Page 85 - The Art and Science of Analog Circuit Design

Signal Conditioning in Oscilloscopes and the Spirit of Invention

response will be used by your competitors to find your weaknesses and
attack your product.
Figure 7-2 defines the terms of the frequency and step responses and
shows the meaning of flatness error. Response flatness is a qualitative
notion that refers roughly to gain errors not associated with the poles that
determine the cutoff frequency, or equivalently to step response errors
following the initial transition. To assess flatness we generally ignore
peaking of the magnitude near the 3dB frequency. We also ignore short-
Figure 7-2. term ringing caused by the initial transition in the step response.
Definition of terms Figure 7-2 illustrates the rough correspondence between the high-
and relationships frequency portions of the magnitude response and the early events in the
between the step response. Similarly, disturbances in the magnitude response at low
frequency magni- frequencies generate long-term flatness problems in the step response
tude and step
responses.
Magnitude
Response (dB)

-3dB

Resonances and
transmission line
reflections

Vc l .. „ Frequency (Hz)
,«~t 1 ' i 2 '•* 3 'A 4 ' */ 5 ' ? 7 » 8 Q 9 10
10 10 10 10 10 / 10? 10 10 hO 10
f(-3dB)

Step
Response

Preshoot

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