An analog switch is two MOSFETs and an inverter — and like most deceptively simple circuits, its real-world behavior lives in the fine print. Signal-chain errors blamed on ADCs or amplifiers often trace back to a multiplexer's charge injection or a switch's temperature-dependent on-resistance. Here is what the specifications mean and how they interact.
On-Resistance: Two Numbers, Not One
RON matters, but RON flatness matters more in precision designs:
- A switch's resistance varies with signal voltage (the NMOS and PMOS halves trade dominance across the input range). This flatness turns RON into a signal-dependent gain error when the switch drives a load — the classic cause of distortion in multiplexed audio and sensor channels.
- Low-RON parts inject more charge — the trade is fundamental. High-speed routers accept 1–5 Ω RON; precision DC multiplexers choose 100 Ω–1 kΩ parts and arrange high-impedance loads.
- Temperature drifts RON by 20–30 % over the industrial range; a switch driving a load directly converts that into gain drift. Buffer after the switch when RON × load is significant.
Charge Injection: The Glitch Nobody Expected
When the switch opens or closes, channel charge dumps into your signal — a voltage step of Q/C on the load capacitance. A 5 pC injection onto 100 pF produces a 50 mV glitch. Mitigations:
- Charge-injection-compensated parts inject a cancelling charge; residual levels of 0.5–2 pC are achievable.
- Increase the load capacitance (the glitch divides by C) at the cost of settling speed.
- Break-before-make timing matters in multiplexers: the part guarantees the previous channel disconnects before the next connects — without it, two sources short through the switch array.
Leakage: A DC Error That Grows With Temperature
Off-isolation and leakage currents look tiny at room temperature — nanoamps — but leakage roughly doubles every 10 °C. A 1 nA leakage into a 1 MΩ source impedance becomes 1 mV at 25 °C and 8 mV at 85 °C. For high-impedance sensors (photodiodes, pH probes, piezoelectrics), spec the leakage at your maximum ambient, not the datasheet's.
Bandwidth, Settling, and Protection Features
- -3 dB bandwidth is quoted for a specific load; settling to 12–16 bits takes far longer than the bandwidth suggests. Check the settling-time specification at your resolution.
- Fault-protected switches keep their outputs safe when inputs exceed the rails — power-off overvoltage protection is worth its premium in hot-swap and industrial input panels.
- Crosstalk and off-isolation matter most when disabled channels carry fast edges; verify at your frequency, not DC.
Matching the Part to the Job
- DC precision multiplexing → moderate RON, lowest leakage and charge injection, buffer the output.
- Audio/video routing → low RON flatness and high bandwidth; distortion spec matters more than RON.
- High-speed data routing → bandwidth and off-isolation first; accept higher charge injection.
- Harsh inputs → fault-protected families with overvoltage clamping.
JTDZ Tech stocks analog switches, multiplexers, and fault-protected variants from TI, ADI, and onsemi with full RON/flatness and injection data. Describe your signal chain and switching rate and we will quote the switch that disappears into the measurement.
