Analog to Digital Conversion. Gary J. Minden October 1, 2013

Transcription

1 Analog to Digital Conversion Gary J. Minden October 1,

2 Mapping Input Voltage to Digital Value Vhigh ,023 Vin Vlow

3 Analog to Digital Conversion Analog -- A voltage between Vlow and Vhigh Digital -- A fraction between 0.0 and 1.0 with K bits In the case of the LM3S1968, K = 10 and there are 1,024 steps. Fraction: Values between 0.0 and Binary Point 2-10 Integer: Values between 0 and 1,023 with scale factor of 2-10 Binary Point 3

4 Challenge #1 -- Compare Voltage Vhigh Vin is input Voltage Range is Vlow to Vhigh Vin V<i> is test Voltage V<i> How to compare Vin to V<i>? That is, is Vin < V<i> or not? Vlow 4

5 Comparator If Vin > V<i>, then VT<i> is True (1) Vin V<i> + - VT<i> A logic signal 5

6 Array of Comparators Vin V<9> V<8> VT<9> VT<8> Set up array of comparators Each comparator tests Vin Each has a different V<i> V<1> + - VT<1> Each comparator generates a logic value, 0 or 1 V<0> + - VT<0> 6

7 Results VT<i> Output of Comparators Logic R<i> Mark Index V<9> V<8> V<7> V<6> V<5> V<4> V<3> V<2> V<1> V<0> R<3> R<2> R<1> R<0>

8 Problems and Approach This does not scale for a large number of steps E.g. 1,024 or 4,096 Successively generate V<i>, a test Voltage, over several trials What sequence of V<i> to use How do you generate V<i>? 8

9 Successive Approximation Approach #1 1.0 V Vin is input Voltage Vin Range is V Assume 1,024 steps 0.0 V V<0> = 0.5 V V<0> is first test Voltage Start in the middle If Vin < V<0>, decrease test voltage If Vin >= V<0>, increase test voltage Vin > V<0>, increase 9

10 Successive Approximation Approach #2 Vin 1.0 V 0.0 V V<1> = 0.75 V Vin is input Voltage Range is V Assume 1,024 steps V<1> is test voltage If Vin < V<1>, decrease test voltage If Vin >= V<1>, increase test voltage Vin < V<1>, decrease test voltage 10

11 Successive Approximation Approach #3 1.0 V Vin is input Voltage Vin Range is V Assume 1,024 steps V<2> = V V<2> is test voltage Start in the middle If Vin < V<2>, decrease V<i> If Vin >= V<2>, increase V<i> 0.0 V Vin >= V<2>, increase 11

12 Successive Approximation Approach # V Vin is input Voltage Vin Range is V Assume 1,024 steps V<9> = V V<9> is test Voltage If Vin < V<9>, decrease test voltage If Vin >= V<9>, increase test voltage Done 0.0 V 12

13 Successive Approach with DAC Vin Vt + - Compare Generate V<i> with a Digital to Analog Converter (DAC) Convert a binary number to a Voltage DAC log 2 (R)-bits SAR Update SAR FSM 13

14 Example with Vin = V i S<9> S<8> S<7> S<6> S<5> S<4> S<3> S<2> S<1> S<0> V<i> Comp D Compare == 1 if Vin >= V<i> 14

15 Example with Vin = V i S<9> S<8> S<7> S<6> S<5> S<4> S<3> S<2> S<1> S<0> V<i> Comp D Compare == 1 if Vin >= V<i> 15

16 Example with Vin = V i S<9> S<8> S<7> S<6> S<5> S<4> S<3> S<2> S<1> S<0> V<i> Comp D Compare == 1 if Vin >= V<i> 16

17 Example with Vin = V i S<9> S<8> S<7> S<6> S<5> S<4> S<3> S<2> S<1> S<0> V<i> Comp D Compare == 1 if Vin >= V<i> 17

18 Example with Vin = V i S<9> S<8> S<7> S<6> S<5> S<4> S<3> S<2> S<1> S<0> V<i> Comp D Compare == 1 if Vin >= V<i> 18

19 Result Value Format Scale Factor 2^-1 2^-2 2^-3 2^-4 2^-5 2^-6 2^-7 2^-8 2^-9 2^-10 1/2 1/4 1/8 1/16 1/32 1/64 1/128 1/256 1/512 1/ Binary Point R = 1/2 + 1/8 + 1/64 + 1/ /1024 = 0.644

20 ADC Architecture Analog Sources LPF Mux S ADC Results FIFO To CPU C 20

21 Successive Approach with DAC Vin Vt + - Compare Generate V<i> with a Digital to Analog Converter (DAC) Convert a binary number to a Voltage DAC log 2 (R)-bits SAR Update SAR FSM 21

