CHAPTER 3 METHODOLOGY 3.1 Overview The Water Monitoring ...

More documents

Recommendations

Info

ADC0802, ADC0803, ADC0804Input Source ResistanceLarge values of source resistance where an input bypasscapacitor is not used will not cause errors since the inputcurrents settle out prior to the comparison time. If a lowpassfilter is required in the system, use a low-value seriesresistor (≤1kΩ) for a passive RC section or add an op ampRC active low-pass filter. For low-source-resistanceapplications (≤1kΩ), a 0.1µF bypass capacitor at the inputswill minimize EMI due to the series lead inductance of a longwire. A 100Ω series resistor can be used to isolate thiscapacitor (both the R and C are placed outside the feedbackloop) from the output of an op amp, if used.Stray PickupThe leads to the analog inputs (pins 6 and 7) should be keptas short as possible to minimize stray signal pickup (EMI).Both EMI and undesired digital-clock coupling to these inputscan cause system errors. The source resistance for theseinputs should, in general, be kept below 5kΩ. Larger values ofsource resistance can cause undesired signal pickup. Inputbypass capacitors, placed from the analog inputs to ground,will eliminate this pickup but can create analog scale errors asthese capacitors will average the transient input switching currentsof the A/D (see Analog Input Current). This scale errordepends on both a large source resistance and the use of aninput bypass capacitor. This error can be compensated by afull scale adjustment of the A/D (see Full Scale Adjustment)with the source resistance and input bypass capacitor inplace, and the desired conversion rate.Reference Voltage Span AdjustFor maximum application flexibility, these A/Ds have beendesigned to accommodate a 5V, 2.5V or an adjusted voltagereference. This has been achieved in the design of the IC asshown in Figure 12.Notice that the reference voltage for the IC is either 1 / 2 of thevoltage which is applied to the V+ supply pin, or is equal tothe voltage which is externally forced at the V REF /2 pin. Thisallows for a pseudo-ratiometric voltage reference using, forthe V+ supply, a 5V reference voltage. Alternatively, a voltageless than 2.5V can be applied to the V REF /2 input. Theinternal gain to the V REF /2 input is 2 to allow this factor of 2reduction in the reference voltage.Such an adjusted reference voltage can accommodate areduced span or dynamic voltage range of the analog inputvoltage. If the analog input voltage were to range from 0.5V to3.5V, instead of 0V to 5V, the span would be 3V. With 0.5Vapplied to the V lN(-) pin to absorb the offset, the referencevoltage can be made equal to 1 / 2 of the 3V span or 1.5V. TheA/D now will encode the V lN(+) signal from 0.5V to 3.5V withthe 0.5V input corresponding to zero and the 3.5V input correspondingto full scale. The full 8 bits of resolution are thereforeapplied over this reduced analog input voltage range. The requisiteconnections are shown in Figure 13. For expandedscale inputs, the circuits of Figures 14 and 15 can be used.V REF /2V REF(5V)9RRAGNDDECODEV+(V REF )ANALOGCIRCUITSDIGITALCIRCUITS8 DGND 10FIGURE 12. THE V REFERENCE DESIGN ON THE ICFSADJ.-+5V3000.1µFZERO SHIFT VOLTAGEFIGURE 13. OFFSETTING THE ZERO OF THE ADC0802 ANDPERFORMING AN INPUT RANGE (SPAN)ADJUSTMENTV IN ± 10VICL7611“SPAN”/22RR2R67V IN(-)20TO V REF /2TO V IN(-)5VV IN(+) V+ 20 +(V REF )ADC0802-10µFADC0804FIGURE 14. HANDLING ±10V ANALOG INPUT RANGE6-14
ADC0802, ADC0803, ADC0804V IN ±5VRR67V IN(-)5VV IN(+) V+ 20 +(V REF )ADC0802-10µFADC0804Full Scale AdjustThe full scale adjustment can be made by applying adifferential input voltage which is 1 1 / 2 LSB down from thedesired analog full scale voltage range and then adjustingthe magnitude of the V REF /2 input (pin 9) for a digital outputcode which is just changing from 1111 1110 to 1111 1111.When offsetting the zero and using a span-adjusted V REF /2voltage, the full scale adjustment is made by inputting V MlNto the V IN(-) input of the A/D and applying a voltage to theV IN(+) input which is given by:(V IN ( + )f SADJV MAX1.5 V MAX– V MIN)= – ----------------------------------------- ,256FIGURE 15. HANDLING ±5V ANALOG INPUT RANGEReference Accuracy RequirementsThe converter can be operated in a pseudo-ratiometric modeor an absolute mode. In ratiometric converter applications,the magnitude of the reference voltage is a factor in both theoutput of the source transducer and the output of the A/Dconverter and therefore cancels out in the final digital outputcode. In absolute conversion applicatIons, both the initialvalue and the temperature stability of the reference voltageare important accuracy factors in the operation of the A/Dconverter. For V REF /2 voltages of 2.5V nominal value, initialerrors of ±10mV will cause conversion errors of ±1 LSB dueto the gain of 2 of the V REF /2 input. In reduced span applications,the initial value and the stability of the V REF /2 inputvoltage become even more important. For example, if thespan is reduced to 2.5V, the analog input LSB voltage valueis correspondingly reduced from 20mV (5V span) to 10mVand 1 LSB