[Paper Review] A Low Cost Discrete Digital Isolator Circuit

T. J. Conway|arXiv (Cornell University)|Mar 25, 2026

Electrostatic Discharge in Electronics0 citations

TL;DR

The paper presents a fully discrete, low-cost digital isolator using general-purpose transistors and PCB trace transformers to achieve >1 kV isolation, ~200 ns delay, and 1 Mbps NRZ data rate.

ABSTRACT

This work presents a fully discrete, low cost digital isolator requiring no specialized ICs and implemented entirely with general purpose transistors and a two layer PCB embedded air core transformer. The design avoids vendor lock in and long term component obsolescence risks, while providing >1 kV isolation, ~200 ns propagation delay, and validated NRZ data rates of 1 Mbps. A modified dual oscillator architecture enables inherent hardware lockout suitable for half bridge gate driver applications. Measured performance and PCB layout guidelines are provided.

Motivation & Objective

• Motivate the need for discrete digital isolation to avoid vendor lock-in and obsolescence risks.
• Propose a fully discrete isolator implemented with general-purpose transistors and PCB‑embedded air-core transformers.
• Demonstrate isolation performance, timing, and data-rate capabilities suitable for UART/SPI-like interfaces and half-bridge signaling.

Proposed method

• Use a cross-coupled MOSFET pair to form a fast startup oscillator on the transmitter side.
• Couple transmitter and receiver using PCB trace spiral coils acting as air-core transformers across two PCB sides.
• Rectify and amplify the induced signal on the receiver side with NPN transistors to produce a digital output.
• Achieve isolation through FR4 PCB material and eliminate the need for high-voltage discrete components.
• Characterize performance with 1 Mbps NRZ signaling and eye diagrams to validate data rate and integrity.

Figure 2: Layout of transmitter (top side) and receiver (bottom side) for 2 layer PCB

Experimental results

Research questions

• RQ1 Can a fully discrete digital isolator achieve practical isolation (>1 kV) without specialized ICs?
• RQ2 What data rate and propagation delay can be achieved with PCB trace transformers and discrete transistors?
• RQ3 Is the design suitable for half-bridge gate-driver signaling with inherent lockout functionality?

Key findings

• Isolation exceeding 1 kV is achieved using FR4 PCB material and discrete components.
• Propagation delay is measured around 200 ns.
• The design supports NRZ data rates of 1 Mbps, with an eye diagram indicating reliable transmission at this rate.
• Prototype PCB layout uses a 2-layer board with 8-turn PCB trace coils and no vias through the transformer path.
• Estimated cost per unit is about 50 cents, with TX power up to 25 mW and RX power up to 5 mW.
• The circuit demonstrates a hardware lockout feature suitable for half-bridge driver signaling.

Figure 3: Measured logic transfer for the prototype PCBs with a 1 Mbps NRZ random data signal