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Like the circuit in the figure below, i have an Amplifier that is driving an ADC with 10 bits (3.3v Ref). Its sampling frequency is 10kHz and the signal is a sine wave with a frequency 1kHz and 1Vp. Does in this situation, where amplifier is just tracking a sinusoidal signal and not a pulse, the settling time of the amplifier affects the signal accuracy ?

I know that if i'm measuring a pulse or a switching between signals, i need to wait the settling time of 0.1% to match the adc resolution(10bits), but when we are talking about sine waves and the transition is not a step 0-1V, how does it work ?

figurecircuit

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  • \$\begingroup\$ If it's a pure sine the most interesting parameters are the things that have "bandwidth" in their names, like Gain-Bandwidth-Product or Unity-Gain-Bandwidth. Since you have unity-gain (gain = 1) and it's 1kHz, I'm going to guesstimate the tracking will be phenomenal in all somewhat easy to find op-amps I can think of, if they are operating within specified conditions. \$\endgroup\$ – Asmyldof Apr 20 '17 at 0:27
  • \$\begingroup\$ Get hold of a free sim and simulate it. There will be a slight phase lag in the output sinewave compared to the input sinewave. That lag will get bigger as frequency rises. \$\endgroup\$ – Andy aka Apr 20 '17 at 7:25
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The ADC likely grabs a bunch of charge as it begins the conversion. That charge is chopped up and compared against other internally generated charges, during the binary-approximation process.

Some OpAmps tolerate that initial "grabs a bunch of charge" and some opamps do not. Adding a RC LowPassFilter between OpAmp and ADC provides the sudden charge demands to the ADC.

The ADC probably has 10pF input sampling capacitor. Or maybe 5pF. An external capacitor of 1,000pF provides room for 100:1 settling, immediately. You'll need to wait a few more Tau for the ADC voltage to reach 10 bits.

An RC using 1Kohm and 1,000pF should be an easy load for the OpAmp, with 1uS Tau.

Is your chosen opamp fast enough to recharge the 1,000pF quickly?

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  • \$\begingroup\$ When you say fast enough, are you referring to the slew rate ? The ADC has a capacitor of 25pF and OpAmp and has a settling time of 3uS, so according to you, i have to have a tau higher than 3, correct ? \$\endgroup\$ – Impe_dancer Apr 20 '17 at 10:53

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