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Data Converters-II

 Analog Electronics (EEE/INSTR  C341)

Date: 23/04/2016

 Dr. Anita Agrawal,

 BITS, Pilani-K.K.Birla Goa Campus

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D/A CONVERTERS

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Sampling frequency: Nyquist criteria/Sampling

theorem which states that:

“In order to represent/convert the analog signalcorrectly, the sampling frequency f sample must be

atleast twice the highest frequency component

f a(max) of the analog signal.” 

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  alias

The highest analog frequency can be no greater than

one-half the sampling frequency

The frequency f a(max) is known as the Nyquist

frequency

f sample ≥ f a (max)

Nyquist criteria contd…… 

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Nyquist criteria contd…… 

• If an input frequency component is greater than the

 Nyquist frequency, then a unique form of distortion

called ‘alias’  is produced which is equal to the

difference in the input frequency and the samplingrate.

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 Amplitude resolution

•   It represents the finest discernible change in the signal and is

often represented in terms of no. of bits.

• Important to note that once the size of the digital word is

chosen and the peak amplitude is fixed, the input signal must

stay within the specific bounds or gross distortion will occur.

For ex……. 

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• Applied to DSP: performs operations on incoming data.

• D/A conversion

• Reconstruction filter : Low pass reconstruction filter to

smoothen the DAC output by eliminating high frequency

contents that result from the fast transitions of the stairstep

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Other D/A converters

• Inverted R-2R ladder D/A converter

• Switched-Capacitor D/A converter

• Serial D/A converter

• Pipelined D/A converter

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Inverted R-2R

 Anita Agrawal

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Serial D/A converter

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Drawbacks• Requires considerable external circuitry to decide

upon which switch is to be open and which to close

during the conversion process.

• Errors due to: –  Parasitic capacitances

• Capacitive parasitic capacitances

• Switch parasitic capacitances

 –  Clock-feed through

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• Conversion speed is slower

Advantages:Can be realised in a small chip area

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Pipelined DAC

• Modified serial charge distribution converter.

• (n+1) equal valued capacitors, 4n+2 switches for n-bit

conversion

• Cascades n+1 serial charge redistribution converters

•  Uses a new clocking scheme.

• Has an improved conversion scheme than the serial… 

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Drawbacks

•  Non-ideal effects in the converter affects the

conversion accuracy.

 –  Capacitor mismatch

 –  Incomplete charge transfer due to non-zero R switch C timeconstants

 –  Clock feedthrough due to overlap capacitances, channel

charges and charge pumping in the switches.

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