[introduction]In this series article, we PCM (pulse encode modulation) the number that regards imitate as signal shows a means. In PCM method, the example of successive wave allows to take certain disperse cost only. Assign a code for these extent next, among them the extent of example of delegate of every code ground. The code word that these regard a number as data is OK get applied below all sorts of circumstances. Modulus converter (ADC) is the equipment that use PCM concept changes imitate signal actually signal of amount to word or circuit. They have a lot of application in industry. For example, a lot of contemporary small controller were deployed inside buy ADC converter. This makes design personnel can join easily with imitate sensor, the environment medium imitate signal changes data of amount to word, undertake handling to its inside small controller in order to be used at all sorts of application.

  

In this series article, we PCM (pulse encode modulation) the number that regards imitate as signal shows a means. In PCM method, the example of successive wave allows to take certain disperse cost only. Assign a code for these extent next, among them the extent of example of delegate of every code ground. The code word that these regard a number as data is OK get applied below all sorts of circumstances. Modulus converter (ADC) is the equipment that use PCM concept changes imitate signal actually signal of amount to word or circuit. They have a lot of application in industry. For example, a lot of contemporary small controller were deployed inside buy ADC converter. This makes design personnel can join easily with imitate sensor, the environment medium imitate signal changes data of amount to word, undertake handling to its inside small controller in order to be used at all sorts of application. Modulus transition can be carried out through all sorts of frameworks, for example successive approachs register (SAR) , collateral (shine put) changeover, Σ – Δ changeover. Digital-to-analogue converter (the task of DAC) and ADC are contrary: It is worth the number changeover to answer successive imitate signal. DAC is used at changing the outcome of several word processing the variable for real world, use at control, information shows or the imitate of other form is handled. Graph the 1 overall block diagram that showed digital processing system.

Graph 1: Use N the phenomenon that ADC and processing system of DAC join number and quantity of imitate world imitate represent real world normally.

In this kind of configuration, main variable involves the such as temperature, physics parameter that waits solely normally, its are changed into voltage or electric current by transducer. Here, use imitate filter will abide by sampling theorem. Placing a filter before ADC is LPF, call fight mix fold filter. This filter is placed before ADC, can eliminate prep above sampling to lead a half (the frequency heft of Fs/2) , during these heft may bring about sampling, appear mix fold. Next, the imitate signal after filter wave changes number word code by ADC module, guide digital processing system, this system can be the data processing of small controller or other form and operation. Later, the digital signal after processing is gone to by feed DAC class, its changeover answers imitate signal. Placing the 2nd filter after DAC module also is LPF, say to attach most importance to compose filter. It still eliminated Kui of prep above Nai this special rate (the frequency of F S /2) . , imitate outputs signal by carry out implement class changeover answers physical world, in order to undertake any farther physics are operated. For example, in configuration of frequency signal processing, the imitate frequency signal that ADC captures microphone changes signal of amount to word, use at be based on the phonic effect processing of the computer. Next the digital signal changeover after DAC will be handled returns imitate form, can broadcast through loudhailer. Be collected in appearance of contemporary electron, instrument, IT, data and transmit, control system, medicine is become picture, with consumptive frequency / in video and computer graph, imitate signal word of changeover amount to already made a primary process. In the article, the crucial performance standard that we discussed the effectiveness in defining ADC to apply in its.

Quantify an error

A variety of error sources exist in change-over circuit. Among them, quantify an error (Q E) or quantifying uncertainty is one of crucial factors that showing influence A/D or D/A converter performance. When be worth for disperse number approximately when successive imitate signal, can appear in modulus changeover quantify an error. In PCM coder, every voltage example already round (quantify) to adjacent usable n, change next for its corresponding binary system code. Contemporary code is changed in decoder place when answering imitate, allow emersion He She to enter an error. Theoretic, changeover forever won’t 100% accurate; That is to say, in transition, the information of limited amount will be lost forever. This is meant when the number states changeover answers imitate, result general and primitive weaveform are different. We will pursue the 2 block diagram that regard 3 A/D as converter.

Graph 2: 3 ADC block diagram

Apparent, 3 ADC have 8 numbers (quanta) n. If the output number result of this system and the comparison that imitate inputs pursue 3 are shown, and the graph wins the bid gave the typical example of Q E.

Graph 3: The number of weaveform of 3 ADC imitate shows

Now, we can see quantified effect. Graph the transmission of the ADC of 4 3 sheet pole that showed full span voltage is 1V is characteristic.

Graph 4: Full span voltage is the characteristic graph of 3 ADC of 1 bend over

Graph 4 express to be quantified 3 times implement, it goes to map of a series of imitate input value only 8 (2 3) potential output number value. If input the peak peak value of signal to be 1 V, criterion every step in a flight of stairs (below good case) the size of edge Y axis is same, formulary definition is 1 LSB under the basis (effective) : Voltage. Below this kind of circumstance, 1 LSB is equal to 1/8 V (or 125 MV) . For example, below these conditions, the value of faultlessly encode 300 MV. Adjacent usable value is a binary system 010, it produces 250 MV. Apparent, gets rounding off result can express in the number in produce a few errors. In ideal hypothesis, the character that changes a system can be a linear diagonal that does not have step. But actually, the finite value that ADC knows associate to be built beforehand chooses individual disperse value to state every imitate inputs example to quantify sampling signal in list. The transfer function that this regulation makes imitate inputs a number to output is had unified ” a flight of stairs ” characteristic. The actual imitate that every sampling dot is in is worth and quantify a number to was worth the perpendicular difference between to define quantify an error (Q E) . Graph 5 quantifying an error mediumly to pursue is by a flight of stairs the real value of function is subtracted the ideal of linear function is worth. The half that quantifies the extent of the error to be equal to quanta level (Q/2) , among them Q is the width with individual long situation. So, q E can be in ± (1/2)LSB or ± (the wave motion inside the limits of Q/2) , if pursue,5 are shown.

