An analog switch can have acceptable insertion loss yet worsen distortion when the signal amplitude increases. Its on-resistance is not necessarily constant across the signal voltage range. Combined with a finite load, that changing resistance creates signal-dependent attenuation. A resistor chosen to imitate the nominal switch resistance will therefore reproduce loss without reproducing the nonlinear error.
Include the actual load
Model the path as the source impedance, switch, and receiving impedance. Analog Devices explains the connection between resistance flatness and distortion in its analog-switch THD design note. Use the selected component’s curves and test conditions; a distortion figure measured into a light load does not establish performance into a much smaller resistance.
As an illustrative first estimate, a resistance change of 2 ohms is a much larger fraction of a 200-ohm path than of a 20-kilohm path. This comparison predicts sensitivity, not a precise harmonic spectrum. The shape of resistance versus signal voltage, the source impedance, and the signal excursion determine which harmonics appear. Supply voltage can also change the resistance curve.
Separate competing distortion mechanisms
Measure a direct connection, a fixed resistor with comparable insertion loss, and the switch under otherwise identical conditions. Sweep signal amplitude at several DC bias levels while keeping the output within the receiver’s linear range. If moving the bias changes distortion without changing peak-to-peak amplitude, inspect where the waveform crosses the switch’s resistance curve.
| Experiment | What it helps distinguish |
|---|---|
| Increase load resistance | Resistance modulation from source distortion |
| Change DC signal bias | Dependence on analog terminal voltage |
| Lower signal frequency | Static nonlinearity from dynamic settling effects |
| Bypass the switch | Instrument and amplifier residual distortion |
Buffering the output can reduce loading, but the buffer adds its own input range, noise, and distortion constraints. A lower-resistance switch may have more capacitance or a different charge-injection behavior. Select against the whole signal path rather than treating the smallest resistance number as the sole objective.
Preserve a reproducible comparison
Save source amplitude at the board, load impedance, switch supply, bias, frequency, and analyzer bandwidth. Check buffer output swing under load before attributing every harmonic to the switch. Use the separate off-isolation and crosstalk review when an unwanted channel contributes a tone; that is a different mechanism from nonlinear attenuation of the selected signal.
Move from reading to a build-ready project
Use the fabrication and assembly checklists to prepare drawings, production files and test requirements.
Open the DFM checklist