FAQ
TL;DR: For a 1.4 V peak, low‑impedance pulse with microvolt‑level decay changes, gate a high‑gain stage with a window so you “only see the variations.” [Elektroda, Anonymous, post #21681579]
Why it matters: This approach avoids rail clipping and lets your ADC capture subtle decay differences in battery, single‑supply designs.
Who this is for: Engineers asking how to amplify tiny decay variations without saturating an op‑amp or losing detail at the ADC.
Quick Facts
- Signal context: ≤1.4 V peak, low impedance; decay variations are only a few microvolts. [Elektroda, Anonymous, post #21681578]
- ADC suggestion: ≥12‑bit resolution and fast sampling to capture the windowed tail accurately. [Elektroda, Anonymous, post #21681586]
- Gating method: Use a “valid window” pulse to enable high gain only during the tail. [Elektroda, Anonymous, post #21681581]
- Clipping recovery: Saturated amplifiers may need a clamp circuit to speed recovery. [Elektroda, Anonymous, post #21681586]
- Low‑noise path: Choose a low‑noise amplifier and the highest precision ADC you can afford. [Elektroda, Anonymous, post #21681587]
How do I amplify only the decay variations without clipping the 1.4 V pulse?
Create a window around the tail, then apply high gain only inside that window. A comparator or logic pulse enables the gain stage so the large leading portion never drives it into saturation. This lets the amplifier operate linearly where variation lives and stay disabled elsewhere. Sample the enabled segment with your ADC. This windowed‑gain technique avoids rail sticking while exposing microvolt‑level changes. “Only see the variations” by selecting the right window thresholds. [Elektroda, Anonymous, post #21681579]
What is a window comparator and why use it here?
A window comparator outputs a pulse when a signal lies between two set thresholds. Use it to mark when the pulse enters the decay region, creating a “valid window” that gates your high‑gain path or ADC capture. This converts slope or level changes into measurable time or amplitude metrics, while keeping the amplifier out of saturation elsewhere. It gives repeatable start and stop points tied to your decay. [Elektroda, Anonymous, post #21681582]
How can I generate a reliable window for the tail?
Detect the trailing edge with comparators set just below the peak and near the tail level. OR: start a timer at the positive edge and delay until the decay region. Either way, produce a clean digital pulse that enables your high‑gain channel or triggers ADC sampling exactly over the tail. Keep thresholds stable and reference them to your single‑supply rail. [Elektroda, Anonymous, post #21681581]
What ADC resolution and speed should I choose?
Use 12‑bit or higher resolution and sample fast enough to cover the full tail window. Higher resolution improves visibility of small amplitude changes after amplification. Fast sampling ensures you don’t miss rapid decay transitions once the window opens. “12 bits or more” is a practical baseline for this measurement style. [Elektroda, Anonymous, post #21681586]
How do I prevent op‑amp clipping from corrupting the measurement?
Let the amplifier clip outside the window, but ensure it is linear inside the window. If the stage must recover from earlier saturation, add a clamp circuit to speed recovery so it’s settled before sampling. Gate the gain only during the region of interest to avoid rail sticking at the measurement instant. “As long as it is linear in the part you’re interested in, you will be fine.” [Elektroda, Anonymous, post #21681586]
What if my pulses don’t always reach 1.4 V?
The windowing method assumes a consistent top level when thresholds reference the peak. If your maxima vary, use absolute thresholds tied to the tail levels or use an edge‑timed delay instead of peak‑based thresholds. Otherwise, the window may open at the wrong time and distort results. Consistency near the maximum simplifies reliable timing. [Elektroda, Anonymous, post #21681579]
How do I trigger the ADC with minimal jitter?
Derive a clean digital window pulse and use it to start ADC conversions. Hardware triggering from a comparator reduces timing uncertainty versus software polling. Keep the comparator hysteresis small to avoid time shift at threshold crossings. Capture only during the window to maximize effective resolution on the tail. A precise trigger was highlighted as key to this task. [Elektroda, Anonymous, post #21681587]
What is a clamp circuit in this context?
A clamp circuit limits how far the amplifier output swings into saturation, shortening recovery time when the signal re‑enters the linear region. Use diodes or active clamps around the op‑amp to bound voltage and speed settling before the window opens. This lowers error from prior clipping and stabilizes tail measurements. [Elektroda, Anonymous, post #21681586]
Do I need a low‑noise amplifier for microvolt‑level decay changes?
Yes. Noise added before or during high gain can mask microvolt‑level variations. Choose a low‑noise amplifier, keep the bandwidth no wider than needed, and minimize input resistance if source impedance is low. Pair it with a high‑precision ADC to preserve dynamic range inside the window. “You might need a low‑noise amplifier.” [Elektroda, Anonymous, post #21681587]
Single‑supply, battery‑powered: any special considerations?
Use rail‑to‑rail stages or bias the signal to mid‑supply so the windowed region sits in the linear range. Maintain stable references for comparator thresholds. Keep quiescent current low to match battery constraints. Windowing reduces average gain duty cycle, saving power and headroom in single‑supply designs. [Elektroda, Anonymous, post #21681578]
Can I measure decay time instead of amplitude changes?
Yes. Place two thresholds across the tail and measure the window pulse width. The pulse length maps to the decay time or slope, converting analog shape into a robust time metric. This avoids heavy analog gain, then you read the pulse width digitally. It works best when the maximum amplitude is consistent. [Elektroda, Anonymous, post #21681579]
What if I still need to see the tiny variations directly?
Insert a high‑pass section before the window gate so DC and large low‑frequency components drop out. This emphasizes decay‑related changes while reducing headroom stress. Gate the resulting AC variation with your window and amplify. The thread suggestion to add a high‑pass path targets “only the variations.” [Elektroda, Anonymous, post #21681579]
Is oscilloscope‑only measurement viable for development?
Yes. Use your window pulse to enable a high‑gain channel or to trigger the scope on the tail. Verify linearity during the window and confirm recovery from any earlier clipping. Once stable, translate the setup to your ADC path for automated capture. This two‑part approach supports iterative tuning. [Elektroda, Anonymous, post #21681581]
Three‑step: How do I implement windowed high‑gain capture?
- Generate a window pulse using comparators or an edge‑timed delay that covers only the decay.
- Gate a high‑gain path with that pulse; add a clamp to speed recovery.
- Trigger the ADC from the window and sample at ≥12‑bit resolution. [Elektroda, Anonymous, post #21681586]
What failure modes should I watch for?
Watch for slow amplifier recovery after clipping, which corrupts early‑window samples. Add a clamp and ensure the window opens after settling. Also avoid noisy gain that buries microvolt‑level changes. Validate that trigger thresholds align with real signal levels over temperature and unit variation. [Elektroda, Anonymous, post #21681586]
