Economic Measurement Techniques With the Comparator_A Module

Transcription

1 Application Report SLAA7 - October 999 Economic Measurement Techniques With the Comparator_A Module Lutz Bierl Mixed Signal Products ABSTRACT This report describes the methods for comparing input signals, and measuring voltage, current, capacitance, and resistance using the Comparator_A module of the MSP4 family. Two measurement principles (charge and discharge) are explained, and schematics and equations are given for different measurement principles. Contents The Comparator_A Comparator_A Attributes and Functions The Control Registers Applications Comparison of Two Voltages Fast Comparator Input Sampling Resistance Measurement Voltage Measurement Digital Motor Control (DMC) Connection of Sensor Bridges Measurement Without Amplification Measurement With Amplification Capacitance Measurement Other Applications Two Independent Measurement Circuits External Use of the Internal Reference Voltage References List of Figures Comparator_A Hardware Comparator_A Control Registers Comparison of Two Input Voltages Variation for the Comparison of Two External Voltages Circuit for Fast Comparator Input Sampling Measurement of Resistors Voltage at Cm During Resistance Measurement Voltage Measurement Voltage Measurement With the Discharge Method

2 SLAA7 Current and Temperature Measurement for a PWM Motor Control Current and Temperature Measurement (Charge Method) Connection of a Measurement Bridge to Comparator_A Voltage V cm During Bridge Measurement (V m and Temperature) Connection of a Bridge With Amplification Measurement of an Unknown Capacity Cx Independent Use of the Two Comparator_A Inputs Use of the Internal Reference Voltage in the External Circuitry The Comparator_A Comparator_A is a module contained in some members of the MSP4xxx family. It is designed for precise analog measurements. Figure shows the versatile hardware of the module.. Comparator_A Attributes and Functions Some of the most important attributes of the Comparator_A module are: Very low input current at the comparator inputs CA and CA It can be switched off to minimize current consumption (control bit CAON) It is controlled by three memory-mapped control bytes (see Section.) Stability with slow-changing input voltages (control bit CAF) can be provided by switching of an analog filter to the comparator output. Interrupt capability for the leading and trailing edges of the output signal CAOUT. Use without interrupt is also possible. Economic Measurement Techniques With the Comparator_A Module

3 SLAA7 PCA CAEX CAON CAF CA CA VCA VCA PCA V + _ Low Pass Filter τ, µs To Internal Modules CAOUT Set CAIFG Flag Internal References CAREF CARSEL VCAREF.5 x.5 x MSP4 V Figure. Comparator_A Hardware Bit CAOUT contains the result of the comparison. The following are the comparison combinations allowed by the hardware: Comparison of two external inputs Comparison of each external input with.5 V CC, or.5 V CC Comparison of each external input with an internal-reference voltage Additional functions performed by Comparator_A are: The internal reference voltages can be output to comparator input pins CA and CA for use by external hardware (see Section.8.). A register allows switching off the port input buffers, which are used for analog purposes. This removes current into the input buffers caused by input voltages that differ from V CC or V SS. Economic Measurement Techniques With the Comparator_A Module

4 SLAA7 The input change-over switch CAEX allows offset-free measurements. The same software can be used for both states of CAEX due to the inversion of the comparator output signal. The above attributes allow simple voltage, current, resistor, and capacity measurements. The main function of Comparator_A is to indicate which one of the two voltages, V CA or V CA, is higher. The output CAOUT is set accordingly: If V CA V CA then : CAOUT else : CAOUT The two voltages V CA and V CA can be external or internal reference voltages. Any combination is possible (see Figure ).. The Control Registers The two control registers, CACTL and CACTL, contain all the control bits necessary to use Comparator_A. See Figure for the function of the control bits. 7 CACTL 59h CAEX CA RSEL CA REF CA REF CAON CAIES CAIE CAIFG rw-() rw-() rw-() rw-() rw-() rw-() rw-() rw-() 7 CACTL 5Ah CACTL.7 CACTL.6 CACTL.5 CACTL.4 PCA PCA CAF CAOUT rw-() rw-() rw-() rw-() rw-() rw-() rw-() rw-() Figure. Comparator_A Control Registers The functions of the Comparator_A control bits, not shown in Figure, are: CAIFG Interrupt flag : No interrupt pending : Interrupt pending CAIE Interrupt enable flag : Interrupt disabled : Interrupt enabled CAIES Interrupt edge select bit : Leading edge of CAOUT sets CAIFG : Trailing edge of CAOUT sets CAIFG CACTL.x: Bits are implemented but do not control any hardware. They can be used for flags. Applications The following sections present some Comparator_A applications. In addition, the Comparator_A hardware allows all applications presented in the MSP4 Application Report Book to be used with the Universal Timer/Port. One of the 6-bit capture/compare registers of Timer_A is used in place of the two 8-bit counters of the Universal Timer/Port. NOTE: The hardware and Timer_A configuration of the MSP4F are used in the application examples. Other MSP4 family members may have slightly-different hardware (ports and Timer_A). 4 Economic Measurement Techniques With the Comparator_A Module

