Tagged hardware
7 pieces
- · 8 min read
Turning a Sensor Off Does Not Turn It Off: Back-Powering Through Shared Bus Pull-Ups
Cut a sensor's supply rail and it can stay awake, drawing current through the pull-up resistors on a shared bus that never switches off. The current arrives through the part's own data pins via its protection diodes. Choosing a lower-power sensor changes nothing, because nothing about this is a property of the sensor. Negligible battery impact is a claim about the topology, not about the part.
- · 7 min read
What a Regulator Asks About Your Biosignal Front End That Your Bench Never Did
Your bench asks whether the signal looks right. A reviewer asks what the system does when it is wrong, across every unit you will ship, in conditions you did not choose. Those are different questions, and the second one is answered in the analog front end months before there is software to test.
- · 9 min read
You Cannot Hire a Gap: Why Cross-Layer Failures Have No Owner
The analog engineer designed a correct reference loop. The actuation engineer designed a correct current source with a safety limit. The mechanical engineer placed the electrodes correctly. Every one of them did excellent work inside their lane, and the device failed, because the failure lived in the loop connecting all three and that loop was on nobody's schematic. A seam is not a gap in anyone's competence. It is the region between all of your competent people.
- · 7 min read
Your Bill of Materials Passes and Your Board Is Still Wrong: Topology vs Parts
Two current-limit resistors in series for redundancy is good design. The same two resistors, both the correct value, placed in parallel instead, quarters your limiting and roughly doubles the current. Every part inspection passes, because both resistors are the right value and the bill of materials is correct. The failure is not in the parts, it is in the topology, and your incoming checks do not look at topology.
- · 7 min read
Common Mode Rejection: How a Biosignal Front End Hears a Whisper at a Concert
When you measure off a body you are not measuring your signal. You are measuring your signal plus the person plus the room, all summed at the amplifier, and the thing you want is microvolts while the mains hum coupling into the person can be a thousand times larger. The entire job of the front end is pulling the small thing out from under the large thing, by throwing away everything that appears equally on both inputs.
- · 8 min read
The Driven Reference Electrode: Your Front End's Hero and Its Hidden Risk
The third electrode is a feedback loop wrapped around a person. It senses the body's common mode and drives against it, which is what turns a hope about electrode symmetry into an actively regulated quantity and gives you rejection figures worth quoting. It is also a deliberately low-impedance connection between your circuit and the body, which makes it a stability problem, a safety-limited path, and the exact node any injected current will come home through.
- · 16 min read
The Return-Path Bug That Kills Your Biosignal Front End
On a closed-loop bio device, the instant you turn the actuator on, the signal can vanish to a flat line, and it is not coupling. The current you inject is a loop that has to come home, and it takes the lowest-impedance path, often the exact electrode protecting your front end. It forces a voltage the amplifier cannot reach, the amplifier rails, and your sensor goes blind. The fix is a decision about where the current returns, not a better part.