Analog to Digital Converters

Posted on Feb 5, 2014

The successive approximation Analog to Digital Converter(ADC) is probably the most common ADC. It requires few parts, and is simple to operate. Also, it always takes exactly the same amount of time to calculate the result. In a nutshell, it consists of a comparator, a digital to analog converter(DAC), and a logic controller. The logic controller u

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ses the DAC to produce a voltage, and checks the output of the comparator to see if it`s over or under the unknown voltage. It then makes an adjustment, and checks again. It always takes exactly the number of steps that there are bits of resolution. IE, an 8 bit successive approximation ADC takes 8 steps to perform the conversion. A 16 bit ADC takes 16 steps, and so on. To be more specific, the controller first sets the most significant bit(MSB) of the DAC. This generates a voltage that is effectively half of the maximum output voltage. It then checks the output of the comparator. If the comparator indicates that the output of the onboard DAC is greater than the unknown voltage, the controller clears the MSB. Otherwise it keeps the MSB set. Then it sets the second to most significant bit, and performs the same test. Each time that a bit is tested, the ADC gets half-way closer to the unknown voltage. In theory it will never quite get there, but in reality it gets there within our measurable margin of error. Once every bit on the input to the DAC has been tested, the input to the DAC becomes the output from the ADC! In this way, the successive approximation ADC will always arrive at the most precise and accurate sample possible in the same amount of time. The ramp ADC: The older ramp type is no longer used, because it is functionally similar to the successive approximation type, but varies in the amount of time required to...

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