5 SLAA7. Comparison of Two Voltages The simplest Comparator_A application is the comparison of two external voltages. Neither internal reference voltage nor Timer_A are necessary. Figure shows the comparison of a divided input voltage V IN with the output voltage V IN of an op amp (amplifier, Schmitt trigger, comparator, and sensor bridge amplifier). It is not necessary to change the comparator setting during the measurement since internal reference voltages are not used. The value of the signal CAOUT at the comparator output is: If V IN R R R V IN then : CAOUT ; else CAOUT Because the output filter is switched on (CAF = ), any change in the comparator output is delayed by approximately. µs. VIN PCA CAEX CAON CAF + _ R VIN x k R VIN CA CA PCA + _ Internal References Low Pass Filter τ, µs To Internal Modules CAOUT Set CAIFG Flag CAREF CARSEL VCAREF.5 x.5 x MSP4 V Figure. Comparison of Two Input Voltages V Economic Measurement Techniques With the Comparator_A Module 5

6 SLAA7 The following software example shows initialization and a test of comparator input voltages to determine which is higher. Figure shows the hardware. ; Initialize Comparator_A for the input voltage test ; MOV.B #CAON,&CACTL ; Define Comp_A mode MOV.B #PCA+PCA+CAF,&CACTL ; Connect CA and CA... ; Proceed with initialization ; ; Compare the two input voltages VIN*k and VIN. k=r/(r+r) ; BIT.B #CAOUT,&CACTL ; VIN*k > VIN? JNZ VGTV ; Yes... ; No, VIN*k < VIN Figure 4 shows other possibilities for the two input voltages. Four input voltages V IN to V IN, selected by an analog multiplexer, are compared with an external programmable-reference voltage. The reference voltage can be switched off for lower current consumption using port P.. The output signal at CAOUT is: If V INx V TLV4 R R4 then : CAOUT ; else CAOUT R4 Where V INx is the particular input voltage selected from the range V IN to V IN. 6 Economic Measurement Techniques With the Comparator_A Module

7 SLAA7 CAF Low Pass Filter To Internal Modules CAOUT τ, µs Set CAIFG Flag Input Selection PCA CAEX CAON VIN VIN VIN VIN In In In In Ctrl Out MUX VIN VIN P... CA CA + _ PCA Internal References Rv P. R TLV4 CAREF R4 CARSEL.5 x VCAREF.5 x MSP4 V Figure 4. Variation for the Comparison of Two External Voltages The following software example includes initialization and a test to determine which of the voltages V IN or reference voltage V IN is higher. Figure 4 shows the hardware. Economic Measurement Techniques With the Comparator_A Module 7

8 SLAA7 ; Initialize Comparator_A for the input voltage test ; MOV.B #CAON,&CACTL ; Define Comp_A mode MOV.B #PCA+PCA,&CACTL ; Connect CA and CA BIS.B #7,&PDIR ; P...: outputs... ; Proceed with initialization ; ; Compare input voltage VIN with the ext. reference voltage ; BIC.B #7,&POUT ; Select VIN BIS.B #6,&POUT ; Switch on VIN... ; Wait for settling of VIN BIT.B #CAOUT,&CACTL ; VIN > VIN? JNZ VGTV ; Yes... ; No, VIN < VIN. Fast Comparator Input Sampling Very fast sampling of sequential input values is often necessary. The following measurement sequence is the fastest way to accomplish this using Comparator_A inputs. After n input checks, a majority test, or its equivalent, can be performed to reach a conclusion. Figure 5 shows the hardware used for this example. The software samples the voltage generated by the current I MEAS over resistor Rm a voltage drop higher than.5 V CC sets CAOUT, a lower voltage drop resets CAOUT. After n samples, the number of sampled s is checked. Any other input combination can also be used. Any of the indirect instructions which read byte CACTL used only two CPU cycles. The contents of control register CACTL does not change during the n samples, so the number of s can be easily found by subtraction of n (initialized register contents). 8 Economic Measurement Techniques With the Comparator_A Module

