US3493742A

US3493742A - Apparatus for extremely rapid determination of ionization and appearance potentials in a mass spectrometer

Info

Publication number: US3493742A
Application number: US487937A
Authority: US
Inventors: Pasquale Martignoni; Robert L Morgan; Lee F Mcclune; Henry A Nappier; Charles M Cason
Original assignee: US Department of Army
Current assignee: US Department of Army
Priority date: 1965-09-16
Filing date: 1965-09-16
Publication date: 1970-02-03
Anticipated expiration: 1987-02-03

Description

P. MARTIGNom ETAL 3,493,742 FOR EXTR Feb. 3, 1970 APPARATUSy EMELY RAPID DETERMINATION 0F IONIZATION AND APPEARANCE POTENTIALS IN A MASS SPECTROMETER 4 Sheets-Sheet l Fi-led Sept. 16, 1965 Feb- 3, 1970 P. MARTIGNONI ETAL 3,493,742

APPARATUS FOR EXTREMELY RAPID DETERMINATION I OF IONIZATION AND APPEARANCE POTENTIALS IN A MASS SPECTROMETER Filed Sept. 16. 196i 4 Sheets-Sheet 2 TRAP l SH EL I 29 I l.I: I-I |I5,=L]/ 3 l0N 25 ANoDE I u I7 2| 24 CATHODS mum I BACKING II V`:\7 I \3| PLATE #nl I5 I9 27 5/ =I|I auNcI-II-:o IoNs `23 'Re v :F' WI 4 PUMP CONTROL i 9 30/ UNIT `\vACUuIIII l: FIG' 4 SIDE 5 CYCLES /sEc. I 7 I I souARING AMPLIFIER IoKcl FLIP FLOPS I 4 4 I L J 5 CYELE`S/gEC -1 42 I l.:

I I 4I I I T 2 5 e Ie 3 2 s4 I 45 I |"\37 l EMITTER FoLLowERs^ v V Y l TO ANALOG POTENTIOMETERS I L I Pasquale MartIgnonI Robert L. Morgan Lee F. McClung FIG. 2 Henryi A. NappIer Char es M. Cason III INVENTOII 4 Sheets-Sheet 5 Pasquale Martignon Robert L. Morgan Lee F. McClun'e Henrr'ANa pler Char es M. CpasonJlI 1NVENTORS. MAN;- 777.

)m @ML P. MART-IGNONI ETAL APPARATUS FOR EXTREMELY RAPID DETERMINATION 0F IONIZATION AND APPEARANCE POTENTIALS IN A MASS SPECTROMETER Feb. 3.1970

Filed sept. le, 1955 ION CURRENT INPUT FIG. v3

Feb- 3, 1970 P. MARTIGNONI ETAI.

'APPARATUS FOR EXTREMELY RAPID DETERMINATION OF IONIZATION AND APPEARANCE POTENTILS `IN A MASS SPECTROMETER Filed Sept. 16, 196,5

NUMBER OF ELECTRONS 4 Sheets-Sheet 4 FIG. 5A

FIG. 5B

I AE

\\ Aie ENERGY I/gcglofcgI/IPONENT ALONG DIRECTION oF P (1s-quale Mangnoni oberr L. Morgan FIG. 5C I ,ee F. McCIun e Henr' A. NcIppIer Chor es M. Cason .III

INVENTO t. )wwf m 1W BY Lfm J. W )W BML WCM United Smtes Patent Office 3,493,742 Patented Feb. 3, 1970 3,493,742 APPARATUS FOR EXTREMELY RAPID DETERMI- NATION OF IONIZATION AND APPEARANCE POTENTIALS IN A MASS SPECTROMETER Pasquale Martignoni, Huntsville, Ala., Robert L. Morgan, Fayetteville, Tenn., Lee F. McClune, What Cheer, Iowa, and Henry A. Nappier, Lacey Springs and Charles M. Cason III, Huntsville, Ala., assignors to the United States of America as represented by the Secretary of the Army Filed Sept. 16, 1965, Ser. No. 487,937

Int. Cl. H01j 39/34 U.S. Cl. Z50-41.9 11 Claims ABSTRACT OF THE DISCLOSURE A time of flight mass spectrometer is automatically controlled by voltage drivers which are in turn controlled by a staircase voltage from a frequency divider in accordance with desired conditions.

The invention described herein may be manufactured and used by or for the Government for governmental purposes without the payment of any royalty thereon.

