Description
Key Technical Specifications
- Channel Count: 8 Differential Input Channels
- Amplifier Type: High Input Impedance Instrumentation Amp
- Sampling Method: Simultaneous Sample and Hold (SS/H)
- Overvoltage Protection: ±30 V (Powered), ±42 V (Unpowered)
- Input Protection: 1 kΩ Series Resistor per Terminal
- Gain Selection: Switch-Selectable (Hardware)
- Scan Modes: Single-Module Parallel, Multiplexed
- Registers: ID (Read-Only), Configuration (Write-Only)
- Software Support: NI-DAQmx, LabVIEW, LabWindows/CVI
- Compatible Chassis: SCXI-1000, SCXI-1001, SCXI-1600
- Dimensions: Standard SCXI Single-Slot Width
- Weight: ~0.2 kg (Module Only)
Product Introduction
You’re trying to correlate vibration data across eight points on a turbine, but your multiplexed DAQ card is smearing the phase relationship because it’s sampling channel 1, then 2, then 3… by the time it gets to 8, the shaft has moved. That’s the NI SCXI-1140’s job to fix. It’s not an ADC; it’s a front-end conditioner with eight independent track-and-hold circuits. When the trigger hits, all eight channels freeze their voltage at the exact same instant, then the downstream DAQ card can leisurely digitize them one by one without losing that critical phase alignment.
Why keep this specific NI SCXI-1140 in your spare kit? Because it solves the “skew” problem that ruins modal analysis and rotating machinery diagnostics. It handles ±30V surges without flinching, and the 1kΩ input protection saves your bacon when a tech accidentally clips a hot wire. Just remember: this module does not digitize. You still need a DAQ card (like a PCI-6251 or SCXI-1600) to actually convert the held voltage to digital. If your readings are bouncing between zero and correct values at 50/60Hz, check your chassis power supply voltage selector before blaming the SCXI-1140—it’s a common field headache.
Troubleshooting Quick Reference
| Fault | Cause | Relevancy | Quick Check | Fix |
|---|---|---|---|---|
| Signal oscillates 0V↔Correct | Chassis power mismatch | 80% | Verify AC voltage selector wheel | Set to correct mains voltage |
| Amplitude varies by channel | Hold droop / Slow sampling | 70% | Check sample rate vs. settling time | Reduce sample rate or increase hold time |
| No signal / Flatline | Bad hold trigger | 60% | Scope HOLDTRIG pin on backplane | Verify DAQ PFI line routing |
| Noisy signal | Ground loop | 50% | Check differential wiring | Ensure true differential connection |
| Module not recognized | ID register fault | 40% | Reseat module, check backplane pins | Clean pins, reseat firmly |
Application Scenarios & Pain Points
- Rotating Machinery Vibration: Phase is everything. If your NI SCXI-1140 isn’t triggering correctly, your orbit plots look like spaghetti. The hold command must be clean; a noisy trigger = noisy data.
- Multi-Axis Modal Testing: Hitting a shaker at 100Hz? Your sample rate needs to be high enough that the hold capacitor doesn’t droop before the ADC grabs it. The SCXI-1140 has a 7µs settling time; don’t ask it to hold for 100µs and expect accuracy.
- Power Line Harmonics: 50Hz/60Hz interference on the hold line causes the signal to gate on/off with the AC cycle. If your NI SCXI-1140 reads zero half the time, check your chassis power cord and voltage setting.
NI SCXI-1140
Compatible Replacement Models
- ✅ Drop-in Replacement: (Exact match). Same 776572-xx part number family. Same jumper settings. No software changes. Just swap and verify gain switches.
- ⚠️ Software Compatible: NI SCXI-1141 (With Filter). Same form factor, same SS/H, but adds programmable low-pass filters. You must update your NI-DAQmx config to enable/disable filters, or you’ll get unexpected phase lag. Worth the upgrade if you’re fighting aliasing.
- ❌ Hardware Incompatible: NI SCXI-1122 (Isolated Multiplexer). No simultaneous sampling. No track-and-hold. It’s just a mux. Will fit the slot, but your phase data will be garbage. Do not use for dynamic analysis.
