Microchip Technology MCP6XXXDM-FLTR User Manual
Common Filter Modifications
© 2006 Microchip Technology Inc.
DS51614A-page 27
4.3
FILTER SECTION ORDERING FOR NOISE AND HEADROOM
FilterLab V2.0 orders the filter sections for good dynamic range performance. Its
default selections are:
• Section pole quality factors (Q
default selections are:
• Section pole quality factors (Q
P
) are ordered from lowest to highest (from
Section # 1 to Section # 4)
- In other words, section damping factors (
- In other words, section damping factors (
ζ = 0.5/Q
P
) go from highest to lowest
(from Section # 1 to Section # 4)
• Gains greater than unity are placed in Section # 1 (for best component
sensitivities)
Some applications may need to alter the default section ordering for special
requirements. To compare different section orderings:
• Check the output headroom of each section’s output (V
requirements. To compare different section orderings:
• Check the output headroom of each section’s output (V
OUT
) using the worst-case
input signal; examples include:
- Minimum and maximum DC levels
- Swept frequency sine wave with maximum magnitude
- Voltage step with maximum step size
- Minimum and maximum DC levels
- Swept frequency sine wave with maximum magnitude
- Voltage step with maximum step size
• Measure the noise performance
- Measure the output with a DC input signal (i.e., at mid-supply), an
oscilloscope, and a high gain low noise amplifier
- Calculate the standard deviation of the output; this is the integrated noise in
V
RMS
- The noise can be improved by scaling the resistors, or by changing the op
amps
• Re-connect the sections in a different order
- Usually it is best to leave the high gain section at the front of the filter
- Re-check output headroom and noise
- Re-check output headroom and noise
4.4
COMBINING LOW-PASS AND HIGH-PASS SECTIONS
Some band-pass and band reject filters can be implemented using cascaded low-pass
and high-pass filter sections. These filters have their pass-band frequencies (f
and high-pass filter sections. These filters have their pass-band frequencies (f
PL
and
f
PH
) far apart (e.g., f
PH
/f
PL
> 5.8). The low-pass and high-pass filters are usually
designed separately, then cascaded together.
The Active Filter Demo Board Kit allows the user to try out these filters on the bench
with little effort. They also help debug this type of design.
The Active Filter Demo Board Kit allows the user to try out these filters on the bench
with little effort. They also help debug this type of design.
4.5
HIGHER FREQUENCY FILTERS
Higher frequency filters (e.g., a low-pass filter with pass band edge at 1 MHz) can have
their design initially verified on these boards. Simply scale either the resistors or capac-
itors to a lower frequency design:
• Increase resistors (or capacitors) by a power of 10
• Choose an op amp that is slower by the same power of 10
• Evaluate response:
their design initially verified on these boards. Simply scale either the resistors or capac-
itors to a lower frequency design:
• Increase resistors (or capacitors) by a power of 10
• Choose an op amp that is slower by the same power of 10
• Evaluate response:
- All frequency parameters are divided by the same power of 10
- All time parameters are multiplied by the same power of 10
- All time parameters are multiplied by the same power of 10
For example, a low-pass filter with a pass-bandfrequency of 1 MHz could be scaled
back to 10 kHz.
The final design must be evaluated on a board capable of supporting higher frequency
signals.
back to 10 kHz.
The final design must be evaluated on a board capable of supporting higher frequency
signals.