9 SLAA7 PCA CAEX CAON CAF RM IMEAS CA CA PCA + _ Low Pass Filter τ, µs e.g. capture input of Timer_A CAOUT Set CAIFG Flag Internal References CAREF CARSEL VCAREF.5 x.5 x MSP4 V V Figure 5. Circuit for Fast Comparator Input Sampling Economic Measurement Techniques With the Comparator_A Module 9

10 SLAA7 ; Fast test for the state of the Comparator_A input CA ; MOV.B #CARSEL+CAREF+CAON,&CACTL ; Define Comp_A mode MOV.B #PCA,&CACTL ; Connect CA to noninv. input MOV #CACTL,R5 ; Prepare pointer to reg. CACTL... ; Sample CAOUT (CAOUT = CACTL.) ; Add next sample... ; Add following samples ; Add sample n ; ; Test if CAOUT showed more than n/ times a positive result ; SUB #n*pca,r5 ; Correct result CMP.B #(n/+),r5 ; R5 (n/+) JGE POS ; More samples... ; More samples or, for an even faster decision: ; Test if CAOUT showed more than n/ times a positive result ; CMP.B #n*pca+(n/+),r5 ; R5 (n*pca+(n/+)) JHS POS ; More samples are... ; More samples are This method allows samples within 5 µs (/4 MHz cycles samples = 5 µs) for an MCLK frequency of 4 MHz. The input CA can be used as an external reference voltage instead of the internal reference voltages.. Resistance Measurement Figure 6 shows the minimum hardware configuration for the measurement of a resistive sensor: the sensor Rsens itself, the reference resistor Rref, and the capacitor Cm. Capacitor Cm is charged to the voltage V CC before each measurement. At the start of capacitor discharge, the contents of timer register TAR (which always counts upwards in continuous mode) is stored. When capacitor Cm voltage reaches the value.5 V CC, the negative edge of CAOUT causes the actual TAR value to be captured in register CCR. The differences between the values in CCR and the start values represent the discharge time intervals tsens and tref, respectively. Comparator_A is not changed during the measurements, but the outputs of Port perform the switching of the resistors to be measured. Economic Measurement Techniques With the Comparator_A Module

11 SLAA7 PCA CAEX CAON CAF RSENS CA CA P.x VCA PCA + _ Low Pass Filter τ, µs A (See Next Page) CAOUT Set CAIFG Flag Internal References RREF P.y CM CAREF VCM VSS CARSEL VCAREF.5 x.5 x MSP4 V Figure 6. Measurement of Resistors Comparator_A Economic Measurement Techniques With the Comparator_A Module

12 SLAA7 To Other Capture/Compare Blocks Timer Bus CCIS CCIS Out Capture/Compare Block OM OM OM Output Unit EQU 5 Capture/Compare Register CCR 5 Comparator 5 Capture/Compare Control Register CCTL Capture Disabled Pos. Edge Neg. Edge Both Edges CCM Capture Mode CCM CCI P./CCIA CAOUT GND A (See Previous Page) Data EQU SSEL SSEL TACLK ACLK SMCLK Input Divider DC to MCLK Timer Clock Timer Bus 5 Timer Register TAR CLK RC Timer Register Block Mode Control INCLK ID ID Pass / /4 /8 POR/CLR Data Set_TAIFG Carry/Zero Stop Up Mode Contin. Up/Down MC MC Equ Timer_A Figure 6. Measurement of Resistors (Continued) Figure 7 shows the voltage Vcm across capacitor Cm during the two measurements. The charge time tc must be between 5τ (for %) and 7τ (for.%), depending on the accuracy required, where τ = Rref Cm. Economic Measurement Techniques With the Comparator_A Module

