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ADC ref Voltage by pass capacitor

Bypass cap on reference voltage?

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For a voltage reference providing a reference to multiple devices, ADC, DAC, instrumentation amp… would there be some benefit in placing bypass caps right at the reference input to these devices even though they have high input impedance?noisevoltage-referencebypass-capacitorshareedit  follow  flag asked 10 hours agozsky333331122 silver badges77 bronze badges

  • 1depends on the device datasheet – Tony Stewart Sunnyskyguy EE75 9 hours ago 
  • 1Well, it depends on many things, including: (1) How accurate you want your applications, say, 8 bit or 24 bit resolution, and if you want as, or more accurate as possible. (2) How many pieces of equipment you are going to make, say 10 pieces, or 5,000. In short, it is an engineering trade off and cost benefit analysis of money, time, reliability, risk etc etc. Now let me come back to your question. (a) You already hinted that the equivalent total device input impedance at the voltage reference is “high”, say all MOS of order 100M, then / to continue, … – tlfong01 9 hours ago   
  • 1Then the by pass cap with a typical value of 10uF would source “negligible” current, and increasing its value to say, 100uF, won’t make much difference in keeping the reference voltage source more stable, therefore the ADC/DAC etc more accurate, except wasting space. In other words, there is not much high frequency voltage spikes for the cap to filter or by pass. But then beside the “by passing” function, there is the cap’s “decoupling” function to consider, / to continue, … – tlfong01 8 hours ago    
  • 1To understand what is the meaning of “decoupling”, I usually recommend to read the following two articles: (R1) “Clean Power for Every IC, Part 1: Understanding Bypass Capacitors – Robert Keim, 2105spe21”: allaboutcircuits.com/technical-articles/…, (R2) “Clean Power for Every IC, Part 2: Choosing and Using Your Bypass Capacitors, Robert Keim, 2015sep27”: allaboutcircuits.com/technical-articles/…. / to continue, … – tlfong01 8 hours ago   
  • 1Ah, lunch time, so I would continue my comments this “Gloomy Locking Sunday Afternoon”: youtube.com/watch?v=9dZj7YW5oFQ, 11,080,814 views 2012nov03. Cheers – tlfong01 8 hours ago    
  • 1I found that to understand “decoupling”, I need to read the third article of the series by the by-pass cap expert Robert Keim. But his third article is too boring for a already gloomy Sunday. So I gave up digging deeper, though I was curious to know if the ADC design engineers do use decoupling capacitors for the reference voltage source. I casually googled the popular MCP3008 ADC datasheet and happily found that those MicroChip enggrs do use bypass caps!. Now if the professional engineers do it, of course me hobbyist do it: i.imgur.com/SWa5rG5.jpeg. I call it a day. Cheers. – tlfong01 3 hours ago    

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1 Answer

ActiveOldestVotes4

Modern ADCs use charge_balancing or charge_eating conversion methods.

That means the VREF must supply big hunks of charge, very fast, or the internal binary_search behaviors will make WRONG DECISIONS.

So yes, you need a bypass capacitor on the VREF signal.

ADCs use approximately 10pF in their internal charge_eating. I’d make the cap at least

  • 10pF * 2^number_bits

Thus for a 16_bit ADC, use 10pF * 2^16 == 10pF * 64,000 == 0.6uF

For delta_sigma (or sigma_delta) oversampling ADCs, the conversion system will grab many hunks of charge during any single conversion. See what the datasheet or the manufacture Apps Engr suggests.shareedit  follow  flag answered 4 hours agoanalogsystemsrf29k22 gold badges1010 silver badges3939 bronze badges

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