in fig1 a circuit embodiment of the present invention shown generally at 200 preferably is used in conjunction with an ac reactance type differential pressure sensor cell , and preferably has a pair of capacitors c 1 and c 2 at least one of which is variable , responsive to a parameter , preferably pressure . fig1 basically is the circuit of fig3 of u . s . pat . no . 4 , 381 , 677 to ruesch , frick and grindheim which is herein incorporated by reference and assigned to the same assignee as the present application , except for minor differences as noted herein . operation of circuit 200 is fully described in u . s . pat . no . 4 , 381 , 677 except as further explained herein . a sum point means 210 also known as current summing node 210 is fed a first discharging signal or , first current i 1 responsive of the capacitance of capacitor c 1 and a second charging signal or , second current i 2 responsive of the capacitance of capacitor c 2 from an amplifier means 212 also known as inverting amplifier 212 for selectively amplifying and inverting second current i 2 . summing node 210 sums the first and second currents as by arithmetically combining the currents . the furst and second currents can be charging or discharging signals in other preferred embodiments . a control means 216 , also known as control amplifier 216 has its non - inverting input ( as indicated by the plus (+) sign ) coupled to circuit common and its inverting input ( as indicated by the minus (-) sign ) coupled to summing node 210 and produces an output signal to control the frequency of an oscillator means 218 , also known as voltage controlled oscillator 218 . oscillator 218 repetitively charges capacitors c 1 and c 2 through a rectification means shown generally at 220 with substantially constant amplitude , variable frequency oscillating voltage signals ( v pp peak to peak amplitude ), the frequency of which is controlled responsive to the changes of the capacitance of c 1 and c 2 . capacitors c . sub . 1 and c 2 repetitively discharge through rectification means 220 responsive to their repetitive charging . thus capacitor c 1 has first signals for charging and discharging and capacitor c 2 has second signals for charging and discharging . rectification means 220 rectifies the currents i 1 and i 2 , representative of c 1 and c 2 which result from the repetitive charging and discharging of c 1 and c 2 . it is noted that the circuit 200 can also operate with a constant frequency varying voltage provided by oscillator 218 , with the voltage controlled by control amplifier 216 . an adjustable capacitor c 3 is coupled to summing node 210 through diodes 242 and 244 to provide a current i cs to summing node 210 through lines 243 and 245 respectively . diodes 242 and 244 may be included in rectification means 220a for increased thermal stability . it is noted that c 3 is coupled to summing node 210 through two separate paths , diodes 242 and 244 and lines 243 and 245 respectively , such that the capacitance of c 3 need not be doubled as stated in u . s . pat . no . 4 , 381 , 677 where capacitor c 3 is coupled to summing node 210 only through diode 244 . coupling capacitor c 3 through both diodes 244 and 242 provides approximately twice the current i cs therefore eliminating the need to double the capacitance . amplifier 212 has an output and an inverting input ( as indicated by the minus (-) sign ), the output being coupled to the inverting input through a first resistor 250 and , the output being coupled to summing node 210 through second resistor 252 , neither of said resistors having been indicated by number in fig3 of u . s . pat . no . 4 , 381 , 677 . an output amplifier 230 having an output and an inverting input ( as indicated by the minus (-) sign ) is coupled at its inverting input to rectification means 220 and provides an output signal v 0 to a suitable implementation means 232 , such as a two wire current control , readout device or the like which draws an output current signal i 0 . amplifier 212 selectively amplifies second current i 2 and capacitor c 3 current i cs . currents i 2 and i cs are preferably amplified by a factor k 1 which is a ratio of an impedance means comprising the resistance of resistor 250 divided by the resistance of resistor 252 . second current i 2 and current i cs are selectively amplified without substantially adversely affecting the biasing or response of control amplifier 216 or other associated circuitry . currents i 1 , k 1 i 2 , and the currents from capacitor c 3 , ( 1 + k 1 ) i cs are fed into summing node 210 and flow through a resistor 214 to a reference , - v ref , producing a voltage at summing node 210 responsive to the first and second signals . the currents , k 1 i cs and k 1 i 2 are different from that shown in fig3 of u . s . pat . no . 