13 SLAA7 VCM Reference Rsens: HiZ Rref: VSS VCA: VCM VCA: VCAREF VCAREF:.5 Rsens: Rref: Sensor Rsens: VSS Rref: HiZ (VSS) VCA: VCM VCA: VCAREF VCAREF:.5 CAOUT gets LO CAOUT gets LO VCAREF =.5 x tc tref tc tsens Time Figure 7. Voltage at Cm During Resistance Measurement Solution of the two exponential equations describing the capacitor discharge for reference resistor Rref and sensor Rsens leads to a simple equation for the calculation of Rsens: Rsens Rref tsens Cm ln V CAREF V CC Cm ln V CAREF V CC tref Rsens Rref tsens tref For highly nonlinear sensors (such as NTC sensors), reference resistor Rref is chosen to be the optimum linearization resistor. It is connected in parallel with the sensor Rsens during sensor measurement. The formula then becomes: Rsens Rref tsens tref tsens The previous calculation formula (written for floating-point package FPP4) is contained in the MSP4 Application Report Book (see Section, Temperature Calculation Example). It is also possible to connect more than one sensor (switched by ports) and two reference resistors, one for the beginning and one for the end of the measurement range. The calculation formulas are contained in Section. of the MSP4 Application Report Book..4 Voltage Measurement Figure 8 shows how to measure an external voltage V IN. The supply voltage V CC is used as a reference. The split configuration of resistor divider and discharge circuit has the advantage of not using input voltage V IN to charge capacitor Cm (normally with large time constants). Instead, the input voltage always has the correct value at input pin CA. Figure 8 shows the position of the comparator switches during the reference measurement. Economic Measurement Techniques With the Comparator_A Module

14 SLAA7 NOTE: The complex formulas developed ahead can always be reduced to relatively simple equations with constant values: V A e x B, and V A e x B The logarithmic functions used are contained in floating-point package FPP4. See Section 5.6 of the MSP4 Application Report Book. If integers are used in the calculation, the exponential function can be emulated by a hyperbola. Then only one division is necessary: V D x C E See Section 5.5 of the MSP4 Application Report Book. 4 Economic Measurement Techniques With the Comparator_A Module

15 SLAA7 VIN PCA CAEX CAON CAF R VIN R CA CA VCA PCA + _ Low Pass Filter τ, µs A (See Next Page) CAOUT Set CAIFG Flag Internal References RREF P.y CM CAREF VCM VSS CARSEL VCAREF V.5 x.5 x MSP4 Figure 8. Voltage Measurement Comparator_A Economic Measurement Techniques With the Comparator_A Module 5

16 SLAA7 To Other Capture/Compare Blocks Timer Bus CCIS CCIS Out Capture/Compare Block OM OM OM Output Unit EQU 5 Capture/Compare Register CCR 5 Comparator 5 Capture/Compare Control Register CCTL Capture Disabled Pos. Edge Neg. Edge Both Edges CCM Capture Mode CCM CCI P./CCIA CAOUT GND A (See Previous Page) EQU Data SSEL SSEL TACLK ACLK SMCLK Input Divider DC to MCLK Timer Clock Timer Bus 5 Timer Register TAR CLK RC Timer Register Block Mode Control INCLK ID ID Pass / /4 /8 POR/CLR Data Set_TAIFG Carry/Zero Stop Up Mode Contin. Up/Down MC MC Equ Timer_A Figure 8. Voltage Measurement (Continued) The voltage range of V CA (seen at the comparator input CA) that can be measured with the previous discharge circuit is limited to the following nominal range: V CAREF V CA V CM(MAX) (Refer to device data sheet) This implies that for a supply voltage V CC =. V, voltages at the inputs (V CA and V CA ) between.85 V (.5. V) and V CM can be measured. 6 Economic Measurement Techniques With the Comparator_A Module

