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ADC Sample Time Calculator

Work out adc sample time instantly with clear inputs, formula shown and shareable results.

About ADC Sample Time Calculator

The sampling capacitor must charge to within half an LSB, which needs (n+1) x ln2 time constants where tau is source impedance times sample capacitance. A 10 kilohm source into 8 pF at 12 bits needs about 0.7 microseconds — far longer than the minimum the datasheet allows.

Formula

ADC settling

tau = Rsource x Csample; sample time >= tau x (n + 1) x ln2

Worked example

With the default values below, ADC Sample Time Calculator returns Minimum sample time of 0.72. Change any input above and every figure here is recalculated in your browser.

Inputs used

Source impedance
10,000 Ω
Sample and hold capacitance
8 pF
ADC resolution
12 bits
ADC clock
14 MHz

Results

Minimum sample time
0.72 µs
Input time constant
0.08000 µs
Sample cycles required
11 cycles
Maximum sample rate
560.00 kHz

Step by step

  1. 1Values used
  2. 2ADC settling
  3. 3Minimum sample time= 0.72 µs
  4. 4Input time constant= 0.08000 µs
  5. 5Sample cycles required= 11 cycles
  6. 6Maximum sample rate= 560.00 kHz

Assumptions and limits

  • The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
  • In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
  • Figures are rounded for display to 5 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

Why does source impedance matter so much?

It sets the charging time constant. A high-impedance sensor read too quickly gives a reading biased toward the previous channel — the classic multiplexer crosstalk symptom.

How do I fix a slow source?

Buffer it with an op-amp follower, or add a capacitor at the ADC pin much larger than the sampling capacitor to supply the charge locally.

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ADC Sample Time Calculator – Calculate Minimum sample time