22 Measured Voltage V Measured = (K/4096) * V Ref VRef Internal V Ref = 3.3 V Vin E.g. if K = 472 then V Measured = V Vi 0.0 V

23 TM4C1294 ADC Features Two ADCs, ADC0 and ADC1 Twenty (20) analog input channels shared by the ADCs 12-bit Successive Approximation Register for each ADC On-chip internal temperature sensor Sample rate of up to two million samples/second Four programmable sample conversion sequences from one to eight entries long, with corresponding conversion result FIFOs Flexible trigger control Controller (software), Timers, Analog Comparators, PWM, GPIO Hardware averaging of up to 64 samples for improved accuracy Converter uses a 3.3V reference Power and ground for the analog circuitry is separate from the digital power and ground 23

24 ADC Top-Level Block Diagram 24

25 LM3S1968 ADC Block Diagram Seq. Ctrl. ADC Analog Mux Ctrl./Status FIFOs Trigger 25

26 Tiva ADC Architecture Results ADC FIFO Seq. FIFO Seq. FIFO Seq. FIFO Seq. To CPU 26

27 ADC Control Enable Interrupt generation DMA operation Sequence prioritization Each sequencer has a priority, 0-3, 0 is highest Trigger configuration Processor (default), analog comparators, an external signal on GPIO PB4, a GP Timer, a PWM generator, and continuous sampling. Comparator configuration External voltage reference Sample phase control 27

28 ADC Sequencer Characteristics 28

29 ADC Initialization unsigned long ulvalue; Enable ADC0 // // Configure ADC<9> to read potentiometer voltage. The // TM4C1294 Tiva processor has two ADCs. We'll use ADC0. // SysCtlPeripheralEnable( SYSCTL_PERIPH_ADC0 ); Config. ADC0/Seq0 Prog. Trig. ADCSequenceConfigure( ADC0_BASE, 0, ADC_TRIGGER_PROCESSOR, 1 ); ADCSequenceStepConfigure( ADC0_BASE, 0, 0, ADC_CTL_IE ADC_CTL_END ADC_CTL_CH9 ); ADCSequenceEnable( ADC0_BASE, 0 ); void ADCSequenceConfigure(unsigned long ulbase, unsigned long ulsequencenum, unsigned long ultrigger, unsigned long ulpriority) Priority void ADCSequenceStepConfigure(unsigned long ulbase, unsigned long ulsequencenum, unsigned long ulstep, unsigned long ulconfig) 29

30 ADC Trigger, Wait, Read Result // // Trigger the sample sequence. // ADCProcessorTrigger(ADC0_BASE, 0); // // Wait until the sample sequence has completed. // while(!adcintstatus(adc0_base, 0, false)) { } // // Read the value from the ADC. // ADCSequenceDataGet(ADC0_BASE, 0, &ulvalue); Start Wait Read Date 30

31 Sensor Value Mapping Sensor is electronic device Thermistor, Photo-Transistor, Microphone,... Each sensor has a physical measurement range E.g. -40 C to 120 C Each sensor maps measurement range a voltage range based on sensor and electronic circuit E.g. Map -40 C to 120 C to 0.0 V to 2.5 V Micro-controller measures voltage E.g. Voltage between 0.0 V and 3.0 V on LM3S1968 Program has to interpret voltage measurement 31

32 Sensor to Voltage Mapping Voltage C -> 0.0 V Temp C 120 C -> 2.5 V 32

33 ADC Voltage to Value Mapping 1000 Value (Int.) Voltage 33

34 Sensor to Voltage Mapping Voltage Temp C -40 C -> 0.0 V 24 C -> 1.0 V 120 C -> 2.5 V 34

35 ADC Voltage to Value Mapping 1000 Value (Int.) V Voltage Assumes V Ref = 3.0 V and 10-bit ADC 35

36 TM4C1294 Temperature Sensor TEMP = ((75 * (VREFP - VREFN) ADCCODE ) / 4096) TM4C1294 Datasheet, pg. 1,068 36

37 TM4C1294 Temperature Sensor 37

38 ADC Voltage to Value Mapping Value (Int.) V Voltage Assumes V Ref = 3.0 V and 10-bit ADC 38

39 ADC Value to Voltage Mapping Value (Int.) V Voltage 666/1024 * 3.0 V = * 3.0 = 1.95 V = V Sensor 39

40 TM4C1294 Temperature Sensor V Sensor = 1.95 V V Sensor = 1.95 V Temp = 1.25 C. Temp = - (((V Sensor V) * 75) + 55) = 1.25 C 40

41 Non-Linear Sensors Voltage Temp C

42 BMP Reading Raw Values 42

43 BMP Calculate Temperature 43

44 BMP Calculate Pressure 44

45 BMP Altitude 45