ADC STUDENT LECTURE Andrew Brown Jonathan Warner Laura Strickland

Table of Contents Signals Applications of ADC’s Types of ADC’s Successive Approximation Example The ADC on the MC9S12C32

Introduction Analog to digital converters convert analog, or “real world” signals to a series of 1’s and 0’s, able to be stored or transmitted through computers or digital systems.

Introduction cont. Reasons why this would be needed: Digital storage of a non-digital signal (ex: recording light intensity of a lightning strike using sensors, mapping a flight path of an aircraft onto a computer for analysis) Transmitting data over a digital system (ex: sending your voice through a telephone system, Skype chatting, etc…)

Analog Signals Analog signals are the smooth, “real”, signals of the world. These signals can contain any and all values needed to represent the data in question.

Digital Signals Digital signals, however, contain series of discrete values, with interpolation occurring between data points to recreate the signal. Digital signals are meant to be used in digital systems, and therefore are composed simply of 0’s and 1’s.

Benefits of Digital Signals over Analog Can be stored in digital system. Can be compressed. Can filter out frequencies you don’t want, analog noise is removed.

How does it work? ADC’s work in two steps: Sampling Quantization

Sampling Let’s look again at our last graph: Our discrete values on the y axis are taken at spaced-out time steps on the x axis. These are the “sampling points”.

Sampling cont. Larger number of sampling points during the same amount of time = smoother looking graph. “Sampling rate” is this frequency at which sampling will occur. Nyquist Theorem: Sampling rate should be 2*highest frequency you want to capture.

Sampling Question If you use a sampling rate of 50,000 Hz for 2 seconds, how many data points are you capturing? What is the distance between each point on the resulting graph of Voltage vs. time?

Quantization “Sampling for y axis” Assigning a binary code value to discrete measurements, stored on a fixed-length variable.

Quantization Noise Since values are rounded to the nearest possible digital value, a certain level of “quantization noise” will occur. Example: In an 8-bit resolution system, a value of will be stored as the digital value 236. Signal to noise ratio measures the noise level by the equation: SNR = 6.02*n dB, for n-bit resolution

Disadvantages of Digital Signals Not a perfect representation of the analog signal Low memory systems give you bad quality output, as resolution or sampling may be low Example: Phone systems use a sampling rate of 8kHz, so all frequencies above 4kHz are canceled. As a result, playing music through a phone sounds muffled and low quality.

Aliasing Aliasing occurs when a signals frequency is above the Nyquist Frequency. The data points captured suggest a lower frequency signal than the one that actually exists.

ADC APPLICATIONS

Sound recording ADC’s are used to convert sound waves into digital signals through the use of computer microphones or sensors. This allows digital storage and transmission of music, voice, and other sound data. Ex: Telephones convert your voice using 8kHz sampling.

Sensors and Data Acquisition Digital sensors output an analog voltage when reading data. Examples: light sensors pressure sensors accelerometers Computers store this data by converting the signal to digital values, used later by computers.

Digital Cameras Photo-sensors on cameras convert photon impacts into voltage outputs. These are then converted to digital values and stored on your camera’s memory card to be recreated later on a computer.

Circuit representation of ADC The general representation of an ADC is shown below. But what is inside the ADC block? How is the data recorded and stored?

TYPES OF ANALOG TO DIGITAL CONVERTERS Jonathan Warner

Overview 1. Parallel Design (Flash) 2. Successive Approximation 3. Dual-Slope 4. Sigma-Delta

Parallel Design (Flash ADC) V ref set to V max Resistors used to divide reference voltage into intervals Comparators used to compare V in to the reference voltages Encoder uses logic gates to convert control logic to binary digital output 2^n-1 comparators

Parallel Design (Flash ADC) Advantages Fastest ADC (gigahertz) Simple Design Can achieve non-linear output Disadvantages 2^n-1 comparers Low resolution Large Die size Prone to glitches (out of sequence output)

Successive Approximation DAC-Based Design Starts by setting MSB D(n-1) to 1 Uses DAC and op amp to determine if bit should remain 1 or be set to zero (greater or less than V res * 2^(n-1)) Next, bit D(n-2) set to 1 and comparison is repeated Output Buffer allows the circuit to read the digital data while the ADC is working on the next sample

Successive Approximation DAC-Based Design Advantages Speed, worst case n clock cycles Conversion time independent of amplitude of Vin Capable of outputting the binary number in serial (one bit at a time) format. Disadvantages Resolution tradeoff with speed

Dual-Slope Integrator-Based Design Switch connects Vin with integrator Switch held for fixed number of clock cycles Analog switched at set time to –V ref T2 clock cycles proportional to Vin V in = V ref x T2/T1

Dual-Slope Integrator-Based Design

Advantages Insensitive to components value errors Can achieve high resolution (but at the cost of speed) Useful for highly accurate measurements Disadvantages Speed, 2^n-1 clock cycles Limited applications

Sigma-Delta Analog signal set to integrator Resulting “sawtooth” waveform compared with zero volts Output either high or low Output converted to positive or negative V res and fed back to be added to next sample’s V in Resulting stream of 0’s and 1’s represents the analog signal average voltage Clock rate used is very high, results in “oversampling” of data

Sigma-Delta Advantages High Resolution No need for precision components Disadvantages Speed, Oversampling Only applicable for low bandwidth

ADC Comparison TypeSpeed (relative) Cost (relative) Resolution Dual SlopeSlowMed12-16 FlashVery FastHigh4-12 Successive Approx Medium – Fast Low8-16 Sigma – Delta SlowLow12-24

