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Converting Analog to Digital

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What issues must we consider when converting analog to digital?

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Signals in the real world are analog: light, sound, etc. Real-world signals must be converted into digital, using a circuit called ADC (Analog-to-Digital Converter), before they can be manipulated by digital equipment.

When you scan a picture with a scanner what the scanner is doing is an analog-to-digital conversion: it is taking the analog information provided by the picture (light) and converting into digital.

When you record your voice or use a VoIP solution on your computer, you are using an analog-to-digital converter to convert your voice, which is analog, into digital information.

Digital information isn't only restricted to computers. When you talk on the phone, for example, your voice is converted into digital (at the central office switch, if you use an analog line, or at you home, if you use a digital line like ISDN or DSL), since your voice is analog and the communication between the phone switches is done digitally.

When an audio CD is recorded at a studio, once again analog-to-digital is taking place, converting sounds into digital numbers that will be stored on the disc.

Whenever we need the analog signal back, the opposite conversion - digital-to-analog, which is done by a circuit called DAC, Digital-to-Analog Converter - is needed. When you play an audio CD, what the CD player is doing is reading digital information stored on the disc and converting it back to analog so you can hear the music. When you are talking on the phone, a digital-to-analog conversion is also taking place (at the central office switch, if you use an analog line, or at you home, if you use a digital line like ISDN or DSL), so you can hear what the other party is saying.

The more sampling points we use - i.e. the higher the sampling rate -, the more perfect will be the analog signal produced by the digital-to-analog converter.

The more samples we capture more storage space is necessary to ...

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1. Using Multisim, construct a simple 0-5V DC digital voltmeter using the ADC (rather than the ADC0804) and the 74185 IC to convert the binary output into BCD, which will be displayed on two 7-segment displays. The 74185 IC will be used to convert a 6-bit binary number to a 2-digit BCD number. Note that the 2 least significant A/D output bits are left unconnected (using only 6 bits of resolution). Calibrate the digital voltmeter by applying 5V to the analog in (measured with a DVM) and adjusting the potentiometer until the 7 segment display reads 5.0 (the decimal point is assumed to be in the middle). Test the accuracy of the digital voltmeter with several different values for analog in.

2. Using Multisim, use the A/D converter to digitize a sine-wave input signal and then reconvert the representative digital signal into an analog signal with the DAC (in Multisim this is called a VDAC). Connect the input of the D/A converter to the output of the A/D converter. Make sure the MSB out is connected to the MSB in. Adjust the input sine wave to 2.5Vp-p with a DC offset of 1.25 volts (i.e., the sine wave goes from 0 to 2.5V). Adjust the Vref/2 voltage using the potentiometer so that the analog input span on the ADC is 0 to 2.5V. Use an oscilloscope to compare the analog input and output waveforms. Set the analog input signal frequency to 200Hz and observe the resultant analog output signal. Increase the input signal frequency and note the effect on the resultant output signal.

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