Graph 5: Quantify error feature graph

The result is zigzag error voltage, expression inputs the white noise in signal to add imitate. Quantifying an error is actual voltage, because it can change signal range. Accordingly, quantify an error to also be called quantify noise (Q N) . The digit that should be used at changeover (when N) is lesser, quantify an error normally bigger, because accurate represent successive signal quantify level less. As the addition of digit, quantify an error to become smaller, can represent primitive imitate signal well and truly thereby. Actually, can reduce the error negligible in a lot of application small cost. Semaphore changes noise to compare (SQNR) is primitive imitate signal (during the power of P S) and modulus are changed, introduce quantify noise (the magnanimity of the rate between the power of P Qn) . However, hypothesis ADC does not have random noise relatively, and can measure changeover easily. Next, can use equality normally 1 calculate for the unit with DB signal and quantify noise to compare (SQNR) .

Formulary 1: Computational semaphore changes noise to compare

The ideal N that is wave of complete amplitude sine in input signal in converter setting, can use formula 2 affirmatory and corresponding SQNR.

Formulary 2: Computational N the SQNR of ADC

This gave out N the good value of converter, make clear 1 to every increase resolution, SQNR can rise make an appointment with 6 DB. With quantify error photograph to compare, SQNR is the main index that evaluates modulus to change quality. SQNR cost is higher, state precision is taller, quantify noise logarithm word denotive impact is less.

A/D and D/A change performance standard

The standards of the norms that affects ADC performance and DAC is similar. Besides SQNR, those who decide D/A and A/D converter function is a few otherer main factor is rate of resolution, sampling, speed, precision and dynamic limits. The face undertakes explanatory to them below. Resolution: In A/D system, resolution is the inputs upright voltage change that the system can detect, change its the corresponding variation that counts word code to output end. Same, to D/A circuit, resolution is the outputs imitate signal change that shows circuit can arise. D/A or A/D IC manufacturer normally in order to count the digit in word code (N) or system are effective (the voltage of LSB) correspondence will appoint resolution. Another kind of method that conveys resolution is the directive quantifies the voltage between level rank jump extent, also call quantify width (Q) . To N DAC, the weight of LSB is 2 -n. For example, when the binary system inputs code to increase a LSB, OK and analytic full span outputs 8 DAC a part in the 2 8 of voltage or 0.39% . So, to full span voltage (V Min) of – of V FS = V Max is equal to 10 bend over, the resolution of 8 systems is bend over of 0.039 (= 10/2 8) . Generally speaking, can adopt formula the 3 formal calculation with voltage.

Formulary 3: Computational N the resolution of ADC

Sampling rate: Sampling rate states every unit time undertakes to imitate signal sampling changes the frequency of number word code. To undertake A/D is changed correctly, sampling rate must be at least by double of the imitate signal frequency of sampling, in order to satisfy Nai Kui this special sampling standard. Be in the sample that collects inside quantum of time surely is more, in order to count word form denotive imitate signal is more accurate. Speed: To A/D converter, speed is appointed to be changeover time, it expresses to finish the time that individual transition place spends, include to undertake to imitate signal sampling, processing is mixed generate digital output. In A/D converter, must consider to change speed and other sequential element to decide the sampling of converter is led. To D/A converter, speed is appointed to be stabilization period, it is input place occurrence binary data and output voltage obtain the delay between stable value. This set the data rate that converter can treat. Precision: Of the output that precision is converter and value of actual imitate signal accord with degree. As a result of quantify a process and produce rounding error, cause as certain as existence of actual imitate value error. As the addition of digit, quantify the pace between level to grow reduce, realize taller precision when be changed between imitate signal and digital signal thereby. For example, 8 words (N = 8) offers 256 different values (2 8) undertakes expression, more different than making appliance has 16 value (4 words of = 2 4) are offerred more imitate signal changeover. Dynamic limits: Dynamic limits is to show ADC is OK limits of extent of accurate in its number output denotive signal, and won’t show loss precision. In other words, dynamic limits is ADC of OK and effective processing with the input the difference between signal n is worth. Dynamic limits expresses to be an input voltage and can detect the rate of voltage, subsequently changeover is decibel. Dynamic limits (the computation of DR) by formula 4 definitions, combined logarithm (DB) and linear (voltage) respect.

Formulary 4: Computational N the dynamic limits of ADC

Full span voltage (V Min) of – of V FS = V Max is the voltage range that ADC uses at stating imitate inputs signal. For example, if ADC uses the referenced tension of bend over of V Ref = 5, input voltage to should fall in the ability inside this limits to realize changeover. Study tension to 12 ADC (n = 12) and ginseng of 5 bend over, dynamic limits can evaluate as follows: Dynamic limits (it is an unit with DB) 72 DB of ≈ of = 20 Log (2 12) = 20 Log (4096) or, dynamic limits (volt) bend over of = 5.2 12 = 5 (4096) = 20480 must remember, electronic component (include converter) the influence that all performance data can be changed by power source voltage and temperature. Data table appoints these parameter below specific temperature and condition of power source voltage normally, in order to provide standardization information. However, in effective system, operation condition likelihood and appoint data to have very big error. Accordingly, real property summarizes in likelihood and data table differ somewhat.

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