Differential Level vs Single-Ended: Thresholds and Noise

Key Takeaways

• A differential receiver decides on the voltage between two lines, not on either line against ground.
• Common-mode noise cancels because it shifts both conductors by the same amount.
• A single-ended receiver compares one line to ground, so ground shift subtracts from noise margin.
• Verify differential output voltage, receiver threshold and common-mode range before blaming the cable.
• An idle differential pair needs failsafe biasing so the receiver holds a defined mark state.

1. What Is a Differential Level?

A differential level is the voltage a receiver compares between two signal conductors rather than against ground.

The receiver acts on the gap between the two conductors, not on either one alone. Because a changing ground shifts both conductors together, that gap survives it.

Core characteristics

• Difference-based decision: the receiver acts on the voltage between the two conductors, so the absolute potential of either one does not matter.
• Defined differential threshold: per TIA/EIA-485-A, a receiver must treat a differential input of plus or minus 200 mV or greater as a valid state, and the band around zero is undefined.
• Finite common-mode range: per TIA/EIA-485-A, the receiver must handle a common-mode voltage from minus 7 V to plus 12 V, and outside that window the output is not guaranteed.
• Complementary drive: the driver pulls one conductor high while pulling the other low, so the differential voltage is roughly twice a single line's swing.

2. How Does a Differential Level Work?

A differential link works by making the receiver decide on a difference, so a disturbance shared by both conductors cancels first.

1. Two complementary outputs. The driver drives one conductor towards the high rail and the other towards the low rail per TIA/EIA-485-A, so the useful signal is the gap between them.
2. Common mode on the pair. A twisted pair makes an external field induce nearly the same voltage on both conductors, and the receiver subtracts that common part.
3. The difference is compared. The receiver measures the voltage between its inputs against a differential threshold, plus or minus 200 mV per TIA/EIA-485-A.
4. No ground reference at the receiver. The receiver never compares an input against its own ground, which is why two ends on different earth potentials can still exchange data inside the common-mode window.
5. Idle pairs need bias. A terminated pair with no driver active sits near zero differential volts, so failsafe biasing holds A above B and the idle bus reads as a steady mark.

3. What Is a Single-Ended Level?

A single-ended level is the voltage of one signal conductor measured against a ground reference shared with the receiver.

The return current travels through the ground connection, which makes ground part of the signal path. Anything that changes ground potential at either end therefore changes the voltage the receiver sees.

Core characteristics

• One line plus a return: the signal occupies a single conductor, and its return current shares the ground path with every other circuit.
• Ground-referenced threshold: the receiver compares the input against a threshold tied to its own supply, so ground potential at the pin shifts the decision point.
• Guaranteed noise margin: available margin is the distance between the driver's guaranteed output level and the receiver's guaranteed threshold, and ground shift consumes part of it.
• Shared return impedance: current from another load in the same ground path develops a voltage across that impedance, which appears as an error at the receiver.

4. Differential Level vs Single-Ended: What Is the Difference?

The difference between the two lies in what the receiver uses as a reference, not in the data carried.

The decision rule: choose a differential level the moment noise margin must survive ground shift or a long return path; keep a single-ended level while both ends share a short, low-impedance ground.

5. Differential and Single-Ended Levels: Key Parameters

The parameters below decide whether a link has usable noise margin, and each one fails in a recognisable way.

• Differential output voltage: per TIA/EIA-485-A, a loaded driver must still deliver at least 1.5 V across a 54 ohm load, and that figure falls as terminations and unit loads are added.
• Receiver threshold and hysteresis: the guaranteed threshold is plus or minus 200 mV with 25 mV or more of hysteresis per TIA/EIA-485-A, and voltages inside that band give intermittent bit errors.
• Common mode range: keep the average of A and B inside minus 7 V to plus 12 V per TIA/EIA-485-A, and errors that appear whenever a motor runs suggest the window is exceeded.
• Failsafe biasing: fit a pull-up on A and a pull-down on B so an idle pair sits above the threshold, otherwise the receiver can report random start bits and the protocol sees phantom frames.
• Termination value and placement: 120 ohm across the pair at each physical end of the trunk, because termination at every node lowers the differential voltage and can push a long link below threshold.
• Idle and mark levels: with no driver active the pair has to hold a defined idle state, and a drifting idle differential voltage means every receiver is effectively listening to noise.
• Cable balance and skew: a consistent twist keeps the induced common-mode component equal on both conductors, while a damaged or untwisted section turns that noise into a differential error the receiver cannot reject.
• Ground offset between grounded nodes: a differential link still references common mode, so two nodes with a large earth offset can exceed the window, and a third conductor or galvanic isolation is the usual answer.