17 SLAA7 With a resistor divider consisting of resistors R and R, the nominal input voltage range for V IN becomes: V CAREF R R V R IN V CC R R R VCM Reference Rref: VSS VCA: VCAREF VCA: VCM VCAREF:.5 Rref: Voltage Rref: VSS VCA: VIN VCA: VCM VCAREF: HiZ CAOUT gets HI VCA CAOUT gets HI VCAREF =.5 x tc t tc tm Time Figure 9. Voltage Measurement With the Discharge Method As shown in Figure 9, capacitor Cm (previously charged to V CC ) is discharged through resistor Rref. The time interval tvcc (from the start of discharge until V CAREF is reached) is measured with Timer_A, as shown before for the resistance measurement. Next, switch PCA is set to (the divided voltage V IN is switched to the noninverting input), the internal reference voltage is switched off with CAREF =, and the discharge of the charged capacitor Cm is repeated. When Vcm reaches the voltage V CA, the comparator output CAOUT is switched high and the accurate time is captured in register CCR of Timer_A. The charge time tc required depends on the measurement accuracy required: 5τ for %, 7τ for.% (τ = Rref Cm). The voltage V IN is calculated from the two measured time intervals tm and tvcc using the following formula (discharge method): V IN V CC R R R tm tvcc ln V CAREF V e CC Because V CAREF =,5 V CC here, the logarithm of the above formula can be replaced by ln.5 = The voltage V IN can also be proportional to a current I IN. This also allows to perform current measurement. Economic Measurement Techniques With the Comparator_A Module 7

18 SLAA7 The charge method must be used when it is necessary to measure voltages or currents down to a value of zero. The reference voltage in this case is V CAREF =.5 V CC. The measurements are taken with capacitor Cm completely discharged. It is then charged until the voltages V CAREF and V CA, respectively, are reached. (See also Section.5, Digital Motor Control, where the charge method is used). Using the two measured time intervals tm and tvcc, the input voltage V IN can be calculated: V IN R R R V CC tm tvcc ln V CAREF V e CC Because V CAREF =.5 V CC here, the logarithm of the above formula can be replaced by ln.5 = Digital Motor Control (DMC) Figure shows an integrated motor control with an MSP4F and an L9. This L9 chip contains two H-bridges in a single package. The direction of rotation is defined by the static output P., and the speed of the motor is determined by the PWM output TA of Timer_A. Motor current and temperature can be measured with simple circuitry at the Comparator_A inputs. Only the charge method can be used because the motor current, which is always positive, must be measured down to a zero value. Capacitor Cm is discharged to V before each measurement, and the time interval until it reaches the voltage V CAREF relative to V SHUNT is measured. Using the measured time intervals tvcc and ti, the absolute value of the motor current Imotor can be calculated. The equation for the motor current Imotor is: Imotor Rshunt V CC ti tvcc ln V CAREF V e CC 8 Economic Measurement Techniques With the Comparator_A Module

19 SLAA7 +. V +4 V N494 M +4 V +5 V Y V Y MSP4F TA A A P.,EN GND IMOTOR / L9 P. VSS CAO CA P. P. Rsens Rref CM VCM Rshunt VSHUNT Reverse Forward V V V V Motor Current and Motor Temperature TA (PWM) Halt Slowly Forward Fast Forward Fast Reverse Slowly Reverse Figure. Current and Temperature Measurement for a PWM Motor Control P. (Direction) The temperature of the motor is measured using temperature sensor Rsens. To allow the use of the reference measurement for the current measurement (which delivers the value tvcc) as well as for the temperature measurement, the sensor Rsens is also measured using the charge method. The formula for calculation of the sensor resistance using the two values tvcc and tsens is: Rsens Rref tsens tvcc Economic Measurement Techniques With the Comparator_A Module 9

20 SLAA7 Figure shows the voltage Vcm at capacitor Cm for the current and temperature measurements. The discharge time td required depends on the measurement accuracy desired, namely 5τ for % and 7τ for.% (where τ = Rref Cm). VCM Current Rref: Rsens:HiZ VCA: VSHUNT VCA: VCM Reference Rref: Rsens:HiZ VCA: VCAREF VCA: VCM Temperature Rref: HiZ () Rsens: VCA: VCAREF VCA: VCM VCAREF Rref: VSS Rsens: VSS VSHUNT td ti td t td tsens td VSHUNT = IMOTOR x RSHUNT Time Figure. Current and Temperature Measurement (Charge Method) It is possible to take frequent measurements because the capacitor Cm is not charged up to the voltage V SHUNT with a high resistance. The frequency of the measurements depends only on the time constant τ = Cm Rref. The current of triac-controlled motors can also be measured using Comparator_A. However, the method previously shown only allows measurement of the positive half wave of the motor current. Economic Measurement Techniques With the Comparator_A Module

