2.2.11.4 General Formula for Sampling Rate

The general formula for calculating both the total system sampling rate and the per‑channel sampling rate is as follows.

Total Sampling Rate

Total Sampling Rate = (Total number of samples) / (Total conversion time of all samples)

Channel Sampling Rate

Channel Sampling Rate = (Total number of samples for the channel / Total number of all samples) × (Total Sampling Rate)

The method used to derive these formulas is illustrated in the following examples.

Example: Equal Weight and Equal Conversion Time

In this example, all channels have an equal conversion time of 2 µs, as shown in the following figure.

Figure 2-32. Equal Weight and Equal Conversion Time
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There are 10 samples that require a total of 20 µs to complete. Therefore, the total system sampling rate is:

10 / 20 µs = 500 kS/s

Each channel is sampled equally, resulting in the following per‑channel sampling rates:

  • Channel 1: 0.20 × 500 kS/s = 100 kS/s
  • Channel 2: 0.20 × 500 kS/s = 100 kS/s
  • Channel 3: 0.20 × 500 kS/s = 100 kS/s
  • Channel 4: 0.20 × 500 kS/s = 100 kS/s
  • Channel 5: 0.20 × 500 kS/s = 100 kS/s
Example: Unequal Weight and Equal Conversion Time

In this example, the channels are not equally weighted in the sampling sequence,as shown in the following figure.

Figure 2-33. Unequal Weight and Equal Conversion Time
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There are again 10 samples that take a total of 20 µs, resulting in a total system sampling rate of 500 kS/s. However, because the channels are sampled with different weights, the per‑channel sampling rates differ:

  • Channel 1: 0.50 × 500 kS/s = 250 kS/s
  • Channel 2: 0.10 × 500 kS/s = 50 kS/s
  • Channel 3: 0.10 × 500 kS/s = 50 kS/s
  • Channel 4: 0.10 × 500 kS/s = 50 kS/s
  • Channel 5: 0.20 × 500 kS/s = 100 kS/s
Example: Unequal Weight and Unequal Conversion Time

In this example, channels have different conversion times and are not equally weighted in the sampling sequence, as shown in the following figure.

Figure 2-34. Unequal Weight and Unequal Conversion Time
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There are 12 samples completed in 20 µs, resulting in a total system sampling rate of 600 kS/s. The per‑channel sampling rates are:

  • Channel 1: (7 / 12) × 600 kS/s = 349 kS/s
  • Channel 2: (2 / 12) × 600 kS/s = 99.6 kS/s
  • Channel 3: (1 / 12) × 600 kS/s = 49.8 kS/s
  • Channel 4: (1 / 12) × 600 kS/s = 49.8 kS/s
  • Channel 5: (1 / 12) × 600 kS/s = 49.8 kS/s

Wait States and the Analog System Controller

The Analog Sample Sequence Controller (ASSC) automatically inserts wait states to prevent processing collisions between samples. Wait states are inserted to ensure the following condition is met:

Current conversion time + ASSC processing time ≥ previous sample processing time (SMEV + SMTR)

The wait‑state duration is calculated as:

(Current conversion time + ASSC processing time) − previous sample processing time (SMEV + SMTR)

This additional time is included in the sampling‑rate calculation and reduces the total achievable sampling rate. When wait states are present, the calculated sampling rate represents an approximation.

Example: Single Channel, Continuous Loop
  • System clock: 40 MHz
  • ADC clock: 10 MHz
  • Acquisition time: 0.2 µs
  • Resolution: 8‑bit
  • Flags: None
  • The conversion time is calculated as: Tconv = Tsync_read + Tsample + Tdistrib + Tcal + Tsync_write = 25 ns + (2 × 100 ns) + (8 × 100 ns) + (2 × 100 ns) + (2 × 25 ns) = 1.25 µs
  • Including ASSC turnaround time: 1.25 µs + 0.25 µs = 1.5 µs
  • Resulting sampling rate: 1 / 1.5 µs = 666 kS/s