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Published byIliana Shepley Modified about 1 year ago

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A NALOG TO D IGITAL C ONVERTERS (ADC S ) & D IGITAL TO A NALOG C ONVERTERS (DAC S )

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A NALOG TO D IGITAL C ONVERSION (ADC) Analog to Digital Conversion is the process by which analog signals are converted to their digitized forms. ADC occurs via three steps: - Sampling. - Quantization. - Encoding.

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T HE IDEA OF S AMPLING The analog signal is sampled at regular intervals of time. The sampling rate must be at least twice the highest frequency of the signal ( Nyquist Criterion ).

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Q UANTIZATION Quantization is defined as the process of converting the continuous sample amplitude into a discrete amplitude. Thus by then, the signal will be discrete in both; time and amplitude.

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Q UANTIZATION E RROR The difference between the actual analog value and quantized digital value.

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E NCODING Encoding is the process which assigns ones and zeros (stream of bits) for every quantization level. The number of bits assigned for each level ( n ) depends on the levels’ number ( L ) ; such that L= 2^n.

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I DEAL ADC Q = Δ=quantization step = full scale / levels’ number. In an ideal analog-to-digital converter, the quantization error is uniformly distributed between –Δ/2 and Δ/2. The resolution of the ADC is the smallest detectable change in voltage.

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T YPES O F ADC S There are many types of ADCs such as: 1. Direct Conversion ADC. 2. Successive Approximation Register (SAR) ADC. 3. Integrating ADCs: Single slope, dual slope, and ramp ADC. 4. Sigma-Delta ADC (over sampled ADC). 5. ADC0804 Analog to Digital Converter. In our experiment ADC0804, and the SAR ADC are explained.

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ADC0804 A NALOG TO D IGITAL C ONVERTER ADC0804 is a 20-pin IC with an 8-bit resolution. The analog input voltage range is from 0 V to 5 V, with 15 mW power consumption and 100 us conversion time.

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SAR ADC It is one of the popular ADCs for 8-16 bits. It has moderate conversion speeds ; the conversion time is around 1 Us. It doesn’t consume a lot of power and its cost is low in comparison with the other types. On the other hand, it requires a sample and hold circuit and it can have missing output codes.

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T HE B LOCK DIAGRAM OF SAR ADC AND ITS FLOW CHART

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D ESCRIPTION OF OPERATION A successive approximation ADC works by using a digital to analog converter (DAC) and a comparator to perform a binary search to ﬁnd the input voltage. A sample and hold circuit (S&H) is used to sample the analog input voltage and hold (i.e. keep a non-changing copy) the sampled value whilst the binary search is performed. The binary search starts with the most signiﬁcant bit (MSB) and works towards the least signiﬁcant bit (LSB). For a 8-bit output resolution, 8 comparisons are needed in the binary search, taking a least 8 clock cycles. The sample and hold circuit samples the analog input on a rising edge of the sample signal. The comparator output is a logic 1 if the sampled analog voltage is greater than the output of the DAC, 0 otherwise.

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Example: Vref=5V, Vs=3.127V, 4-bits.(b1b2b3b4) 1.Set b1=1 all b2b3b4=000 >> 1000 Vdac=8*5/(2^4)=2.5V (Vdac=2.5)>(Vs=3.127) No!! b1=1 2. Set b2=1 all b3b4=00 >> 1100 Vdac=12*5/(2^4)=3.75V (Vdac=3.75)>(Vs=3.127) Yes b2=0 3. Set b3=1 b4=0 >> 1010 Vdac=10*5/(2^4)=3.125V (Vdac=3.125)>(Vs=3.127) No!! b3=1 4. Set b4=1 >>1011 Vdac=11*5/(2^4)=3.4375V (Vdac=3.4375)>(Vs=3.127) Yes b4=0 Final Result=1010

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D IGITAL T O A NALOG C ONVERSION ( DAC) Digital to analog converter (DAC) is the device which converts digital signals to analog ones. Most of the DACs consist of a network of resistors and analog switches. The switches control the currents or voltages that are derived from a particular reference voltage and provide analog output current.

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T YPES O F DAC S There are many types of DACs such as: 1. Weighted resistor DAC. 2. R-2R DAC. 3. General purpose DAC ( DAC 0800). 4. Frequency to voltage converter. 5. Pulse width modulation. In our experiment, R-2R DAC and the general purpose DAC are explained.

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R-2R DAC The Block diagram of R-2R DAC is as shown below: The analog output is given by the following equation: The highest resistance value is only 2R and thus requires an area inside the IC less 8 times than that of the weighted resistor DAC which highest resistance is 16R.

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The DAC analog output is represented by current. This current is the ratio of the input code to the full scale voltage (Vref) as given in the following equation: So, an operational amplifier is needed to convert the current to voltage level.

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T HE G ENERAL P URPOSE ( DAC 0800) The block diagram of DAC 0800 is as shown below: It’s clear that it’s a 8-bit DAC with 16 pins IC. The conversion time is around 100ns.

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B IPOLAR / U NIPOLAR DAC S For unipolar DACs, the output voltage is given by: For bipolar DACs, the output voltage is given by:

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