21 SLAA7.6 Connection of Sensor Bridges It is also possible to measure sensor bridges connected to Comparator_A. There is a difference between applications with and without amplification. Both possibilities are shown..6. Measurement Without Amplification The circuitry of Figure can be used when no amplification is required,. The two capacitors Cm and Cm are charged up to the voltages V m and V p of the two midpoints of the bridge legs. Then they are discharged to reference voltage.5 V CC while taking two adjacent measurements. The time intervals measured, tm and tm, are used on the calculation of voltages V p and V m. The references are represented by time intervals tvcc and tvcc for the capacitors to discharge from the supply voltage V CC down to.5 V CC. The equations for the two bridge voltages Vp and V m are: V p V CC e tvcc tm tvcc ln V CAREF V CC V m V CC e tvcc tm tvcc ln V CAREF V CC The value of most interest Rb/Rb (the relative change of the bridge resistor Rb with pressure) can be calculated using the former two equations: RB RB Vp V m V CC A complete measurement cycle consists of four voltage measurements: V m, V p, V CC. Figure shows the switch positions of the comparator for the measurement of voltage V p. Switches PCA, PCA, CARSEL, and CAEX are toggled to measure V m. This ensures that both voltages are measured with the same comparator offset. Economic Measurement Techniques With the Comparator_A Module

22 SLAA7 CAF Low Pass Filter To Internal Modules P.w CAOUT P.x Set CAIFG Flag P.y τ, µs RB RSENS PCA CAEX CAON VP Vm RV RV RREF RREF CA CA + _ To Timer_A P.z PCA Internal References CM CM CAREF Measurement Bridge VSS CARSEL VCAREF.5 x.5 x MSP4 V Figure. Connection of a Measurement Bridge to Comparator_A Figure shows the voltage curve for the bridge leg connected to comparator input CA; this input measures the voltage V m and the temperature. Economic Measurement Techniques With the Comparator_A Module

23 SLAA7 The charge time tc required depends on the measurement accuracy desired namely, 5τ for %, 7τ for.%, where τc = Rref Cm. The same is valid for time interval tce, where the time constant is τce = (Rref+Rv+Rb/) Cm. VCM Voltage Rref: VSS Rsens:HiZ VCA: VCM VCAREF:.5 Reference Rref: VSS Rsens:HiZ VCA: VCM VCAREF:.5 Rref: Temperature Rref: VSS Rsens:VSS VCA: VCM VCAREF:.5 CAOUT gets LO CAOUT gets LO CAOUT gets LO Vm Charge up to Vm VCAREF tm tc t tc tsens tce Time Figure. Voltage V cm During Bridge Measurement (V m and Temperature) The temperature of the bridge can also be measured as shown in Figure. The temperature is necessary for the compensation of the bridge. The value Rsens of a temperature sensor is measured the normal way. Resistor Rref must be selected so it can also be used for linearization of the temperature sensor. This is necessary because resistor Rref cannot be switched off completely. The equation for the sensor resistance Rsens is: Rsens Rref tsens tvcc tsens Calculation of the components The following steps are used to calculate the components shown in Figure. Components Rref, Cm, and Rv have the same values as components Rref, Cm and Rv calculated below.. Rsens = f(t) is given This defines the optimum linearization resistor Rlin. Rlin equals Rref. The time constant τ is chosen to reach a given minimum resolution n for a given Rb/Rb Rref Cm 4. With τ, the capacitor Cm is: Cm Rref n ƒmeas ln.5 Rb Rb ln.5 Rb Rb 5. Resistor Rv should impose minimum load to the bridge: Rb << Rv Rref Economic Measurement Techniques With the Comparator_A Module

24 SLAA7 Where: n Minimum resolution for a given Rb/Rb Rlin Optimum linearization resistor for the temperature sensor [Ω] fmeas Measurement frequency [Hz] EXAMPLE: a bridge system needs to be designed using the following data: Rlin = kω, n = 5 for Rb/Rb = Ω/ kω, Rb = kω, fmeas = MHz. Rlin defines Rref: Rref = kω. Time constant τ: 5 MHz ln.5 ln.5.78 s 5 MHz.6. Capacitor Cm: Cm.78 s 78 nf Cm nf k 4. Resistor Rv: kω << Rv kω Rv = kω This results in a resolution n for Rb/Rb = Ω/ kω: n ƒmeas ln.5 ln.5 n nf k MHz Measurement With Amplification An op amp must be used if the resolution of the circuitry of Figure is not sufficient. This op amp amplifies the voltage difference of the two bridge legs. It is connected to Comparator_A as shown in Figure 4. The measurement method and the equations are identical to those shown in the Voltage Measurement section. The value of interest, Rb/Rb (assuming Rb << R) is now: RB RB V BR R v V CC R V BR V CC A complete measurement cycle consists of two voltage measurements: One measurement with the bridge unloaded (that is, without pressure) One measurement with the bridge loaded The difference voltage V BR resulting from the two measurements is calculated. If a temperature measurement is necessary, it is done as described in the Resistance Measurement section. 4 Economic Measurement Techniques With the Comparator_A Module

