How to Scale an Analog to Digital Converter PLC Signal (4-20mA to RAW)

Converting a physical analog signal to a digital register value inside a Programmable Logic Controller (PLC) is the foundational step of industrial instrumentation. If you are feeding a standard 12 mA signal (the exact midpoint of a 4–20 mA loop) into a standard 16-bit signed analog to digital converter PLC input, the direct converted raw integer answer depends entirely on your hardware brand. For an Allen-Bradley CompactLogix (e.g., 1769-IF4 module), the converted raw value is 16,383. For a Siemens S7-1200 (SM 1231 module), the converted raw value is 13,824. For a 12-bit AutomationDirect module, it is 2,047.

The universal formula used to derive these values is:

Because field signals fluctuate, here is how the raw integer shifts across a ±20% range around our 12 mA baseline, comparing the two most common PLC architectures:

The Assumptions That Fix Your ADC Conversion

The raw integer output of an analog to digital converter PLC module is not a universal constant; it is strictly bound by the hardware's bit-depth resolution and the manufacturer's proprietary memory mapping. The primary assumption that fixes your answer is whether the PLC uses a 12-bit, 13-bit, or 16-bit Analog-to-Digital Converter (ADC) chip, and whether the register is signed or unsigned.

Siemens standardizes their analog scaling around the number 27,648 (which represents 100% of the nominal range in their normalized scaling blocks). Allen-Bradley typically utilizes the full span of a 16-bit signed integer (32,767) for 4-20mA loops. Lower-cost micro-PLCs often rely on 12-bit ADCs, capping out at 4,095. Below is a data-dense breakdown of real-world module specifications that dictate your scaling math:

If you assume a 16-bit resolution but wire your sensor into a 12-bit module, your calculated engineering units will be off by a factor of eight, potentially causing catastrophic over-pressurization or thermal runaway in controlled processes. Always verify the exact module part number in the manufacturer's hardware reference manual before writing your NORM_X and SCALE_X logic blocks.

Mains Voltage Irrelevance vs. Signal Step-Down (120V/230V/3-Phase)

A common point of confusion for electricians transitioning into industrial instrumentation is asking how the ADC conversion shifts for 120V vs 230V vs 3-phase systems. In AC power theory, calculating current from power shifts drastically depending on whether you are dividing by 120V (single-phase), 230V (single-phase), or multiplying by √3 for 480V 3-phase systems.

In the low-voltage DC domain of a PLC analog to digital converter, the mains voltage is entirely irrelevant to the raw ADC count. The ADC chip only "sees" the 0-10V DC or 4-20mA DC signal presented at its terminal block. It does not know or care if the facility is powered by a 120V residential panel or a 480V 3-phase industrial switchgear.

However, the mains voltage does matter if you are using an AC voltage transducer to feed the PLC. For example, if you are monitoring a 3-phase motor's line voltage using an AC-to-DC isolator transducer:

• Scenario A (120V System): A transducer scaled 0–150V AC to 0–10V DC will output 8.0V DC when measuring 120V AC.
• Scenario B (230V System): You must swap to a 0–300V AC to 0–10V DC transducer. At 230V AC, it outputs 7.66V DC.
• Scenario C (480V 3-Phase): You use a 0–600V AC to 0–10V DC transducer. At 480V AC, it outputs 8.0V DC.

The PLC ADC raw count for 8.0V DC remains identical across all three scenarios (e.g., 22,118 on a Siemens 0-10V module). The shift happens in the transducer's scaling ratio, not the PLC's internal ADC math. According to the Rockwell Automation 1769 Analog I/O Manual, the ADC strictly processes the isolated DC signal presented to it, relying on external signal conditioners to handle mains step-down.

When the Conversion Becomes Meaningless

Calculating the raw integer is only half the battle. The conversion becomes functionally meaningless—and potentially dangerous—under three specific field conditions:

1. Missing Engineering Unit Mapping (The "RAW" Trap)

A raw value of 16,383 means nothing to an HMI operator. If you fail to map this integer to a physical range (e.g., 0–100 PSI) using a scaling instruction, the conversion is useless. In Siemens TIA Portal, this requires chaining a NORM_X block (to convert the raw INT to a 0.0–1.0 REAL float) followed by a SCALE_X block (to map the float to 0.0–100.0 PSI). Without this, your HMI displays raw hex/decimal counts instead of actionable process data.

2. Impedance Mismatch on Voltage Inputs

If you attempt to feed a 4-20mA current signal into a PLC channel configured for 0-10V DC, the conversion will fail entirely. Current requires a closed loop; voltage requires a parallel potential difference. To make this conversion meaningful, you must install a precision 250-ohm shunt resistor across the PLC's voltage input terminals. By Ohm's Law (V = I × R), 20mA flowing through 250Ω yields exactly 5.0V DC. If your PLC expects 0-10V, your usable ADC resolution is instantly cut in half, and the 4mA baseline will register as 1.0V instead of 0V, requiring an offset in your scaling math.

3. Ground Loops and Common-Mode Voltage

Analog to digital converters are highly susceptible to ground potential differences. If your 4-wire transmitter is grounded at the field sensor, and the PLC analog common (0V) is grounded at the panel, a ground loop can introduce 1V to 3V of common-mode noise. In a 0-10V system, a 2V ground loop means your 0V baseline actually reads as 2V (or ~5,529 raw counts on a Siemens module). As detailed in the Siemens S7-1200 System Manual, exceeding the common-mode voltage limit (typically ±10V to ±15V depending on the module) will cause the ADC to saturate, pegging the raw value at its maximum overflow limit (e.g., 32,767) regardless of the actual physical signal. Always use isolated analog input modules or 4-20mA loops to reject common-mode noise in industrial environments.