The invention relates generally to the automation of a technique for determining the ionization or appearance potentials of a compound by using a mass spectrometer. Specifically the present invention relates to the replacement of all manual controls of Foxs Retarding Potential Difference technique by automatic electronic controls. Also this invention provides for the plotting directly of the ionization etliciency curve.

Foxs Retarding Potential Difference (R.P.D.) technique is generally regarded as one of the most accurate and sensitive techniques used to obtain ionization (IP) and appearance potentials (AP). It is, however, also one of the most frustrating, time consuming and laborious techniques. Many variables have to be controlled and set up by hand to carry out the R.P.D. technique. The trap current must be maintained constant while varying the electron energy control grid and repeller grid voltages in a linear pre-selected manner. The plots must be compared and their difference plotted to get the desired (IF curves). Doing all this manually requires a considerable amount of time. Further, the element of human error is ever present-especially after many hours of doing the same task.

It is therefore an object of this invention to provide a system for automatically determining the ionization and appearance potentials of a compound.

A further object of the present invention is to replace the manual controls of Foxs R.P.D. technique with automatic electronic controls.

A still further object of the invention is to provide for the plotting directly of a final ionization efiiciency curve in which AI is plotted versus E.

The present invention relates to a modification of Foxs R.P.D. technique, in which all the variables are electronically controlled. The data is automatically analyzed and the results are plotted on an x-y recorder. The total time for a complete determination is 3.8 sec. The curves are accurate, reproducible, and show fine details. The sensitivity of this system is such that IPs may be obtained from water and oxygen background peaks when the total pressure in the mass spectrometer is between 6x10*8 and 1x10-9 torr. The electronic apparatus is divided into two major divisions: the control loop and the processing loop. The control loop functions to maintain the trap current constant while varying the electron energy control grid and repeller grid voltages in a linear pre-selected manner. The processing loop takes a voltage proportional to an ion current and compares this voltage with the one obtained from the previous increment of electron energy and repeller grid voltage. This ion current difference signal is then plotted as a function of the electron energy; giving one an ionization efficiency curve.

The invention further resides in and is characterized by various novel features of construction, combinations, and arrangements of parts which are pointed out with particularity in the claims annexed to and forming a part of this specification. Complete understanding of the invention and an introduction to other objects and features not specifically mentioned will be apparent to those skilled in the art to which it pertains when reference is made to the following detailed description of a specific embodiment thereof and read in conjunction with the appended drawing. The drawing, which forms a part of the specification, presents the same reference characters to represent corresponding and like parts throughout the drawing, and wherein:

FIGURE l shows a functional block diagram of the invention;

FIGURE 2 is a schematic diagram illustrating the binary electronics of the present invention;

FIGURE 3 shows a schematic illustration of the spectrometer control circuit and control data processing circuit of this invention;

FIGURE 4 is a schematic showing of a time of flight mass spectrometer and its connections to the system; and

FIGURES 5A, 5B and 5C are waveforms illustrating the electron energy present in the ionization chamber of the spectrometer.

The study of the ionization (IP) and appearance (AP) potentials resulting from electron impact on various gaseous molecules has been of considerable scientific interest. The IP is an important property of molecules and has been used in the interpretation of electronic and molecular structures. The AP has, also, been extremely useful in the determination of bond energies and electronegativities and as a measure of reactivity in organic reactions. The AP can be treated as an ordinary chemical reaction so that for the reaction,

where Hf is the thermodynamic heat of formation and E is excess energy formed in the process. An equivalent statement is where D is the bond strength of A-B.

A considerable number of techniques are currently in use to determine the IP and AP by electron impact with a mass spectrometer. The best one, and the one used in our system, is the Retarding Potential Difference method (R.P.D.). The R.P.D. technique consists of introducing an extra grid (the repeller grid 1 of FIGURE 4) between the filament 3 and the ionization chamber 5. The potential of repeller grid 1 is made slightly negative with respect to filament 3 to provide a sharp low energy cut-off for the electron beam 7. This can be better understood by referring to FIGURES 5A, 5B and 5C. FIGURE 5A shows the energy distribution of electrons emitted from the filament. It may be noted that the energy distribuution is exponential on the repeller grid. A sharp energy cut-off in the low range is achieved as shown in FIGURE 5B. When the retarding potential of the repeller grid is changed by a small amount a slice of electrons having only a small change in electron energy AE is repelled. See FIGURE 5C. If the change in the voltage on the repeller grid is made very small, then the energy band AE of the electrons can be considered to be monoenergetic beam of electrons (within the limits of the given apparatus). The difference in the ion current caused by the two conditions of the repeller grid is measured. It can readily be seen from FIGURES 5B and 5C that any change in the ion current can only be caused by the beam of electrons having the energy band AE. Whenthe repeller grid is changed to give one, FIGURE 5C, there are two possibilities for the behavior of the ion current: First, no change; second, a small change. If the potential between the control grid 9 and the ionization region 5 (FIGURE 4) is less than that necessary for ionization of the molecules under investigation, no change in the intensity of the ion current occurs. If the potential difference is greater than the ionization potential, a change in ion current equal to the ions formed by the slice (AE) of electrons is noted. By increasing the potential between the ionization region and grid 9 by small increments (also the initial potential of the repeller grid 1 is increased by the same amount) and noting the difference in the ion current caused by changing grid 1 by the small amount, a measure of the ion current as a function of energy AE is obtained.

