max imum useful signal vol- tage at the input to the first ampl i f ie r wi l l amount to 2-3 V, and the total signal vol tage and the contact potent ia l difference to 8-10 V. An ampl i f ie r which has a l inear ampl i tude character is t ic up to 8-10 V and operates into a high-resis tance load can be easily designed to work with transistors.

An exper imenta l verif icat ion of the contact potent ia l difference compensat ion according to the c i rcui t in Fig. 2

has shown that the zero drift referred to the second ampl i f ie r input (see Fig. 2) does not exceed 10 mV in 30 rain for a stored vol tage of 10 V in a 5000 pF capaci tor type K72-1. Thus, the interval between consecut ive compensat ion

instants can be raised up to 30 min (instead of 1 rain used in the method of storing the vol tage at the input of the first ampl i f ier ) , and, therefore, any losses of useful information can be considerably reduced,

1.

2.

L I T E R A T U R E C I T E D

G, D. Prinz, D C amplif iers with automat ic zero adjustment and input current compensation, Journ. of Scient. Instr., 24, No. 12 (1947). D, E, Polonnikov, Electronic Amplif iers of Automat ic Compensators [in Russian], F izmatgiz (State Press for Physical and Mathemat ica l Literature), Moscow (1960).

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