Checking Internal Headroom in a Cascaded Active Filter

A cascaded filter can have acceptable overall gain while an intermediate stage clips. Later sections may attenuate the clipped waveform enough that the final output remains comfortably inside its voltage range. Review each stage’s internal signal level, especially when a high Q section appears early in the chain.

Map the gain through each section

List the response of every section and the cumulative response from the input to that stage’s output. The Analog Devices AN649 filter design note warns that section peaking can exceed amplifier dynamic range and discusses ordering lower Q sections before higher Q sections to help manage that problem.

Treat this as a reason to evaluate ordering, not as proof that one order is always best. Noise, offset, and loading requirements also affect a practical choice.

Use the complete input spectrum

Check the largest wanted input and the largest realistic unwanted input. An interferer close to one section’s resonance can create a larger intermediate voltage than a full scale tone at a convenient passband frequency.

For illustration, a 0.5 V peak input followed by a section with a magnitude gain of four at one frequency produces 2 V peak at that node in the linear model. A later attenuating section cannot restore linearity if the first stage lacks the required swing.

Include bias and amplifier limits

Add the signal excursion to the stage’s DC operating point. Compare positive and negative peaks with output swing under the actual load, input common mode limits, and relevant slew behavior. A symmetric AC waveform around an unsuitable bias can clip only on one side.

Evaluate startup, input steps, and expected overload events separately from a small signal frequency sweep. Capacitor initial conditions can produce a temporary internal excursion that the steady sinusoidal response does not reveal.

Measure intermediate nodes carefully

Observe each accessible stage while applying the relevant frequencies and amplitudes. Use probes that do not materially load the network. Compare the onset of distortion with the calculated headroom and inspect recovery after an overload.

Save stage order, per-stage maximum excursion, bias levels, load, and measured distortion. If sections are reordered or gain is redistributed, repeat the complete response and noise checks because the final ideal transfer function alone does not describe those practical changes. A successful review establishes linear operation throughout the chain under the defined input conditions. A final output that never reaches a supply rail is not sufficient evidence that the entire filter remains linear.