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DAC Speed and Glitches

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C) Finite Amplifier Speed

Suppose you put a water glass under an open faucet. Eventually, the glass overflows. But it doesn't overflow instantaneously -- it has to fill up first. Similarly, when a potential changes on an amplifer input, there is a delay, then a transition time, before the output tracks the input. Thus, even after the glitches are gone, the output amplifier on the ladder network takes some time to respond to the changed setting. How long? If R in the various diagrams is ~ 10 kΩ and the amplifier input capacitance is 1 pF, RC = 10-8 s. After 50 ns, the amplifier has settled to with e-5 of its final value or 0.67%. That's 1 part in 148, or just a little better than 1 part in 27 = 1 part in 128. For a 10 bit converter, to settle to 1 part in 210 means waiting until enough RC time constants have passed to be within 1 part in 1024 or 0.0977%. That's about 7 RC time constants or 70 ns.

Exercise. How long must one wait for a 16 bit DAC to settle? Assume there are no glitches, only RC delays in the amplifer circuit with 10 kΩ and 1 pF.

 

   
 

 

 

 

 

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