Analog in and out

Analog inputs and outputs

Nine ADCs, six DACs and two PWM drivers — the bridge between the digital variable table and the 4–20 mA, 0–10 V world industrial sensors and actuators live in.

Inputs

You need	Reach for
the workhorse: 16-bit, four channels	`ADS1115`
the same, faster and coarser	`ADS1015` (12-bit)
eight channels	`ADC128S102`, `MCP3008`
a single channel, minimal wiring	`MCP3221`, `MCP3421`
load cells and strain gauges	`NAU7802` (24-bit, built-in amplifier)
a wide input range	`ADS1118`

ADS1115 is the default for good reasons: 16 bits is enough for almost anything, four channels covers a small equipment, and its programmable gain amplifier handles small signals without extra parts.

Reading 4–20 mA

The industrial standard, and it needs one resistor: current through a precision resistor becomes a voltage the ADC can read. 4–20 mA through 165 Ω gives 0.66–3.3 V, which suits a 3.3 V input.

Why the world uses current rather than voltage is worth knowing. A broken wire reads 0 mA — outside the 4–20 range, and therefore detectably wrong. A 0–10 V sensor with a broken wire reads 0 V, which is a perfectly valid "zero".

Use the dead band below 4 mA

Alarm on anything under about 3.5 mA. That is a wire fault, a dead sensor or a power failure at the transmitter — and being able to tell is what you paid for when you chose 4–20 mA.

Outputs

DACs — MCP4725, MCP4728, DAC7678, GP8403 — produce a real voltage for a drive input, an analog setpoint, a proportional valve. GP8403 is worth noting: it outputs 0–10 V directly, without the amplifier the others need.

Digital potentiometers — MCP4461, MCP47A1 — replace a trimmer in somebody else's circuit.

PWM drivers — PCA9685 (16 channels), TLC59208F — produce a switching signal. With an RC filter it approximates an analog voltage; without one it drives LEDs, heaters and servos directly.

You need	Reach for
0–10 V for a drive	`GP8403`
a clean low-voltage analog signal	`MCP4725`, `MCP4728`
many channels of dimming or heating	`PCA9685`
to replace a trimmer	`MCP4461`

3.3 V is not 10 V

Most DACs output at most their supply voltage. A drive expecting 0–10 V sees a third of the setpoint you think you sent and runs at a third of the speed — which looks like a scaling bug in the program, and is not. Either use GP8403, or add an amplifier.

Scale in the variable, not in the logic

Give the variable a scale and a unit and let it convert counts to bar or volts. The logic then reads bar, the screen shows bar, and nobody has to remember what 24 576 meant.

The chips

Analog inputs

Chip	Bus	What it is
`ADC128S102`	SPI	SPI 12-bit ADC, 8 single-ended channels (TI) — raw counts per channel; two frames per conversion (address, then data). addr = CS pin.
`ADS1015`	I2C	I2C 12-bit ADC, 4 single-ended channels (AIN0–3 = pin 0–3, INPUTS only), address 0x48–0x4B — the faster 12-bit sibling of the ADS1115 (same registers, conversion in 12-bit steps of 16: raw keeps the ADS1115 scale). Optional key range = full scale in volts, stored as PGA code+1 like the ADS1115.
`ADS1115`	I2C	I2C 16-bit ADC, 4 single-ended channels (AIN0–3 = pin 0–3, INPUTS only), address 0x48–0x4B. Optional key range = full scale in volts (6.144/4.096/2.048/1.024/0.512/0.256, default 4.096) — stored in PeriphRec.config as PGA code+1 (0 = default). The raw variable value is the i16 conversion register; engineering units come from the variable gain/offset.
`ADS1118`	SPI	SPI 16-bit ADC (the ADS1115 over SPI) — 4 single-ended channels, raw conversion counts at ±2.048 V; units via variable gain/offset. addr = CS pin.
`MCP3008`	SPI	SPI 10-bit ADC, 8 single-ended channels — raw counts per channel (units via variable gain/offset). addr = CS pin.
`MCP3204`	SPI	SPI 12-bit ADC, 4 single-ended channels — raw counts per channel (units via variable gain/offset). addr = CS pin.
`MCP3221`	I2C	I2C 12-bit ADC, 1 channel, no configuration at all — channel 0 = raw counts (units via variable gain/offset). Address 0x48–0x4F (factory-coded).
`MCP3421`	I2C	I2C 18-bit delta-sigma ADC, 1 differential channel — channel 0 = raw converter value; engineering units via variable gain/offset. PeriphRec.config: bits 0–2 = PGA code +1 (YAML gain: 1/2/4/8), bits 4–6 = resolution code +1 (YAML bits: 12/14/16/18, default 16 — 15 SPS). Address 0x68–0x6F (factory-coded).
`NAU7802`	I2C	I2C 24-bit ADC for strain-gauge bridges (weighing) — channel 0 = raw converter value; kilograms come from variable gain/offset (scale calibration = two known points). PeriphRec.config: bits 0–3 = PGA gain code +1 (default x128, YAML key gain: 1..128), bits 4–7 = rate code +1 (default 10 SPS, YAML key rate: 10/20/40/80/320). The internal 3.0 V LDO powers the bridge (AVDD).

Analog outputs

Chip	Bus	What it is
`DAC7678`	I2C	I2C 12-bit DAC, 8 channels (OUTPUTS) — raw = (variable - offset) / gain clamped to 0..4095, written on change per channel. Address 0x48–0x4B.
`GP8403`	I2C	I2C 2-channel 0–10 V DAC (DFRobot GP8403) — channels 0/1 are OUTPUTS: raw = (variable - offset) / gain, 12-bit over the range. Optional key range = 5 or 10 (V full scale, default 10) — stored in PeriphRec.config as 1 = 5 V. Address 0x58–0x5F. Verify on the bench — written from the DFRobot library, not a datasheet.
`MCP4461`	I2C	I2C quad digital potentiometer — channels 0–3 = volatile wipers, raw 0..256 (OUTPUTS). Address 0x2C–0x2F.
`MCP4725`	I2C	I2C 12-bit DAC — channel 0 is an OUTPUT: raw = (variable - offset) / gain, clamped to 0..4095. Variable gain/offset therefore defines the engineering units (e.g. gain 10.0/4095 for a 0–10 V module). Written only on change.
`MCP4728`	I2C	I2C 12-bit DAC, 4 channels (OUTPUTS) — raw = (variable - offset) / gain, clamped to 0..4095; all four channels go out in one fast write, only on change. Address 0x60–0x67.
`MCP47A1`	I2C	I2C 6-bit DAC, 1 channel (OUTPUT) — raw 0..63; the cheapest analog trim output there is. Fixed address 0x2E.

PWM outputs

Chip	Bus	What it is
`PCA9685`	I2C	I2C 16-channel 12-bit PWM — every pin is an OUTPUT, the variable value is duty in % (0–100, like the on-chip PWM service). Optional key freq_hz = PWM frequency 24–1526 Hz (default 200) — stored directly in PeriphRec.config. Address 0x40–0x7F (0x70 is the all-call address — avoid it with other chips).
`TLC59208F`	I2C	I2C 8-channel 8-bit LED PWM — every channel is an OUTPUT, the variable value is duty in % (0–100). Address 0x20–0x27 (configurable wider; the check covers the common range).

Every entry above is in the editor: pick the chip, give it an address or a chip-select pin, and its channels become variables. See Adding a peripheral for the click-through.