6. When to Use a Differential Level and When Not To

Good fit for a differential level

• Cable runs that leave the enclosure or bridge separately earthed equipment.
• Noisy plants with drives, contactors or welding near the cable.
• Multi-drop buses where several receivers resolve the same small differential voltage.

Good fit for a single-ended level

• Chip-to-chip links inside one board, where the ground plane is the reference.
• Configuration, debug and bootloader ports within one enclosure.
• Cost-sensitive products where one conductor and a shared ground are enough.

Poor fit for a differential level

• Installations with no defined return path or common-mode reference.
• Star or stub-heavy wiring, where reflections break the defined level.
• Very short board links, where a transceiver adds cost for no gain.

Poor fit for a single-ended level

• Any run that leaves the enclosure, where ground shift competes with noise margin.
• Heavy common-mode noise that the receiver cannot reject.
• Multi-drop networks, where every extra load and return path adds error.

7. Real-World Applications

In industrial automation and energy monitoring, the common pattern is a differential field bus with a single-ended host behind it. The host MCU drives TTL on the board, while a transceiver or radio module presents the differential or isolated interface to the cable. The MA01-AXCX4020 is a Modbus RTU serial IO module with an RS485 interface, 4 digital inputs and 2 relay outputs, placing a differential port at the field end.

The E831-RTU(8080T-485) adds dry and wet contact acquisition and relay output control on the same RS485 interface, with DC 8-28 V operation for rail installation. For radio links that replace the cable, the E220-400R30D is an RS485 module using the LLCC68 chip and LoRa spread spectrum on 410.125-493.125 MHz (default 433.125 MHz), rated 30 dBm with a 10 km figure on a DC 5.0 V supply.

Where a host has no transceiver, the E22-400T22D-V2 is a Semtech SX1268 DIP LoRa UART module, 22 dBm and up to 5 km line-of-sight, with a TTL UART compatible with 3.3 V and 5 V MCU IO. The E90-DTU(400SL44) carries RS232 or RS485 transparent transmission over an ultra-long range on the same band. Board-level margins say little about installed performance, so the antenna, feed line and a defined ground at the radio deserve as much attention as the module.

8. FAQ: Differential Levels in Practice

Q1: What is a differential level in serial communication?

It is the voltage a receiver measures between two conductors and compares to a defined threshold, not one line against ground. Per TIA/EIA-485-A a differential input of plus or minus 200 mV or more is a valid state, so the useful signal is the gap between the pair.

Q2: Why does my differential link fail when a nearby drive runs?

Common-mode noise from the drive shifts the pair's average voltage, so the receiver no longer meets its guaranteed threshold. Work through these checks.

• Errors under load -> measure the common-mode voltage at the receiver and confirm it stays inside minus 7 V to plus 12 V per TIA/EIA-485-A.
• Errors grow with cable length -> check the differential voltage at the far end, because termination and stubs reduce it.
• Random bytes when idle -> add failsafe biasing so the pair holds a defined state.

Q3: How do I calculate noise margin on a single-ended link?

It is the difference between the driver's guaranteed output level and the receiver's guaranteed threshold, minus any ground shift between the ends. A driver guaranteeing 4.5 V into a receiver needing 2.0 V has 2.5 V nominal margin, and a 1 V ground offset leaves 1.5 V.

Q4: Do I need failsafe biasing if the bus is terminated?

Yes. A termination resistor alone holds the idle pair near zero differential volts, inside the undefined threshold band, so the receiver output is not guaranteed. Add a pull-up on A and a pull-down on B sized for the node count and cable.

Q5: Can I connect a single-ended UART pin straight to a differential pair?

Not without a transceiver. A UART pin drives one conductor against ground, while a differential pair needs two complementary outputs and a receiver that compares them. An RS485 driver performs that conversion, so the level standard is a separate decision from the data format above it.

Q6: What is the minimum differential voltage my receiver needs?

Per TIA/EIA-485-A the receiver must recognise plus or minus 200 mV or greater, with at least 25 mV of hysteresis. Design so the smallest differential voltage reaching the receiver stays above that figure after termination, stub loading and cable loss.

9. Practical Checklist Before You Trust the Level

1. Measure the differential voltage at the receiver, not at the driver, with the bus loaded and terminated, then compare it against the plus or minus 200 mV threshold per TIA/EIA-485-A.
2. Verify common-mode voltage stays inside minus 7 V to plus 12 V per TIA/EIA-485-A with every known noise source running.
3. Fit and size failsafe biasing so the idle pair reads as a defined mark, then recheck the idle level after adding nodes.
4. Terminate the two physical ends with 120 ohm, keep stubs short, and re-measure the differential voltage after any change.
5. On a single-ended link, subtract the measured ground shift from the nominal noise margin, and move to a differential interface if the remainder is small.