25 SLAA7 under CAF Low Pass Filter P.z CAOUT Measurement Bridge RB R V PCA CAEX CAON τ, µs Set CAIFG Flag Vp Vm R R + RSENS VBR CA CA + _ To Timer_A P.x PCA RREF P.y Internal References CM VCM CAREF VSS CARSEL VCAREF.5 x.5 x MSP4 V Comparator_A Figure 4. Connection of a Bridge With Amplification Economic Measurement Techniques With the Comparator_A Module 5

26 SLAA7.7 Capacitance Measurement Figure 5 shows the circuit used to measure a capacitor Cx using a reference capacitor Cref. Output P.y connects capacitor Cref to V SS during the reference measurement, and output P.z connects the unknown capacitor Cx to V SS during the capacitance measurement. Both ports are otherwise switched to high impedance. Cs represents the stray capacitance of the measurement circuit. CS: Circuit Capacitance R P.x MSP4 CA +. V CA CX CS CREF P.y P.z VSS V Figure 5. Measurement of an Unknown Capacity Cx The voltage curve for the two measurements is the same as that shown in Figure 7 (resistance measurement). The equation for Cx is: Cx tx (Cref Cs) Cs tref Where tx and tref represent the measured discharge intervals for Cx and Cref. Capacitor Cm can be used for reference purposes in the existing circuitry for resistance and voltage measurement. Comparator input CA can also be used for other purposes. See the following sections..8 Other Applications Two of many possible applications of the Comparator_A module are described below..8. Two Independent Measurement Circuits It is possible to connect two completely independent measurement circuits to the Comparator_A inputs CA and CA. It is only necessary that both circuits use the internal reference voltages. This allows, for example, measuring voltage with one channel and checking a threshold on the other one. Figure 6 shows the connection of two independent measurement circuits for resistance measurement and fast sampling of an input signal (see also Sections. and.). The positions of the internal switches are shown for resistance measurement. To measure the circuit at input CA, all switches, with the exception of filter switch CAF, must be toggled. 6 Economic Measurement Techniques With the Comparator_A Module

27 SLAA7 CAF Low Pass Filter To Internal Modules CAOUT Set CAIFG Flag τ, µs PCA CAEX CAON RM IMEAS CA CA + _ To Timer_A PCA Internal References RSENS P.x CAREF RREF P.y CM CARSEL.5 x VCM VSS VCAREF.5 x MSP4 V Figure 6. Independent Use of the Two Comparator_A Inputs.8. External Use of the Internal Reference Voltage Sometimes it can be convenient to have access to the internal reference voltage in Comparator_A that is used for the comparison. This allows the use of the exact reference value in the external circuitry. Figure 7 shows the switch positions required to make the internal reference diode voltage available at the input CA and at the inverting comparator input. Economic Measurement Techniques With the Comparator_A Module 7

28 SLAA7 CAF Low Pass Filter To Internal Modules CAOUT Set CAIFG Flag τ, µs PCA CAEX CAON + _ VIN CA CA + _ PCA + Internal References VINT VCAREF CAREF V External Circuitry CARSEL VCAREF.5 x.5 x V Figure 7. Use of the Internal Reference Voltage in the External Circuitry 8 Economic Measurement Techniques With the Comparator_A Module

29 References SLAA7. MSP4xxx Family User s Guide SLAU49. MSP4x4xx Family User s Guide SLAU56. MSP4 Application Report Book SLAA4 4. MSP4FA Data Sheet SLAS4 5. MSP4 4-Bit Analog-to-Digital Converter Application Reports SLAA45, SLAA46, SLAA47, SLAA48, SLAA5 Economic Measurement Techniques With the Comparator_A Module 9

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