Data rate, and the bits that are noise

Where the quiet comes from

This is a delta-sigma converter, which means it samples its input far faster than it reports — 250 000 times a second internally — and then averages those samples down to whatever output rate you asked for.

So the rate setting is really an averaging setting. Ask for 8 conversions a second and each one is an average of about 31 000 internal samples. Ask for 860 and each one is an average of 290.

The lower bars are every rate at the range you have selected, so the shape of the trade is visible at a glance: flat across the slow half, falling away at the top.

What TI actually measured

With the inputs shorted together — so the only thing being measured is the converter's own noise — the peak-to-peak figures come out like this at ±2.048 V:

Up to 128 SPS the noise is exactly one count. The converter is as quiet as its own resolution, and the last bit means something. Above that it stops being true, and it gets worse in the narrow ranges: at ±0.256 V and 860 SPS, under fourteen bits are real.

You are still reading a sixteen-bit number. The last digits of it are just random.

Which to pick

Start at 8 or 16 SPS. A battery, a temperature, a water level, a load cell in a scale — none of these move fast enough to care, and the slowest settings are free precision. 8 SPS also lines up neatly with 50 and 60 Hz mains, which is part of why TI quotes its best rejection figures there.

Move up when something forces you to. Four channels read in turn at 8 SPS takes half a second per sweep. If that is too slow, 128 SPS costs nothing in bits and gets the sweep down to 31 ms.

Above 250 SPS, know what you are buying. It is the right corner for a current spike or a waveform, and the wrong one for anything you are going to put a unit on.

Single-shot, and the 150 µA

The chip powers up in single-shot mode, which is its default: it sits in a power-down state drawing about half a microamp, wakes in 25 µs when asked, converts once, and goes back to sleep. Continuous mode never sleeps and draws about 150 µA.

For a data logger taking a reading every minute, single-shot at 8 SPS is essentially free — 125 ms of work an hour. That is what the mode is for, and most libraries use it by default without mentioning it.