In order to better understand the operation of the system set forth in FIGURE 1, a description of its specific components and their functions is first presented.

In the mass spectrometer 10` (shown schematically in FIGURE 4) a sample of the gas from the compound to be analyzed is placed in the ionization chamber and kept at a particular pressure by apparatus not shown. The ions are produced by bombardment of the sample with pulsed electron beam 7. The beam is pulsed at the rate of 10,000 times per second. Electrons are made available by continuous emission from a hot wire filament 3. Control grid 9 is biased negatively with respect to the lament, thus preventing electrons from passing continuously into the ionizing region. When control grid 9 is pulsed positively, a burst of electrons leaves the filament area and passes through the narrow slits in the grids and forms a beam in the chamber 5 which is directed towards the electron trap. Grids 4 and 6 are focusing grids. Molecules of the sample are now bombarded by the electrons and break down into positive and negative ions and neutral radicals. These ions will collect in the ionizing region 5.

Immediately after the electron beam is shut off, by making control grid 9 negative again, the ion focus grid 15 is pulsed negatively to withdraw positive ions. The ions are attracted toward this grid and into the accelerating region 17. Ions entering this region are very strongly attracted to the highly negative ion energy grid 19 giving them an impulse of kinetic energy such that they are di rected down a field-free drift tube 21 towards a collector 23. Since all ions receive an equal energy impulse, their respective velocities vary according to their mass-tocharge ratio. Since all ions leave the accelerating region practically simultaneously and are allowed to drift some distance prior to striking the collector, those of equal mass will tend to bunch and collectively separate from non-similar mass bunches. The lighter masses 22 have higher velocities than the heavier masses 24. As each bunch strikes the ion cathode 23, electrons are knocked from its surface and attracted to the slot between the field strip 25 and the dynode strip 27 of multiplier 29. Proper voltage levels are provided by control unit 30. Electrons cycloid down the multiplier and are collected on the anode 31 producing a negative voltage pulse which is the output of the mass spectrometer and represents the ion current flow.

FREQUENCY DIVIDER Frequency divider 35 is shown in detail in FIGURE 2. The frequency divider continuously divides the operational frequency of the spectrometer (10 kc.) by 2X103. This action generates an output of 5 cycles/second. The frequency divider consists of an eleven binary counter which adds Set inputs to the 8 and 16 counters when the 512 counter is in the true state. This addition of 24 counts to the chain effectively subtracts 24 counts during the second cycle of the counter thereby causing it to emit an output pulse for every thousand counts instead of every 1024 counts. These counters need not be reset since they continually give division by 2X103. The division by 2X1()3 determines the switching time of the repeller grid 1. This time is automatically divided by 2 by the 1 flip-flop of the digital to analog converter 37; therefore the energy step time increment 11-0 is a division of the 10l kc. by 4X103. With this arrangement, each electron energy step will be long enough for four thousand ion trains to drift down the flight tube and be analyzed. Two thousand ion trains are produced with the repeller grid at some voltage, approximately 0.5 v. with respect to the electron energy. The next two thousand ion trains will be produced with the repeller grid switched to 0.6 v. The difference between the number of ions in these bunches is equivalent to the number of ions which would be produced by all the electrons of AE from 0.5 to 0.6 in electron energy distribution function. The present system has one hundred steps of increasing electron energy. The voltage on the repeller grid is repeatedly switched from negative to more negative as described with each of these steps.

