Establishing requirements for circuit design

Requirements are difficult because customers describe needs vaguely, needs conflict, and later changes are costly. Good requirements are complete, unambiguous, measurable, testable and traceable. The circuit design is built from them step by step.

Establishing requirements

1. Identify the stakeholders (customer, user, manufacturer, maintenance, regulator) and collect needs by interview, questionnaires, studying existing products and observation.
2. Define the function: what the system must do, inputs and outputs (signals, ranges), and operating modes.
3. Define the specifications in measurable numbers: accuracy, resolution, speed/bandwidth, power, supply, size, weight, cost, operating temperature, humidity.
4. Define constraints: standards (EMC, safety), environment, reliability (MTBF), maintainability, availability of parts, time and budget.
5. Separate "must" from "nice to have", resolve conflicts, and agree with the customer on a written requirement specification (SRS) with acceptance tests.

Converting to a circuit design

 Requirements -> Specification -> Block diagram
      -> Component selection -> Detailed schematic
      -> Simulation -> Prototype -> Test -> PCB

1. Block diagram (architecture): divide the function into blocks (sensor, signal conditioning, ADC, processor, display, power) and define the interfaces.
2. Trade-offs: choose among alternatives (microcontroller vs discrete logic, analog vs digital) by cost, performance, power and risk.
3. Component selection: select parts that satisfy the specification with margin (tolerance, temperature, derating, availability).
4. Detailed design: schematic, calculations for every block (gain, filter, power budget), worst-case analysis.
5. Simulation and prototype: verify with SPICE, build a breadboard/prototype, and test against each requirement.
6. Layout, review and documentation: PCB design with noise rules, design review, final test and a traceability matrix showing that each requirement is met.

Reviews with the customer at each stage reduce rework.