Single-ended vs differential inputs
A grounded source measured in RSE mode closes a ground loop, and a floating source on a differential input drifts to a rail unless it has a bias resistor.
Most multifunction DAQ devices let you wire each analog input in more than one way, and the choice sets how many channels you get, how much noise you see and whether some signals can be measured at all. NI calls the modes RSE, NRSE and differential, while MCC and LabJack say single-ended and differential. The modes differ in what the input voltage is measured against.
Single-ended inputs: RSE and NRSE
A single-ended input measures one terminal against a reference shared by every channel. NI splits this into two modes:
- RSE (referenced single-ended) measures each channel against AI GND, the device’s own ground.
- NRSE (non-referenced single-ended) measures each channel against AI SENSE, a shared input that you connect to the ground of your signal sources, so the reference can follow a remote ground that sits a little away from the DAQ’s ground.
The single-ended mode on MCC and LabJack devices corresponds to RSE. On a LabJack T7, for example, a single-ended channel is measured against GND.
NI’s connection guidance considers single-ended acceptable for signals above 1 V, leads shorter than 3 m (10 ft), and sources that can share a common reference. It recommends differential for signals under 1 V, longer leads, sources that need their own reference, and leads routed through noisy environments.
Differential inputs
A differential input measures the voltage between two terminals, AI+ and AI−, and rejects the voltage they have in common. That shared part is the common-mode voltage, Vcm = (V+ + V-) / 2. Noise picked up equally on both wires of a twisted pair is common mode, and so is a ground offset between the source and the DAQ. How much of it leaks through is set by the common-mode rejection ratio (CMRR). The NI USB-6211 specifies 100 dB from DC to 60 Hz, which turns 1 V of common-mode voltage into about 10 µV of error.
Common-mode range
Rejection only works while both inputs stay inside the amplifier’s operating range relative to the DAQ’s ground. Outside it the reading is wrong, usually with no warning. The limits on three USB devices:
| Device | Channels | Allowed voltage on each input, relative to ground |
|---|---|---|
| MCC USB-1208FS-Plus | 8 SE / 4 DIFF | −10 V to +20 V for linear operation in differential mode |
| LabJack T7 | 14 SE / 7 DIFF | −11.5 V to +11.5 V for valid readings |
| NI USB-6211 | 16 SE / 8 DIFF | ±10.4 V of AI GND, signal plus common mode |
The differential inputs on these devices share the DAQ’s ground and have no isolation. A signal riding on 48 V, on a 24 V loop supply or on anything mains-referenced can’t be measured differentially on them. It needs an isolated input, an isolation amplifier or a divider designed for the job.
Channel count in differential mode
A differential channel uses two input terminals, so the count drops from 8 to 4 on the USB-1208FS-Plus, from 14 to 7 on the LabJack T7 (paired AIN0/AIN1, AIN2/AIN3 and so on through AIN12/AIN13), and from 16 to 8 on the NI USB-6211. On the T7 and the USB-6211 the mode is set per channel, so you can run thermocouples differentially and keep high-level signals single-ended on the same device.
On the USB-1208FS-Plus the mode also changes resolution. Single-ended mode offers one ±10 V range at 11-bit resolution, about 9.8 mV per step. Differential mode gets the full 12 bits and ranges down to ±1 V. If you have the terminals to spare, differential is the better mode on that device even for clean signals.
Pseudo-differential inputs
NI describes a pseudodifferential input as combining characteristics of a differential channel and an RSE channel. The negative input connects to ground through a relatively small impedance, while the positive and negative sides are separated by a much larger one. Dynamic signal modules use this arrangement. On the NI-9234, AI− (the shield of the BNC) connects to chassis ground through 50 Ω, with a ±2 V maximum common-mode range. Floating sources, which in NI’s description include most accelerometers, get a ground reference with no extra parts, and a grounded source can be measured as long as its ground sits within that ±2 V.
Floating and grounded sources
The right mode depends mostly on how the source is grounded.
A grounded source is referenced to building ground: a bench supply with its negative strapped to earth, a function generator with an earthed BNC shell, a sensor powered from an earthed supply. A USB DAQ plugged into a desktop PC is earthed too, through the PC. The two grounds meet at different points in the building wiring, and the voltage between them can be millivolts to volts, usually with mains frequency in it. Measuring a grounded source in RSE mode closes a ground loop. Current flows in the signal’s ground wire, and its voltage drop adds to your reading. Use differential or NRSE.
A floating source has no connection to ground: an ungrounded thermocouple, a battery, an isolated transmitter output, a transformer secondary, many battery-powered instruments. RSE works with these because the DAQ provides the reference. On a differential input nothing ties the source to ground, so its common-mode voltage drifts on leakage and input bias currents until it leaves the input range and the reading wanders or sticks at a rail.
Bias resistors
A floating source on a differential input needs a bias resistor from the negative input to ground. NI’s guidance is to connect the negative lead directly to AI GND for DC-coupled sources under 100 Ω. For higher source impedances, connect it through a resistor of about 100 times the source impedance, which keeps the two signal paths closer to balanced so that noise couples onto both equally. A second resistor of the same value from the positive input to ground balances the path fully, at the cost of loading the source with both resistors in series. LabJack’s thermocouple example uses about 100 kΩ from AIN1 to GND on a differential pair.
Input mode by signal type
| Signal | Mode |
|---|---|
| High-level (above about 1 V), short leads, floating or sharing the DAQ’s ground | Single-ended (RSE) |
| Many high-level signals from one remote system with its own ground | NRSE, with AI SENSE tied to that ground |
| Thermocouples and other millivolt signals | Differential, with a bias resistor if the sensor floats |
| Bridge sensors (strain gauges, load cells) | Differential; the output sits at about half the excitation voltage |
| Grounded source at another location | Differential, inside the common-mode range |
| Shunt resistor on a current loop | Single-ended with the shunt on the low side, otherwise differential |
| IEPE accelerometers and microphones | A pseudo-differential dynamic signal input with IEPE excitation |
The thermocouple guide and the post on 4–20 mA current loops cover the wiring for those two cases.
Troubleshooting noisy or drifting readings
Work through these in order, quickest checks first.
- Short the input at the terminals. Connect AI+ to AI− (or a single-ended input to ground) and watch the reading. If it is still noisy with the sensor disconnected, look at the range, rate and USB connection.
- Look for slow wandering or a stuck rail on a differential channel. That pattern usually means a floating source without a bias resistor.
- Change the ground arrangement. Unplug the laptop’s charger, or try another USB port or computer. If the 50 or 60 Hz noise changes, you have a ground loop. Measure differentially, ground at one point, or isolate.
- Measure the common-mode voltage with a meter. Check each input against the DAQ’s ground. Both must be inside the device’s range.
- Look at a spectrum of the noise. Peaks at 50 or 60 Hz and their harmonics are mains pickup. A peak at an unexpected frequency that moves when you change the sample rate is an alias of something faster (see sample rate and aliasing).
- Check for crosstalk between channels. On a multiplexed DAQ, a high-impedance source can’t charge the input fully before the next channel is converted, so one channel’s value bleeds into the next. Lower the source impedance, buffer the signal or slow the scan. LabJack lists 1 kΩ as the maximum source impedance for the T7.
- Tidy the wiring. Twist signal pairs, route them away from motor and mains cables, and ground shields at one end only.
- Use the smallest range that holds the signal. Small signals on a large range waste resolution and pick up the full offset error of that range.
Steps 3 and 5 go faster with a live spectrum next to the strip chart, and DAQAtlas, which is in development, can put both on one dashboard. The DAQ device finder filters hardware by specification if you need different input modes or common-mode range.