ADS1256 Sampling

The ADS1256 is a 24-bit delta-sigma ADC with an internal 8-channel MUX. The firmware uses it in differential mode to read three geophone channels and packs the results into the RS-422 data stream at the configured sample rate.

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Channel Assignment

ADS1256 Differential Pair	SEED Channel	Geophone Orientation
AIN0 / AIN1	EHZ	Vertical
AIN2 / AIN3	EHN	North–South
AIN4 / AIN5	EHE	East–West

Channel assignments are defined in config.yml via the adc_channel field (0, 1, 2) and are pushed to the MCU at startup as part of the Settings Handshake.

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The 4-Call MUX Cycle

The ADS1256 has a one-conversion pipeline delay: when you write a new MUX register the change takes effect on the next conversion, not the current one. This means a naive 3-call loop would read:

Call 1 → MUX=Z → ADC still settling from previous → WRONG
Call 2 → MUX=N → reads Z
Call 3 → MUX=E → reads N

The firmware solves this with four calls per output sample, discarding the last result:

1. cycleDifferential(AIN0/AIN1) — MUX set to Z; ADC returns previous channel (discard on very first loop).
2. cycleDifferential(AIN2/AIN3) — MUX set to N; ADC returns Z ✅.
3. cycleDifferential(AIN4/AIN5) — MUX set to E; ADC returns N ✅.
4. cycleDifferential(AIN0/AIN1) — MUX back to Z; ADC returns E ✅. Discard this result.

After the 4th call the MUX is primed at channel 0 again, and the cycle repeats cleanly.

Never average on the MCU

The ADS1256’s internal sinc³ filter already performs anti-alias decimation at the hardware level. Applying a software box-filter average on top would degrade the frequency response and introduce a poor rectangular-window roll-off. Always transmit raw samples. Decimation and filtering belong in the daemon.

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Timing Budget

The ADS1256 output data rate (DRATE) must be fast enough to complete three differential conversions within one output sample period.

Each conversion takes 1 / DRATE seconds. With 3 channels (plus the discard call = 4 total) and a 10 % SPI overhead margin:

Rearranging:

The settings validator in rpi-seism-common enforces the stricter ×13 rule (9 filter cycles × 1.10 safety margin → rounds up to 13) to guarantee stable settling:

Output rate	Minimum DRATE	Recommended setting
100 Hz	1300 SPS	DRATE_2000SPS (index 11)
200 Hz	2600 SPS	DRATE_3750SPS (index 12)
500 Hz	6500 SPS	DRATE_7500SPS (index 13)

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PGA (Programmable Gain Amplifier)

The ADS1256 includes an integrated PGA that sets the input voltage range. For the GD-4.5 + LT1167 signal chain with ~×5.5 analog gain, the geophone signal rarely exceeds ±20 mV. The default setting is:

Enum	Index	Input range	Use with
PGA_64	6	±78.125 mV	GD-4.5 + LT1167 (~×5.5 gain)
PGA_32	5	±156 mV	GD-4.5 direct or low-gain amp
PGA_8	3	±625 mV	High-output piezo sensors

Tip

If a proposed rev2 FDA stage (ADA4932-1, ×100) is fitted, drop to PGA_1 (±5 V) or PGA_2 (±2.5 V) since the amplifier will already be driving the ADC near full-scale.

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VREF

The ADS1256 reference voltage directly sets the ADC full-scale range and appears in the StationXML sensitivity calculation:

The default vref = 2.5 V with PGA_64 yields approximately 215,384,678 counts/V before the instrument sensitivity factor.

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Self-Calibration

On startup (before the first reading) the firmware should trigger an ADS1256 self-calibration (SELFCAL command) to zero out internal offset and gain errors. This is particularly important after power-on temperature transients. The calibration takes ~2 ms at 2000 SPS and is invisible to the daemon due to the 2 s boot delay in the handshake sequence.