4 , 381 , 677 . the capacitance of capacitors c 1 and c 2 change in response to a difference in pressure . the output signal i 0 is of the form : the output signal i 0 is essentially a first , second and third order function of differential pressure which can be approximated in terms of capacitance as : ## equ1 ## where : i 0 = output signal of amplifier 230 the capacitances of c 1 and c 2 can be expressed as : ## equ2 ## where : a = active rest capacitance of c 1 equation 5 can be simplified using letters g , h , i , j , k and l as coefficients corresponding to the actual coefficients of pressure p in equation 5 : ## equ4 ## where : g = a - d + c - f for equation 6 to be linear with respect to pressure p , the following two equations should be satisfied : ## equ5 ## equation 8 is derived by setting l = 0 in equation 6 and dividing the numerator in equation 6 by the denominator in equation 6 until a first order pressure term appears leaving a remainder which is set equal to zero . varying k 1 and varying the capacitance of capacitor c 3 , provides sufficient flexibility to substantially satisfy equations 7 and 8 . adjustment of the capacitance of c 3 provides a predominantly third order adjustment which in the past has been used to adjust the linearity of output signal i 0 over selected ranges of pressure . in fig2 a graph of typical data , where the output signal i 0 is a 4 to 20 milliampere signal ( as for example in fig3 of u . s . pat . no . 4 , 381 , 677 ), expressed as terminal based linearity of output signal i 0 versus pressure is shown with capacitor c 3 adjusted to minimize third order dependence . a second order differential pressure dependency remains having a maximum deviation from linear of approximately - 0 . 29 percent . adjustment of k 1 by selectively trimming resistors 250 and 252 results in a predominantly second order dependency control . in one embodiment resistors 250 and 252 preferably each comprise at least two resistors coupled in parallel , at least one of which is trimmable as by a laser to vary the parallel resistance . k 1 is then the ratio of the equivalent resistance of the parallel resistors of resistor 250 divided by the equivalent resistance of the parallel resistors of resistor 252 . when capacitor c 3 , resistor 250 and resistor 252 are correctly selected , a substantially linearized function results and equations 7 and 8 are substantially satisfied . resistors 250 and 252 can also be variable as a function of pressure or other parameter affecting second order differential pressure dependency . the present invention provides enhanced linearity from lower to upper pressure range limits allowing adjustment of span , zero and range ( standard process measurement terminology ) of the differential pressure transmitter without readjustment of linearity . linearity is also enhanced when compared to linearizing by only utilizing third order adjustments . to partially compensate for second order linearity by adjusting capacitor c 3 , the third order dependence is not minimized , but optimized to account for both second and third order dependence , leaving both a third order dependence and a second order dependence . by selectively choosing resistors 250 and 252 and capacitor c 3 , both second and third order dependencies are substantially independently minimized resulting in a substantially linear relationship between the output signal i 0 and pressure from lower to upper pressure range limits , enabling span and zero changes to be made over the entire range of pressure from the lower range limit to the upper range limit without need for readjustment of linearity . in fig3 a graph of terminal based linearity of output signal i 0 ( a 4 to 20 milliampere signal ) from a circuit constructed according to fig1 shows enhanced terminal based linearity of output signal i 0 with capacitor c 3 and resistors 250 and 252 selected to improve linearity . the maximum deviation from linear is less than 0 . 04 percent . capacitor c 3 is selected to minimize third order dependence and resistors 250 and 252 are selected to minimize second order dependence . comparing fig2 and 3 by subtracting their respective maximum deviations from linear (- 0 . 29 -(- 0 . 04 )=- 0 . 25 percent ) shows that unadjusted second order linearity is the largest uncorrected dependence after third order dependence is compensated . it has been found that a k 1 of between 0 . 900 and 1 . 100 , is sufficient to compensate for second order nonlinearity without adversely affecting associated circuitry . preferably , k 1 , varies between 0 . 975 and 1 . 025 . with k 1 individually selected between 0 . 975 and 1 . 025 second order dependence is minimized in virtually every case . the above constructed circuit comprised resistor 250 having an equivalent resistance of 27 , 807 ohms and resistor 252 having an equivalent resistance of 28 , 088 ohms , resulting in a k 1 of 0 . 990 . in a further preferred embodiment of the present invention as seen in fig1 of u . s . pat . no . 4 , 381 , 677 , resistors 54 and 56 are varied to provide second order linearity adjustment and enhancement of the output signal . resistors 64 and 66 also can similarly be varied . in fig2 of u . s . pat . no . 4 , 381 , 677 , it is within the scope of the present invention to adjust c 3 and amplify i 1 , i 2 or both in a manner described above to enhance the output signal .