DIGITAL TO ANALOG CONVERTER The digital to analog converter 37 is shown in detail in FIGURES 2 and 3. It contains AND gate 39, flip-flop 41, and switch 42 to connect and disconnect counter 44. The converters seven stage binary counter 41, whose input is taken from the output of the AND gate 39 automatically further divides the frequency by 2. The outputs of each binary stage are fed by an amplifier to its corresponding potentiometer shown in FIGURE 3. The potentiometers are set in the same ratio of impedance as the value of the stage with which it is associated.

The output from the frequency divider is applied to one input of an AND gate 39, while its other input is supplied by the output of the single stage bistable multivibrator or ip-op 41. To start, single stage flip-op 41 is set true by momentarily closing the switch 42 between it and the squaring amplifier 4.3 contained in the frequency divider. When tiip-op 41 is set, its output signal to the input of the AND gate is true and five cps. output from the frequency divider is counted on the seven stage register. The 1, 4, 32, and 64 flip-flop have their not true outputs connected to

0R gates

45 and 47 while the other flip-flops of the counter 41 have their true outputs connected to OR gate 45. With these connections, there will be no change in the output from OR gate 47 until only flip-

fiops

1, 4, 32, and 64 are set and flip-flops 28" and 16 are reset. Therefore, OR gate 47 only resets ip-fiop 41 after a count of 100. When flip-flop 41 is reset by OR gate 47, AND gate 39 loses one of its inputs and removes the frequency divider signal from the seven stage register 44.

ELECTRON ENERGY DRIVER The electron energy driver 51 is shown in FIGURE 3 as having an operational amplifier 53 which accepts three inputs: the voltage staircase 113 generated by the digital to analog converter 37; the initial electron energy setting circuit 55; and the output of the trap current regulator 57.

CONTROL GRID DRIVER The control grid driver 59 (FIGURE 3) contains amplifier 61 which accepts two inputs: a fixed input, grid bias 63, which is used to adjust the initial bias; and the output from the digital to analog converter. The control grid and electron energy drivers work together in keeping a constant bias voltage on the control and electron energy electrodes. The voltage staircase generated by the digital to analog converter raises the energy of the ionizing elec- 5 trons by discrete steps which have been determined by the settings of the analog potentiometers.

REPELLER GRID DRIVER In FIGURE 3 the repeller grid driver 65 is shown as having an operational amplifier 67 which accepts three inputs: one input from potentiometer 69 is used to clamp the signal 110 from the frequency divider; a second input is the 5 cps. output of the frequency divider; and the third input is taken from the digital to analog converter and is used to step the repeller grid voltage simultaneously with the electron energy and grid bias voltages. The potentiometer 69 determining the second input is adjusted so the frequency divider signal is to -0.1 volt. The fixed voltage potentiometer 69 is adjusted to give +0.05 volt. This combination generates a i005 volt output from the amplifier 67 which drives the repeller grid.

TRAP CURRENT REGULATOR The trap current regulator 57 takes its voltage signal directly from a cathode follower meter `driver 73. This voltage is divided by two at amplifier 75 (shown in FIG- URE 3). The phase of one of the inputs is reversed to eliminate a 150 v. potential from the mass spectrometer. The output of amplifier 77 represents the cathode difference voltage of the meter driver. The output of amplifier 77 is applied to the input of amplifier 53 of electron energy driver 51 and thus regulates the electron energy to a preset value as the trap current regulator automatically forces the difference output of the meter driver to zero.

DATA PROCESSOR In FIGURE 3 the data processor 79 is shown in detail. The 5 cycle output from frequency divider 35 is fed to amplifier 80. Amplifier 80 and amplifier 82, the inverter, drive diode switches 84 and 85 of the

switch amplifiers

86 and 88 as well as the sample and hold

amplifiers

91 and 93. The -phase of the 5 cycle signal is so adjusted that the switch amplifier 86 and the sample and hold amplifier 91 are on when switch amplifier 88 and the sample and hold amplifier 93 are off. This switch voltage must be larger than the largest signal fed to all these amplifiers; otherwise, it will not determine the switching of these amplifiers. It will be noted that the switches are driven in synchronism with the repeller grid and also that the switches will occupy both of their two possible states during each step of the electron energy.

The operation of the data processor begins at the onset of the ionizing voltage at the initial ionization potential, the repeller grid voltage is high by the AE where the AE is determined by the potentiometer setting. The ion current signal output 95 of spectrometer 10 (see FIGURE 1) at this electron energy is applied to amplifier 97 and switch amplifier 86 simultaneously. The output of amplifier 97 tries to drive switch amplifier 88, but the diode switch 84 is reset by the 5 cps. source. Switch amplifier 86 being Set has an inverter output equal to the magnitude of the input voltage. The output of amplifier 86 drives amplifier 100 which serves as an inverter and as a zero adjustment for the output of amplifier 86. The output of amplifier 100 drives the sample and hol amplifier 91 which is in the sample state and acts as a gain of 1 inverter. The output of amplifier 91 drives amplifier 97 which also has the same ion current signal thereon. The output voltage from amplifier 91 is inverted and equal in magnitude to the ion current input signal. The output of amplifier 97 is zero when the experiment first begins.

