Constant gain signal amplifier

~f~37~73 ~59-790 BACRGROUND OF THE IN_ENTION 1. Field oE the ~nvention The present invention relates to signal amplifiers.

More specifically, the present invention is directed to a signal ampliEier system utilizing a method and apparatus for compensating a variable gain amplifier to achieve a constant overall system signal gain.

SUMMARY OF THE INVENTION An object of the present invention is to provide an improved method and apparatus for compensating a signal amplifier system having a gain amplifier to achieve a constant overall system signal gain.

In accomplishing this and other objects, there has been provided, in accordance with the present invention a method for compensating an amplifier system comprising the steps of applying an input signal to a variable ga;`n analog amplifier means, converting an output of the amplifier means representative of an amplified version of the input signal to a digital representation thereof, supplying the digital representation to a digital computer to be used in computing a digital output from the digital computer as an output from the amplifier system in accordance with a predetermined overall system signal gain, developing a gain selection control signal by the digital computer for selecting a signal gain of the amplifier means to maintain the predetermined overall system signal gain and applying the gain selection control signal to the variable gain analog amplifier to select a suitable signal gain of the analog amplifier means.

~LZ37~73 ~159-790 In accordance with the present :invention, there is provided a signal amplifi.er system compris:ing analog-to-digital converte.r means Eor converting an analog input signal into a digital equivalent, said converter means having a plurality of selectable signal gains for correspondingly amplifying the analog input signal, a non-volatile memory for st,oring data on the available signal gains of said converter means and on a desired overall signal gain of the amplifier system, and digital computer means connected to said converter means and said memory for computing a digital output signal from said amplifier system representative of the analog input signal in accordance with the overall predetermined system signal gain and a control signal for selecting a signal gain of said converter means to maintain the overall predetermined system signal gain.

BRIEF DESCRIPTION OF THE DRAWING A better understanding of the present invention may be had when the following detailed description is read in connection with the accompanying drawlng, in which the single figure i~ a block diagram illustration of an example o~ an apparatus embodylng the method of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT Detailed Description Referring to the single figure drawing in more detail, there is shown an example of an apparatus for providing a compensation of a signal amplifier system having a variable gain amplifier 4 utilizing the method of the present invention to achieve a constant overall system signal gain. The variable gain amplifier 4 is supplied with an input from a multiplexer 6 which.is arranged to sequentially connect selected ones of a plurality of inputs, input 1, input 2,...input n to the amplifier 4. Additionally~ tle multiplexer 6 is also arranged - 3 ~;2;3l71'73 ~159~790 to periodically connect a zero input to the ampliEier ~ to provide a means for monitoring the trans:ient noise or oEfset of the amplifier system. The output of the variable gain amplifier 4 is applied to an analog-to-digital (A/D) converter 8 to be converted to a digital representation thereof. The output oE the A/D converter 8 is applied as an input to a digital computer, e.g., microprocessor 10.

- 3~ A non-volatlle memory, e.g., PROM l2, Is connected to the mlcroprocessor l0 to store the Instructlons used by the mlcroprocessor l0 In effectlng the method of the present Invention.

The mlcroprocessor l0 also controls the input selectlon of the multlplexer 6 via Input select llne l4 and the gain of the varlable galn amplIfter 4 vla galn select line l6.

An output of the mlcroprocessor l0 provldes a dlgltal representatlon of the selected Input of the multtplexer 6 after compensatlon by the ]- mlcroprocessor l0 to effect a constant galn of the signal amplifylng system.

Thls microprocessor output Is applled to a dlgltal-to-analog (D/A) converter l8 to be converted to a conventional 4-20mA output slgnal which can be used to produce a control functlon, as Is well-known In the art, for associated devlces (not shown).

M oJ2.E_Q.EQ.eE~ A Tl Q i~i The method utlllzed by the present Inventlon baslcally involves the operatlon of a two-siage ampllfier system whlch has a constant value of overall slgnal galn but where the dlstrTbutton or allocatlon of the slgnal galn Is dynamlcally and automatlcally shlfted between the fIrst and second stages as requlred because of the changlng amplItude of the Input stgnal.

Thus, this shlft of the slgnal galn produces the best overall performance of the system whlle malntainlng a constant slgnal galn.

In the illlstrated embodlment of the Inventlon, the flrst stage of the amplifler system Includes a vartable gain analog am~pl1fler ~ havlng a pluraltty of signal gain values whlch can be Indlvldually selected slnce the posslble range of the Input slgnal amplItude coupled wlth the resolutlon and slgnal-to-noise requlrements of the ~37:173 system are such that no slngle value of amplifler galn can be reallstlcally used.

