ABB AI825 Analog Input Module: 4-Channel Isolated I/O for Process Control

AI825: Per-Channel Isolation on a 4-Channel Budget

The AI825 solves a specific problem: you have four 4-20 mA loops whose ground potentials are unknown or unequal, and you cannot justify eight channels to get group isolation. It delivers four individually galvanically isolated channels with 14-bit-plus-sign resolution on the S800 platform. Per-channel isolation means a ground loop on loop 3 cannot corrupt the reading on loop 1 — with a group-isolated four-channel module, one ground fault takes out the whole group.

Update cycle time is under 10 ms, which is fast enough for closed-loop cascade control on a slow process loop and more than adequate for supervisory process monitoring.

Input impedance is 10 MΩ for voltage inputs and 50 Ω for current inputs. Power dissipation is typically 2.5 W with a 3.2 W maximum; +5 V consumption is typically 70 mA (max 100 mA) and +24 V typically 90 mA (max 110 mA). Operating temperature is 0 to +55 °C, storage -40 to +70 °C, protection class IP20, weight 0.22 kg, dimensions 45 x 119 x 102 mm.

AI825 Against the Other ABB Analog Input Modules

Channel count and isolation topology are the selection drivers. If you need more channels than four, the AI825 is the wrong part regardless of isolation; if eight channels on one group is acceptable, the AI845 is cheaper per point:

All ABB parts · I/O Modules · AI895 (intrinsically safe)

Channel Flexibility and Mixed Signal Types

Each channel can be individually configured for current or voltage, so mixed signal types within the same module are possible — a 4-20 mA pressure transmitter on channel 1, a 0-10 V temperature signal on channel 2. Temperature drift varies by signal type and range: max 47 ppm/°C over 0-10 V, 34 ppm/°C over ±10 V, 78 ppm/°C over 0-20 mA and 57 ppm/°C over ±20 mA. For high-accuracy current-loop work, that difference is worth accounting for in the accuracy budget.

Mounting on S800 Mechanics

Mount the AI825 on an S800 series MTU such as the TU811 or TU831, observing keying code DA, and confirm the module is fully seated with the connector engaged before applying power. Field wiring lands at the MTU termination points. The module communicates over Modulebus to the ABB controller, and the module can be swapped without disturbing field conductors.

Corrosion Protection and Environmental Limits

IP20 protection class and ISA-S71.04 G3 corrosion resistance cover typical control room and mild process building environments. Operating temperature is limited to 0 to +55 °C — if the module will sit in an unheated enclosure in a cold climate, this limit is the constraint that decides whether the AI825 is suitable or a wider-temperature variant is needed.

Troubleshooting the Two Common Faults

No communication on Modulebus: check the Modulebus connection and both power feeds (+5 V and +24 V), and confirm the module is fully seated on the MTU. A module that is half-seated will power up and report nothing.

Incorrect readings: verify field wiring matches the configured signal type (current or voltage) for that specific channel. Because channels are individually configurable, a single channel configured for the wrong signal type will read wrong while the other three read correctly — which is a more confusing fault pattern than a module-wide failure.

Supply & terms: Stock status: In Stock · Condition: New · Warranty: One Year · Origin: Switzerland. Listed by AutoDCS Tech Parts Team · Last updated 2026-10-07.

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