at the V REF /2 input becomes 5mV. As can beseen, this reduces the allowed initial tolerance of the referencevoltage and requires correspondingly less absolutechange with temperature variations. Note that spans smallerthan 2.5V place even tighter requirements on the initial accuracyand stability of the reference source.In general, the reference voltage will require an initialadjustment. Errors due to an improper value of referencevoltage appear as full scale errors in the A/D transfer function.IC voltage regulators may be used for references if theambient temperature changes are not excessive.Zero ErrorThe zero of the A/D does not require adjustment. If theminimum analog input voltage value, V lN(MlN) , is not ground, azero offset can be done. The converter can be made to output0000 0000 digital code for this minimum input voltage by biasingthe A/D V IN(-) input at this V lN(MlN) value (see Applicationssection). This utilizes the differential mode operation of the A/D.The zero error of the A/D converter relates to the location ofthe first riser of the transfer function and can be measured bygrounding the V IN(-) input and applying a small magnitudepositive voltage to the V IN(+) input. Zero error is the differencebetween the actual DC input voltage which is necessary tojust cause an output digital code transition from 0000 0000 to0000 0001 and the ideal 1 / 2 LSB value ( 1 / 2 LSB = 9.8mV forV REF /2 = 2.500V).where:V MAX = the high end of the analog input range,andV MIN = the low end (the offset zero) of the analog range.(Both are ground referenced.)Clocking OptionThe clock for the A/D can be derived from an external sourcesuch as the CPU clock or an external RC network can beadded to provIde self-clocking. The CLK IN (pin 4) makesuse of a Schmitt trigger as shown in Figure 16.RCLK INCCLK R194ADC0802-ADC0804CLKFIGURE 16. SELF-CLOCKING THE A/D1f CLK ≅1.1 RCR ≅ 10kΩHeavy capacitive or DC loading of the CLK R pin should beavoided as this will disturb normal converter operation.Loads less than 50pF, such as driving up to 7 A/D converterclock inputs from a single CLK R pin of 1 converter, areallowed. For larger clock line loading, a CMOS or low powerTTL buffer or PNP input logic should be used to minimize theloading on the CLK R pin (do not use a standard TTL buffer).Restart During a ConversionIf the A/D is restarted (CS and WR go low and return high)during a conversion, the converter is reset and a new conversionis started. The output data latch is not updated if theconversion in progress is not completed. The data from theprevious conversion remain in this latch.Continuous ConversionsIn this application, the CS input is grounded and the WRinput is tied to the INTR output. This WR and INTR nodeshould be momentarily forced to logic low following a powerupcycle to insure circuit operation. See Figure 17 for details.6-15
Page 1 and 2:
CHAPTER 3METHODOLOGY3.1 OverviewThe
Page 3 and 4:
3.2.2 Analog To Digital Conversion
Page 5 and 6:
Figure 3.5: Source Code for Convers
Page 7 and 8:
3.2.4 Water Monitoring System Circu
Page 9 and 10:
3.3 Programming.Programming is the
Page 11 and 12:
ISP flash programmer in Figure 3.11
Page 13 and 14:
The Visual Basic program will start
Page 15 and 16:
Distance(cm)ReadingVoltage Trial1 (
Page 17 and 18:
4.2 Signal ConversionA sensor has s
Page 19 and 20:
4.3 User InterfaceBased on Table 4.
Page 21 and 22:
Hardware DesignBasically, the const
Page 23 and 24:
CHAPTER 5CONCLUSION5.1 SummaryWater
Page 25 and 26:
This project can be expanded by usi
Page 27 and 28:
APPENDICES41
Page 29 and 30: cjne a,volt,xsamejmp startxsame:mov
Page 31 and 32: xsame19:jnc more19mov dptr,#XVALIDc
Page 33 and 34: etrx:jnb ri,$clr rimov a,sbufretdel
Page 35 and 36: ElseShape8.FillColor = vbWhiteEnd I
Page 37 and 38: Private Sub Timer3_Timer()Label3.Ca
Page 39 and 40: APPENDIX EData Sheet AT89S5253
Page 41 and 42: Features• Compatible with MCS-51
Page 43 and 44: AT89S52Block DiagramP0.0 - P0.7P2.0
Page 45 and 46: AT89S52weakly pulled high. Setting
Page 47 and 48: AT89S52Table 3a. AUXR: Auxiliary Re
Page 49 and 50: AT89S52Watchdog Timer(One-time Enab
Page 51 and 52: AT89S52Figure 6. Timer 2 Auto Reloa
Page 53 and 54: AT89S52Baud Rate GeneratorTimer 2 i
Page 55 and 56: AT89S52Oscillator CharacteristicsXT
Page 57 and 58: AT89S52frequency should be less tha
Page 59 and 60: Figure 15. Flash Programming and Ve
Page 61 and 62: AT89S52Table 9. Serial Programming
Page 63 and 64: AT89S52Absolute Maximum Ratings*Ope
Page 65 and 66: AT89S52External Program Memory Read
Page 67 and 68: AT89S52Serial Port Timing: Shift Re
Page 69 and 70: Packaging Information44A, 44-lead,
Page 71 and 72: August 1997SemiconductorADC0802, AD
Page 73 and 74: ADC0802, ADC0803, ADC0804Absolute M
Page 75 and 76: ADC0802, ADC0803, ADC0804Electrical
Page 77 and 78: ADC0802, ADC0803, ADC0804Typical Pe
Page 79: ADC0802, ADC0803, ADC0804The device
Page 83 and 84: ADC0802, ADC0803, ADC0804Figures 19
Page 85 and 86: ADC0802, ADC0803, ADC0804IRQ (4)†
show all

CHAPTER 3 METHODOLOGY 3.1 Overview The Water Monitoring ...

Create successful ePaper yourself

Delete template?

Save as template?