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AD8628 Op Amp Non-Inverting Amplifier Not Providing Expected Gain with 0-3V Rail

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  • #1 21682925
    Scott Siler
    Anonymous  
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  • #2 21682926
    Martin Rowe
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    Martin Rowe
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    David Ashton
    Anonymous  
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    Richard Gabric
    Anonymous  
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    Scott Siler
    Anonymous  
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    Scott Siler
    Anonymous  
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    Sanjay Meena
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    David Ashton
    Anonymous  
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  • #10 21682934
    Richard Gabric
    Anonymous  
  • #11 21682935
    Scott Siler
    Anonymous  
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    David Ashton
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    Steven George
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Topic summary

✨ An aerospace engineer is attempting to amplify the 0-36 mV output from a commercial strain gauge load cell powered by a 12 V excitation to better utilize the input range of an NI 6289 DAQ, which has a minimum input range of ±100 mV and 18-bit resolution. The engineer used an AD8628 precision op amp in a non-inverting amplifier configuration with a gain of 3 but observed no expected gain; the output voltage was lower than the input. Responses clarified that strain gauges are passive devices requiring excitation and typically produce differential outputs from a Wheatstone bridge, necessitating a differential or instrumentation amplifier rather than a single-ended non-inverting amplifier. The common mode voltage from the 12 V excitation is about 6 V, which must be considered in the amplifier design. Suggestions included using a differential amplifier stage, ensuring proper wiring, and considering ratiometric measurement techniques where the ADC reference drives the load cell to improve stability and accuracy. It was also noted that the load cell output after amplification might exceed the DAQ input range, so gain or excitation voltage adjustments may be necessary. Temperature stability and noise were considered manageable in the described environment. Additional advice included reviewing op amp tutorials on differential amplifiers and carefully designing the front-end to match the required resolution and stability.
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FAQ

TL;DR: If your 12 V–excited load cell outputs 0–36 mV, a simple non‑inverting stage won’t work; “You need to work through an error budget.” 18‑bit ≈ 4 ppm. Use a differential/instrumentation front end and watch input range. [Elektroda, Anonymous, post #21682934]

Why it matters: This FAQ helps DAQ users fix low‑level load‑cell amplification issues on 0–3 V rails without losing accuracy.

Quick Facts

Why doesn’t my AD8628 non‑inverting amplifier give the expected gain on a 0–3 V rail?

You are trying to amplify a floating Wheatstone bridge with a single‑ended stage. The bridge outputs two lines with no ground reference. A basic non‑inverting op‑amp assumes the source is ground‑referenced, so gain collapses or clips. Use a differential or instrumentation amplifier that accepts floating inputs and sets gain precisely. “A strain gage has two outputs neither of which is connected to ground.” [Elektroda, Anonymous, post #21682928]

Can I just set gain = 3 to stretch 0–36 mV into the NI 6289’s ±100 mV range?

Be careful. Gain × max signal = 36 mV × 3 = 108 mV. That can exceed the ±100 mV input span, depending on polarity. Over‑range causes clipping and lost data. Reduce gain slightly or reduce excitation to keep within limits, or center with offset circuitry if needed. “Beware…you will get…0 to 108 or 0 to −108 mV.” [Elektroda, Anonymous, post #21682933]

What input mode should I use on the NI 6289 for a load cell?

Use differential input mode. Your DAQ supports differential and the load cell bridge is inherently differential. This improves noise rejection and matches the floating bridge outputs. The thread author states using differential inputs with the NI 6289 successfully before attempting extra gain. [Elektroda, Anonymous, post #21682931]

Do I even need an external amplifier with an 18‑bit NI 6289?

Maybe not, if you operate near capacity and accept less than full 18‑bit performance. First build an error budget: excitation stability, temperature drift, and noise. One post notes 18 bits ≈ 4 ppm, which demands excellent stability. Evaluate whether your measurement environment and supply can support that before adding analog gain. [Elektroda, Anonymous, post #21682934]

What is an instrumentation amplifier, and why is it recommended here?

An instrumentation amplifier is a precision differential amplifier with high input impedance and excellent common‑mode rejection. It measures small differential signals on large common‑mode voltages, like a strain‑gauge bridge at ~6 V common‑mode, and sets gain with one resistor. It solves the floating‑source problem your single‑ended stage has. [Elektroda, Anonymous, post #21682928]

How do I size excitation and gain to avoid DAQ over‑range?

Compute Vout_max = Sensitivity × Excitation × Gain. With 3 mV/V, 12 V, and Gain = 3, Vout_max = 108 mV. Target ≤95 mV for margin. Options: reduce gain (e.g., 2.6–2.8), lower excitation, or add a small offset to center. Over‑range is a common failure mode during calibration. [Elektroda, Anonymous, post #21682933]

What is a Wheatstone bridge common‑mode voltage in this setup?

With 12 V excitation, the bridge midpoints sit near half the supply, around 6 V common‑mode. Your front end must tolerate and reject that common‑mode while extracting the tens of millivolts differential signal. Plan the amplifier’s input range and CMRR accordingly. [Elektroda, Anonymous, post #21682934]

Will the AD8628 work for precision DC load‑cell amplification?

It’s a precision op‑amp, but system performance depends on surrounding components and topology. For load cells, a purpose‑built differential or instrumentation front end is preferred. One contributor also mentions fully differential drivers used with high‑resolution ADCs when applicable. Choose topology before the device. [Elektroda, Anonymous, post #21682934]

How should I think about resolution vs. stability for 18‑bit readings?

An 18‑bit code width is tiny. “18 bits is around 4 ppm.” Temperature shifts and supply drift can dominate. Decide how many effective bits you truly need, then match excitation stability and amplifier drift to that requirement. Don’t chase bits your analog front end cannot support. [Elektroda, Anonymous, post #21682934]

Quick 3‑step: how do I add safe gain to a load cell into ±100 mV?

  1. Measure raw full‑scale: confirm ~0–36 mV at your actual excitation.
  2. Set differential/instrumentation amp gain so VFS ≤95 mV (e.g., 2.6–2.8).
  3. Verify polarity; avoid 108 mV peaks that exceed span. “Reduce your gain or reduce the excitation voltage.” [Elektroda, Anonymous, post #21682933]

What is a DAQ and why does input range matter here?

A DAQ (data acquisition device) digitizes analog signals for a computer. The NI 6289’s lowest differential range is ±100 mV. If your amplified signal exceeds that, it clips. If it uses only a small fraction, you lose usable resolution. Matching span maximizes effective bits. [Elektroda, Anonymous, post #21682931]

Does using a higher‑capacity load cell help avoid overload and still keep resolution?

It can prevent mechanical overload but reduces electrical output per unit load, lowering signal‑to‑noise and effective resolution. The thread author accepted this tradeoff when not measuring near capacity. Confirm if your resolution target still holds after downsizing the signal. [Elektroda, Anonymous, post #21682935]
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