The Measurement of DC Voltage Signal Using the UTI

Transcription

1 he Meaurement of DC Voltage Signal Uing the. INRODUCION can er an interface for many paive ening element, uch a, capacitor, reitor, reitive bridge and reitive potentiometer. By uing ome eternal component, can er an interface for the meaurement of DC voltage ignal alo. hi i very ueful for the application of the thermocouple ignal meaurement. In thi ection, we preent a meaurement concept for the DC voltage ignal uing the.. MESUREMEN CONCEP he capacitance meaurement function of the can be employed to meaure voltage ignal. an eample, Figure (a) how a chematic diagram circuit for the meaurement of voltage ignal uing the mode C3 of the. In thi circuit, denote an unknown voltage (meaurand), V i a known erence voltage. he capacitor and witche,,, ample the voltage ignal and V alternatively and convert the voltage ignal into charge alternatively. he will convert the charge into the period of the output of the. he three-ignal technique [3] i applied to enure the accuracy and reliability of the circuit by eliminating the gain and et parameter of the interface ytem. o perform thi threeignal technique, a erence ignal (V ) and a contant part (including et voltage) are meaured in eactly the ame way a the enor ignal ( ) during two additional phae. Figure (b) how one complete cycle, coniting of three phae, and. V SEL SEL SEL3 SEL4 PD ES SF µc (a)

2 V out GND cycle B C D Figure (a) he chematic diagram of the meaurement ytem. (b) Output ignal V out of the. (b) In the following, we eplain the meaurement principle: When work at mode C3, pin i the input of the charge amplifier in, pin E and F are grounded internally, and pin B, C and D of the output quare-wave ignal (ee figure (b)). he period of thee quare-wave ignal are modulated by the charge in the capacitor. hee quare-wave ignal alo control the tate of the ampling witche. During pin B output the quare-wave ignal the initial et of the i meaured. During pin C output the quare-wave ignal the erence voltage i meaured. nd during pin D output the quare-wave ignal the meaurand (a voltage ignal) i meaured. he duration of each phae i proportional to the ignal which i meaured during that phae. hey are given by: = KC, () K = V C +. () CC = K V V C +. (3) where i the power upply voltage. K i the linear tranfer coefficient of the. C i a initial contant part (including et capacitance). With equation (), () and (3), the meaurand i found from V = V CC. (4)

3 Formula (4) how that the value and inaccuracie of the capacitance and voltage power upply and the error of the linear tranfer coefficient K do not affect the meaured reult becaue of the applied three-ignal technique. he meaurement principle decribed above i alo uitable for the mode of C5, C, CMUX and C NONIDELIIES Many nonidealitie are eliminated by uing the three-ignal technique. However, ome effect cannot eliminated by thi technique, a dicued below.. he Effect of the Paraitic Capacitor Figure how ome critical paraitic capacitance C p, C p, C p and C pin in the voltage meaurement ytem. C p C p C D Cp C pin V Figure he paraitic capacitance in the voltage meaurement ytem. ) he paraitic capacitance C pin decribed in. [], the paraitic capacitance C pin will caue a nonlinearity of the converion of the input ignal to the output period. For a nonlinearity of 300 ppm, the paraitic capacitance C pin of up to 50 pf i allowed. ) he paraitic capacitance C p and C p he influence of the paraitic capacitance C p and C p on the reult can be epreed by the following equation: V V C V C cc p cc p V V C + V C. (5) Equation (5) how that even after applying the three-ignal technique the paraitic capacitor C p and C p caue a gain error and an et on the reult. hee ytematic error can be eliminated after calibration while uing a compenation algorithm in the microcontroller. For uch a compenation, it i neceary that the paraitic do not change after calibration. B. Output impedance R o, R o of erence voltage ource and he output impedance R o or R o of the erence ource or meaurand and capacitance aociated with reitor R o and R o form an RC circuit with a time contant of τ i ( R ON + Roi)( + Cp ), (6) 3

4 where R oi repreent the output impedance R o or R o. Further, R ON i the witch-on reitance of witche S and S. hi effect reult in a relative error for the reult, p ε NL ep, (7) t Eample: for a relative error of, for intance maller than 0-5, with R ON = 00 Ω, p = 0 µ and +C p = 0 pf, R oi mut be maller than 43 kω. C. Switch clock feedthrough It i common practice to reduce the effect of witch clock feedthrough by compenation. Becaue witche and, and are controlled by oppoing clock ignal, the effect of the clock feedthrough i canceled partly. Moreover, it i poible to obtain a further reduction of the clock feedthrough by uing CMOS witche. D. Noie In the voltage meaurement, by uing the ytem hown in Figure (a), the noie originate mainly from two part: the ocillator noie and the quatization noie caued by ampling in the microcontroller. he tandard deviation of the relative error caued by thee two noie ource can be epreed by, repectively: i ε no C pin = C intυeq VccN f C, (8) ε nqt = 3 Np fc, (9) where f i the bandwidth of the O, υ eq i the RMS value of the equivalent input noie (V/ Hz) of the O, f c i the ampling frequency of the microcontroller and N i the meaured period number. 4. OPIMIZION OF HE MESUREMEN RNGE In order to guarantee that the work in it optimum linear meaurement range, the meaurement range for the voltage ignal,range and the value of the ampling capacitor are determined by the following relationhip: V C V C, (0), range cc, range where C,range i the capacitance meaurement range of the. For intance, for the mode C3 of, C,range = pf. When the ampling witche are upplied by a power upply V DD which i larger than the power upply of the, the meaurement ytem decribed by Figure (a) can meaure a voltage ignal larger than. 5. HE MESUREMEN OF VOLGE SIGNL USING MODES C ND C5 he voltage ignal can alo be meaured by uing mode C and C5 of the. Figure 3 how the chematic diagram circuit for the meaurement of voltage ignal uing mode C and C5 of the. 4

5 V Figure 3 chematic diagram circuit for the meaurement of voltage ignal uing mode C and C5. Figure 4 how the output ignal of the at mode C and C5. V out GND cycle Figure 4 Output ignal of the. 6. HE MESUREMEN OF VOLGE SIGNL USING MODE CMUX Figure 5 how the chematic diagram circuit for the meaurement of voltage ignal uing mode CMUX of the. Control ignal from µc V Figure 5 chematic diagram circuit for the meaurement of voltage ignal uing mode CMUX. 5