The repeller grid next decreases in potential by AE and all switches take the opposite state. The 5 cycle reference switch causes amplifier 91 to store the ion current input signal it sampled before the switching action took place. The stored ion current signal is subtracted from the new ion current signal by amplifier 97. This difference, AI, is

applied to switch amplifier 88, which is now Set, and its output is fed to the sample and hold amplifier 93. The output of amplifier 93 is applied to the y axis of an x-y recorder. Each new difference is then immediately plotted in succession and held until the next cycle occurs. This process continues until the scan of all 100 steps of the electron energy is completed. The x axis of the plotter obtains its signal from digital to analog converter 37. The resulting graph is AI vs. electron voltage.

OPERATION The overall operation of the invention may be best understood with reference to FIGURE 1. The mass spectrometer is turned on by switch means not shown. With the spectrometer on, frequency divider 35 is fed the operational frequency (10 kc.) and divides it into 5 cps. at its output 110. However, the system does not start until the digital to analog converter 37 is switched on (by switch 42, FIGURE 2). When converter 37 is switched on, a staircase output is produced at its output 113. This output 113 is fed to electron energy driver 51, control grid driver 59, and repeller grid driver 65; therefore keeping the relative voltage difference between the cathode and the grids constant, while raising the voltage level of the cathode. This means that, due to the converter, the electron energy in the ionization chamber will increase in proportion to the staircase wave from the output of converter 37. However, at the same time the repeller grid driver 65 has a constant bias voltage switched in and out of its circuit at twice the frequency of the staircase output. This causes the relative voltage between the cathode and the repeller grid to have a first and a second value during each staircase level. This means that the electron energy will also have two values during each staircase level (E and E-AE; see FIGURE 5C); where AE is a constant value.

Grid bias unit 63 is pulsed at 10 kc. to cause control grid driver 59 to alternate between driving the grid to a positive value set by the staircase output and a negative value. When the control grid is negative, no current fiows;

therefore, the electron energy supply to the ionization chamber is a pulsed supply at 10 kc. The trap current regulator 57 senses the trap current output of the spectrometer and provides a control output signal 115 in order to regulate the trap current to a constant value.

The output of spectrometer 10 is a current output which represents the ion current produced in the spectrometer. The spectrometer will have four thousand pulse trains at its output at each staircase level. Two thousand pulse trains will be due to the ion current caused by electron energy E and the other two thousand will be `due to electron energy E-AE, as explained above. The data processor 79 receives the two sets of pulses, sums each of them and subtracts one sum from the other. The output of the data processor, therefore, represents the ion current difference, AI, at each staircase voltage level. This is fed into the y axis of x-y plotter 117. The x axis of the x-y plotter is fed the staircase voltage out of converter 37; therefore giving a AI vs. electron energy plot. Other read-out devices could be used in place of the x-y plotter. For example, an analog to digital converter could be used so that the information would be in a form directly readable by a computer.

The output of the spectrometer is also fed to an analog output unit 121. Unit 121 provides measurements of mass peaks representing either more or less than one ion per cycle. Its output is fed to an oscilloscope 122,. By the control of the units gating circuit, not shown, any of the mass peaks may be monitored by the oscilloscope. A detailed description of analog output unit 121 is not given as its details form no part of the invention. The unit is the Analog Output System for the Bendix Mass Spectrometer, Model 14 Series. Also, the total mass spectrum may be viewed on oscilloscope 124 which is connected directly to the output of the spectrometer.

A preferred embodiment of the invention has been chosen for purposes of illustration and description. The preferred embodiment illustrated is not intended to be exhaustive nor to limit the invention to the precise form disclosed. It is chosen and described in order to best explain the principles of the invention and their application in practical use to thereby enable others skilled in the art to best utilize the invention in various embodiments and modifications as are best adapted to the particular use contemplated. It will be apparent to those skilled in the art that changes may be made in the form of the apparatus disclosed without departing from the spirit of the invention as set forth in the disclosure, and that in some cases certain features of the invention may sometimes be used to advantage without a corresponding use of other features. It is, therefore, to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described. Accordingly, it is desired that the scope of the invention be limited only by the appended claims.