For example, a hlgh slgnal galn Is deslrable for low amplitude Input slgnals whlle a low signal galn Is requlred for hlgh amplttude Input signals to prevent saturatlon of the amplIfler 4.

The second stage of the ampllfler system Is a dlgltal computer represented by the mlcroprocessor lO.

Dependlng on the value of the output of the A/D converter 8, the computer lO selects the appropriate galn value of the variable galn amplIfler 4.

Concurrently, using stored data In the PROM 12 whlch defInes the performance of the analog amplifier 4 at each of the selected galns~ an output dlgTtal value from the computer l0 is computed whlch is equivalent to a signal which would have been produced by the varlable galn amplIfier 4 operatlng at Its hlghest galn.

Thts digltal output slgnal ls then appl led to the D/A converter 18 to be converted and used as an Input for assoclated equlpment.

Thus, the analog ampllfier 4 always operates at tts optlmal galn value wlth respect to the amplitude of Its Input slgnal,~and any calculatlons by the dlgltal computer lO are slmplIfied slnce they do not need to be~dTf~ersnt for each of the analog galns whereby~ separate caltbration curves for each gain are elImlnated.

In general, any analog-to-~dlgltal converslon method can be deflned by a group of equatlons.

Using a speciflc type of A/D and amplifler combinatlon, the followlng set of equatlons can be written to equate digltal output of the A/D and amplifler palr to the analog Input signal thereto.

The equatlons used by the ~computer 10 are~ effectlva to provlde a gain correction for high, medium ~ ;

-5 7~'73 and low amplltud~ Input slgnals, e.9., an amplIfler 4 and A/D converter palr 8 havlng three selectable slgnal galns (G), I.e., hlgh medlum and low.

Thus, the equatlot)s are:

GC~I = .25 GH(Vln ~ VlnOff) ~ 5 ~ VOutOff GCM = .25 GM~VIn + VlnOff) ~ 5 + VOutOff GCL = .25 GL(Vln + VlnOff) + 5 + VOutOff whtch Vlnoff and Voutoff are the offset slgnal levels for the input and output slgnals from the amplIfler 4. 1rue zero Input slgnals (Vln = O) can be measured by selectlng the zero Input whlch would simpllfy the above equatlons to:

GCzH = .25 GH VlnOff + 5 + VOutOff GCzM = .25 GM VlnOff + 5 + VOutOff GCzL = .25 GL VlnOff + 5 ~ VOutOff Uslng these equatlons, the galn Is corrected as follows:

GCH-GCzH = .25 GHVln = GCHCorr. (1) GCM-GCZM = .25 GMVIn = GCMCorrO GCL-GCzL = .25 GMVIn = GCLCorr.

Solvlng for Vln In each of these three equatlons:

Vln = GCHcorr~/-25 GH Vln = GcMcorr~/~25 GM (2) Yln = GCLcorro/-25 GM (3) and flnally substltutlng equations (2) and (3) for Vln In equatlon (1 ):

GCHCorr. = ~- ~ GCMCorr.

~G MJ GCHcorr. = ~ H~GCLCorr.

~GLJ ~3~ 3 The ratlos GH and G~l are characterlzed GM GL data stored tn the PROM 12 durlng the manufacturtng of the system to be used by the digltal computer 10 during the system compensatlon operatlon, In a typlcal example, the amplIfler 4 would have three selectable galns, e,g., 20, 5 and 1.25.

Inltlally, when the system is turned on, the galn of the amplIfler 4 Is InltlalIzed to tts lowest value.

Subsequently, 1he fIrst input Is selected ~y the multlplexer 6, and the output of the A/D converter 8 Is read by the computer 10.

Based on the value of that output, the determination Is made as to whether or not, the inltlal amplIfier galn In correct.

The system can either compensate from the lowest galn level or select the next gain level to Improve resolui-ion of the Input slgnal and compensate from the htgher galn level.

The aforesald compensatlon Is preferably performed for each Input selected by multlplexer 6.

Further, the cornputer 10 can store each selected gain for each Input in another memory and, subsequenfly, selects the prlor stored galn when the correspondlng Input Is again selected~ Thus, the overall system galn Is matntatned whlle the amplltier galn Is varled for optlmum operatlon whlle the dlvlslon of~the system gain Is automatlca~lly allocated by the computer 10 between the analog and dlgltal stages of the system.

Accordlngly, It may be seen that there has been provlded, In accordance wlth the present Inventton,~an Improved method of compensatlng an amplIfler system havtng a varlable galn ampltfter to achieve a constant overall system slgnal gatn and an apparatus utlllztng the same.

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