4

At low frequencies, the value required for the frequency-

setting resistors can be excessive. Resistor values above

about 5M

Ω can react with parasitic capacitance causing

below 5M

Ω . When f

An external 5.49k

Ω resistor, R

eliminate the need for external capacitors. At the other

when using an inverting pole-pair configuration or when

using a noninverting configuration with Q < 0.57.

PP1 (Noninverting pole-pair subcircuit using internal gain-

selection mode, this configuration is used for all band-pass

filter responses. This configuration allows the combination

of unity pass-band gain and high Q (up to 400). Since no

external gain-set resistor is required, external parts count is

minimized.

PP2 (Noninverting pole-pair subcircuit using an external

used when the pole-pair Q is less than 0.57.

PP3 (Inverting pole-pair subcircuit)—See Figure 9A. In the

automatic topology selection mode, this configuration is

used for the all-pole low-pass and high-pass filter responses.

PP4 (Noninverting pole-pair/zero subcircuit)—See Figure

10. In addition to a complex pole-pair, this configuration

produces a j

ω-axis zero (response null) by summing the low-

in the UAF42. In the automatic topology selection mode,

this configuration is used for all band-reject (notch) filter

responses and Inverse Chebyshev filter types when

Q > 0.57. This subcircuit option keeps external parts count

PP5 (Noninverting pole-pair/zero subcircuit)—See Figure

11. In addition to a complex pole-pair, this configuration

produces a j

ω-axis zero (response null) by summing the low-

in the UAF42. In the automatic topology selection mode,

this configuration is used for all band-reject (notch) filter

responses and Inverse Chebyshev filter types when

Q < 0.57. This subcircuit option requires an external gain-set

PP6 (Inverting pole-pair/zero subcircuit)—See Figure 12. In

addition to a complex pole-pair, this configuration produces

a j

ω-axis zero (response null) by summing the low-pass and

UAF42. This subcircuit is only used when you override the

automatic topology selection algorithm and specify the in-

verting pole-pair topology. Then it is used for all band-reject

(notch) filter responses and Inverse Chebyshev filter types.

The program automatically places lower Q stages ahead of

higher Q stages to prevent op amp output saturation due to

gain peaking. Even so, peaking may limit input voltage to

less than

±10V (V

for each filter design is shown on the filter block diagram.

If the UAF42 is to be operated on reduced supplies, the

maximum input voltage must be derated commensurately.

To use the filter with higher input voltages, you can add an

input attenuator.

The program designs the simplest filter that provides the

desired AC transfer function with a pass-band gain of

1.0V/V. In some cases the program cannot make a unity-

gain filter and the pass-band gain will be less than 1.0V/V.

In any case, overall filter gain is shown on the filter block

diagram. If you want a different gain, you can add an

additional stage for gain or attenuation as required.

To build the filter, print-out the block diagram and compo-

nent values. Consider one subcircuit at a time. Match the

subcircuit type referenced on the component print-out to its

corresponding circuit diagram—see the Filter Subcircuits

section of this bulletin.

The UAF42 Filter Component Values print-out has places to

display every possible external component needed for any

subcircuit. Not all of these components will be required for

any specific filter design. When no value is shown for a

component, omit the component. For example, the detailed

schematic diagrams for complex pole-pair subcircuits show

external capacitors in parallel with the 1000pF capacitors in

the UAF42. No external capacitors are required for filters

above approximately 10Hz.

After the subcircuits have been implemented, connect them

in series in the order shown on the filter block diagram.

FILTER SUBCIRCUITS

Filter designs consist of cascaded complex pole-pair and

real-pole subcircuits. Complex pole pair subcircuits are

based on the UAF42 state-variable filter topology. Six varia-

tions of this circuit can be used, PP1 through PP6. Real pole

sections can be implemented with the auxiliary op amp in

the UAF42. High-pass (HP) and low-pass (LP) real-pole

sections can be used. The subcircuits are referenced with a

two or three letter abbreviation on the UAF42 Filter Compo-

nent Values and Filter Block Diagram program outputs.

Descriptions of each subcircuit follow:

POLE-PAIR (PP) SUBCIRCUITS

In general, all complex pole-pair subcircuits use the UAF42

in the state-variable configuration. The two filter parameters

that must be set for the pole-pair are the filter Q and the

needed to set Q.