We claim:

1. A control system comprising first, second, and third voltage driving means each having an output and at least one control input; each of said driving means having a bias voltage input; voltage stepping means having an output connected to said control inputs so that each of said driving means will have an output equal to the Sum of the voltage output of the voltage stepping means and its bias voltage; an electron beam producing means having a cathode, first grid, second grid, and an anode; said first driving means being connected to said cathode so as to supply it; said second and third driving means being connected to said first and second grids respectively so as to control the operation of the electron beam producing means; the bias voltage of said second driving means is pulsed at a selected first frequency so as to cause a beam produced by said electron beam producing means to be pulsed at the same frequency; a frequency divider means connected to sense said first frequency and to produce an output which is voltage having a Second frequency which is a selected division of said first frequency; and means connecting the output of said frequency divider means to a control input of said voltage stepping means whereby said stepping means has a staircase voltage output which is stepped at a frequency proportional to said second frequency.

2. A control system as set forth in claim 1, wherein said first grid is a control grid; and wherein said second grid is a repeller grid.

3. A control system as set forth in claim 2, wherein the output of said frequency divider means is connected to said third voltage driving means so as to cause its bias voltage to be switched in and out of its circuit as said second frequency.

`4. A control system as set forth in claim 3, further comprising a regulator connected to the beam producing means so as to sense the current output of the beam; said regulator having an output connected to said first voltage driving means so as to keep the beam current constant.

5. A control system as set forth in claim 4, wherein said beam producing means is set in an ionization chamber of a mass spectrometer; and said beam causes the ionization of a gas which is to be analyzed.

6. A control system as set forth in claim 5, wherein said mass spectrometer has means therein whereby an output signal is produced which is representative of the number of ions produced by each pulse of said beam and is at the same frequency as the pulsed beam; a data processor; and said signal being applied to an input terminal of said data processor.

7. A control system as set forth in claim 6, wherein said data processor comprises a first sample and hol circuit which has an input connected to an output of a first switching means by way of an inverter means; said switching means being switched at the frequency of said frequency divider means by connections thereto; said switching means having an input connected to said input terminal of the data processor; an amplifier having an input connected to the output of said sample and hold circuit, and an output connected to an output terminal of the data processor; and connecting means connecting the input terminal of the data processor to the input of said amplifier whereby its total input will be zero when the switching means is on and will be the difference between the signals present value and its last Value when the switching means is off.

8. A control system as set forth in claim 7, wherein said data processor further comprises a second switching means and a second sample and hold circuit; said second switching means being connected between the output of said amplifier and an input of said second sample and hold circuit; said second switching means being switched at the frequency of the frequency divider means and is phased with respect to the first switching means such that it is in an on condition when the first switching means is in an off condition; and an output of said second sample and hold circuit being connected to said output terminal of the data processor.

9. A control system as set forth in claim 8, further comprising an x-y plotter wherein its x input is connected to the output of said voltage stepping means, and its y input is connected to the output terminal of said data processor.

10. In a data processor having an input terminal which receives a signal having a frequency which is greater than the frequency of its change in information; the improvement comprising an inverter means; a switching means; a first sample and hold circuit which has an input connected to an output of said switching means by way of sald inverter means; said switching means being switched at the frequency of the change of information and having an input connected to said input terminal; an amplifier havlng inputs connected to said sample and hold circuit, and an output connected to an output terminal of the data processor; and connecting means connecting the input terminal of the data processor to the inputs of said amplifier whereby its inputs will be equal and opposite when the switching means is on and whereby the amphfiers inputs will be the difference between the signals present information value and its last information value when the switching means is off.

11. data processor as set forth in claim 10, further comprising a second switching means and a second sample and hold circuit; said second switching means being connected between the output of said amplifier and an input of said second sample and hold circuit; said second switching means being switched at the frequency of the change of information and being phased with respect to the first mentioned switching means such that it is in an on condition when the first mentioned switching means 1s in an off condition; and an output of said second sample and hold circuit being connected to sald output terminal of said data processor.

References Cited UNITED STATES PATENTS 2,810,075 10/1957 Hall et al Z50-41.9 3,012,139 12/1961 Hanson et al. Z50-41.9 3,154,747 10/l964 Kendall Z50- 41.9 X 3,307,033 2./1967 Vestal 250-41.9

WILLIAM F. LINDQUIST, Primary Examiner U.S. Cl. X.R.