Stacking Acute Logic Analyzers with Third-Party Oscilloscopes

Why Stack a Logic Analyzer with an Oscilloscope

Mixed analog and digital debugging is one of the most common scenarios in embedded systems development. You need to see what the firmware is doing on digital buses while simultaneously observing what the hardware is doing on power rails, clock signals, or analog sensor outputs. An oscilloscope alone gives you 2-4 analog channels but no protocol decode. A logic analyzer alone gives you dozens of decoded digital channels but no analog waveforms. Stacking combines both in a single time-correlated view.

Typical scenarios where stacking is essential:

• SPI communication with power rail droop: An SPI transaction causes a current spike that droops the supply rail, corrupting the transfer. You need to see the SPI decode and the analog voltage droop simultaneously to correlate cause and effect.
• Clock jitter during bus activity: An I2C master’s clock frequency varies when the CPU is under load. Analog capture of SCL shows the jitter; digital decode shows which transactions are affected.
• PWM-controlled power supply debug: A digital PWM signal drives a buck converter. You need to see the digital control signals alongside the analog output voltage ripple and inductor current waveform.
• ADC validation: Comparing the analog input signal to the digitized values read out over SPI or I2C confirms ADC linearity and timing.

Compatible Oscilloscopes

Acute logic analyzers can stack with most oscilloscopes that provide an external trigger output (TRIG OUT) and an external trigger input (EXT TRIG or AUX). This includes instruments from all major oscilloscope manufacturers. The key requirement is a BNC trigger output that provides a TTL-level pulse synchronized with the oscilloscope’s trigger event.

Some specific compatibility notes:

• Most modern oscilloscopes output a TTL-level trigger pulse on the rear-panel TRIG OUT BNC. Verify your oscilloscope’s manual for the output voltage level and polarity.
• USB-based oscilloscopes that lack a physical TRIG OUT require an alternative synchronization method (discussed below under software triggering).
• For highest timing accuracy, use oscilloscopes with trigger output jitter below 1 ns.

Trigger Cable Connections

The fundamental concept is simple: one instrument triggers the other so both start capturing at the same moment.

Method 1: Oscilloscope Triggers the Logic Analyzer

This is the most common configuration. The oscilloscope’s analog trigger is typically more flexible for catching analog events (voltage thresholds, edge rates, pulse widths).

1. Connect a BNC cable from the oscilloscope’s TRIG OUT to the Acute logic analyzer’s EXT TRIG IN.
2. Configure the oscilloscope to trigger on your analog condition of interest (e.g., falling edge of a power rail crossing below 3.1V).
3. In the Acute software, set the trigger source to External, edge-sensitive, matching the polarity of your oscilloscope’s trigger output.
4. Arm the Acute logic analyzer first, then arm the oscilloscope. When the oscilloscope triggers, it sends a pulse to the logic analyzer, which begins capture simultaneously.

Method 2: Logic Analyzer Triggers the Oscilloscope

Use this when you want to trigger on a digital event (e.g., a specific I2C address or SPI chip-select assertion) and observe the analog consequences.

1. Connect a BNC cable from the Acute logic analyzer’s TRIG OUT to the oscilloscope’s EXT TRIG IN.
2. Configure the Acute trigger on your digital condition (protocol trigger, pattern, or edge).
3. Set the oscilloscope to trigger on the external input.
4. Arm the oscilloscope first (in single-shot or normal mode), then arm the logic analyzer. When the digital event occurs, the logic analyzer’s trigger output fires, and the oscilloscope captures the analog waveform.

Method 3: Common External Trigger

For maximum flexibility, derive a trigger from a test point on the target board (e.g., a GPIO toggled by firmware at the start of a transaction). Connect this signal to both the logic analyzer’s EXT TRIG IN and the oscilloscope’s EXT TRIG IN using a BNC T-connector. Both instruments trigger on the same hardware event with minimal skew.

Synchronization Setup

After connecting trigger cables, you need to align the time bases. The two instruments have independent time bases and sample clocks, so a single trigger event provides a common time reference, but any offset between the trigger event and each instrument’s first sample must be accounted for.

Measure and compensate trigger delay: Connect the same fast-edge signal (a 1 MHz square wave from a signal generator) to both an oscilloscope channel and a logic analyzer channel. Trigger both instruments and measure the apparent time difference of the rising edge. This offset is your system’s trigger latency. Note it and apply it when correlating waveforms.

Match time windows: Set both instruments to capture the same time duration centered on the trigger. For example, if you are investigating a 10 ms event, set both instruments to 20 ms total capture with 50% pre-trigger.

Match time scales in the Acute software: The Acute software’s stacking view allows you to import an oscilloscope screenshot or waveform data file (CSV or proprietary format) and overlay it with the logic analyzer capture, aligned by trigger timestamp. Use the Tools > DSO Stacking feature to import and align waveforms.

Using Combined Views for Analog+Digital Correlation

Once both captures are aligned, the Acute software displays logic analyzer channels (with protocol decode) above or below the imported oscilloscope waveform. You can:

• Place a single cursor that spans both the analog and digital domains, reading time-correlated values from each
• Zoom synchronously across both domains
• Search for protocol events in the digital capture and instantly see the corresponding analog behavior

Practical Example: SPI with Power Rail Monitoring

Consider a scenario where an SPI flash read occasionally returns corrupt data:

1. Connect the oscilloscope’s Channel 1 to the 3.3V power rail supplying the SPI flash, and Channel 2 to the SPI clock (for analog quality reference).
2. Connect the logic analyzer to MOSI, MISO, CLK, and CS# for SPI decode.
3. Configure the logic analyzer to trigger on a SPI data pattern matching the known corrupt read value.
4. Use Method 2 (logic analyzer triggers oscilloscope) so the scope captures the power rail at the moment of corruption.
5. After capture, examine the 3.3V rail. If you see a voltage dip below the flash’s minimum operating voltage coinciding with the corrupt read, you have found the root cause: insufficient decoupling or a high-current event on the shared rail.

This type of cross-domain analysis is nearly impossible without time-correlated analog and digital capture, and stacking Acute logic analyzers with your existing oscilloscope provides it without requiring a dedicated mixed-signal oscilloscope purchase.

Summary

Stacking an Acute logic analyzer with a third-party oscilloscope gives you the best of both worlds: deep digital channel count with protocol decode and high-resolution analog waveforms. The setup requires only a BNC trigger cable and a few minutes of configuration. For teams that already own a quality oscilloscope, stacking is a cost-effective alternative to replacing it with an integrated MSO, while often providing more digital channels and more capable protocol analysis than an MSO’s built-in logic analyzer.