Successive Approximation Example Given: 8 bit ADC V in = 0.2 V V ref = 2 V BitVoltage n = 2 8 = 256 V res = V ref / 256 V res = V (Resolution) Find: n bit digital output

Successive Approximation Example (cont.) < < > > < < > > Digital Output

The ATD10B8C on the MC9S12C32 Input Pins ATD10B8C

MC9S12C32 Block Diagram

The Basics of the ATD10B8C Resolution: 8- or 10-bit (manually chosen) 8-channel multiplexed inputs Successive Approximation architecture Can perform single or continuous sampling Can sample single or multiple channels Conversion time: 7 µs (in 10-bit mode) Optional external trigger

ATD10B8C Block Diagram PinPurposes AN7/ ETRIG/ PAD7 Analog input channel 7/ External trigger for ADC/ General purpose digital I/O AN6/PAD6 – AN0/PAD0 Analog input/ General purpose digital I/O V RH, V RL High, low reference voltages V DDA, V SSA Supply power for analog circuitry

Control Register 2 PinDescription 70 – Power down ATD; has recovery time period 1 – Normal ATD functionality 60 – Normal clearing (read CCF before reading result register) 1 – Fast Flag Clearing (auto-clear CCF after result register is accessed) 50 – Continue running in Wait Mode 1 – Halt conversion and power ATD down while in Wait Mode 40 – External Trigger Edge 1 – Trigger Level 30 – Low/falling trigger polarity 1 – High/rising trigger polarity 20 – Disable external trigger mode 1 – Enable external trigger mode 10 – ATD Sequence Complete Interrupt Request disabled 1 – ATD Sequence Complete Interrupt Request enabled 00 – No ATD interrupt occurred 1 – ATD sequence complete interrupt pending

Control Register 3 PinDescription 6 - 3Controls the number of conversions per sequence 20 – ATD Conversion calculation goes to corresponding result register 1 – Current ATD conversion put in consecutive result registers; wraps around sequentially at end 1-0Determines how ATD responds to a breakpoint (see Table 8.5)

Control Register 4 PinDescription 70 – 10-bit resolution 1 – 8-bit resolution 6-5Selects the length of the second phase of the sample time in units of ATD conversion clock cycles. (See Table 8-7) 4-0ATD Clock Prescaler (PRS) (5 bits long). ATD conversion clock frequency is calculated by:

Control Register 5 PinDescription 70 – Data in the result registers is left-justified 1 – Data in the result registers is right-justified 60 – Result register data is unsigned 1 – Result register data is signed 5Continuous Conversion Sequence Mode 0 – Single conversion sequence 1 – Continuous conversion sequences (scan mode) 4Multi-Channel Sample Mode 0 – Sample only one channel 1 – Sample across multiple channels 2-1Selects the analog input channel(s) whose signals are sampled and converted to digital codes (See Table 8-12)

Single Channel (MULT = 0) Single Conversion (SCAN = 0) Port AD ATD Converter Result Register Interface ATDDR0 ATDDR1 ATDDR2 ATDDR3 ATDDR4 ATDDR5 ATDDR6 ATDDR7

Single Channel (MULT = 0) Continuous Conversion (SCAN = 1) Port AD ATD Converter Result Register Interface ATDDR0 ATDDR1 ATDDR2 ATDDR3 ATDDR4 ATDDR5 ATDDR6 ATDDR7

Multiple Channel (MULT = 1) Single Conversion (SCAN = 0) Port AD ATD Converter Result Register Interface ATDDR0 ATDDR1 ATDDR2 ATDDR3 ATDDR4 ATDDR5 ATDDR6 ATDDR7

Single Channel (MULT = 1) Continuous Conversion (SCAN = 1) Port AD ATD Converter Result Register Interface ATDDR0 ATDDR1 ATDDR2 ATDDR3 ATDDR4 ATDDR5 ATDDR6 ATDDR7

Status Register 0 PinDescription 70 – Conversion sequence not completed 1 – Conversion sequence completed (set to 1 after each sequence complete when SCAN mode is on) 50 – No external trigger overrun error has occurred 1 – External trigger overrun error has occurred 40 – No overrun in results 1 – A overrun in results 3-0Conversion Counter (read-only; points to result register that will receive the result of the current conversion)

Status Register 1 PinDescription 7-0Conversion complete flag (one bit is set at the end of every conversion in a conversion sequence, going from CCF0 in order to CCF7) 0 – Conversion # x is not completed 1 – Conversion # x is completed and results are available

Left-Justified Result Register Right-Justified Result Register is similar. Each register has a high and a low byte. 8 Result Registers total ($ $009F)

Setting Up the ATD Step 1: Power-up the ATD and define settings in ATDCTL2 ADPU = 1 powers up the ATD ASCIE = 1 enables interrupt Step 2: Wait for ATD recovery time (~ 20μs) before proceeding Step 3: Set the number of successive conversions in ATDCTL3 S1C, S2C, S4C, S8C determine the number of conversions (see Table 8-4)

Setting Up the ATD Step 4: Configure the resolution, sampling time, and ATD clock speed in ATDCTL4 PRS0, PRS1, PRS2, PRS3, PRS4 set the sampling rate (see Table 8-6) SRES8 sets the resolution to 8-bit (= 1) or 10-bit (= 0) Step 5: Configure the starting channel, single/multiple channel, SCAN setting and whether result data should be signed or unsigned in ATDCTL5 CC, CB, CA determine input channel (see Table 8-12) MULT sets single (= 0) or multiple (= 1) inputs SCAN sets single (= 0) or continuous (= 1) sampling DJM sets output format as left-justified (=0) or right-justified (=1) DSGN sets output data as unsigned (=0) or signed (=1)