Numerous applications exist in industry, particularly with control systems, in which it is desirable to remove all but the lowest frequency components from a signal to effectively yield a dc voltage. This voltage may, for example, serve as a setpoint to a PID controller in a process-control or an HVAC application, in which the cable that is carrying the analog signal is exposed to a wide spectrum of noise, including low-frequency noise components from various sources. These sources could include variable-speed drives, ballasts, transients from switching gear, and motors. In many cases, noise reduction using a conventional lowpass filter can create adverse effects in the response time of the system, even if you use a multipole filter. As an alternative, the circuit of Figure 1 is ideally suited to provide extensive noise reduction for applications such as these without impairing a system's ability to track rapid changes in signal level. The concept involves a lowpass filter with a slewing mechanism that has significant performance advantages over other nonlinear-lowpass-circuit topologies, given its ability to discriminate step changes in signals from noise.

The basic operation of the circuit is to momentarily increase the corner frequency of the lowpass filter formed by R₆ and C₃, using an analog switch, IC₁, upon detection of a step change in signal, allowing V_OUT to track V_IN with little delay. IC₁ has an on-resistance of about 100Ω, so when it closes across R₆, the corner frequency of the circuit changes from 0.016 Hz to approximately 160 Hz, which is ample bandwidth for the target applications for this circuit. IC_2B, along with R₄, R₅, and C₂, operates as an error amplifier with a corner frequency of f_CERR=1/2πR₅C₂=1.59 Hz. The amplifier generates an error signal, V_ERR, that IC_2A measures in reference to V_OUT. IC_2A acts as a floating window comparator that places the lowpass filter into slew mode when V_ERR exceeds the predetermined threshold that zener diode D₃ establishes. For values of V_ERR that are greater than V_OUT, diode D₂ conducts, causing the noninverting input of IC_2A to track this signal. IC_2A compares the signal to a threshold voltage of approximately V_OUT+5.2V at its inverting input. When a negative-step change (V_IN<V_OUT) to the input, V_IN, is of sufficient amplitude such that V_ERR becomes approximately 5.7V (accounting for the barrier potential of D₂), IC_2A's output switches high. This action activates IC₁, causing a short circuit across R₆, thus allowing V_OUT to track V_IN.

For values of V_ERR below V_OUT, the action is similar, except that the inverting input of IC_2A tracks V_ERR through D₃ and D₄, and the comparator's output toggles high at the point at which V_ERR is approximately 5.2V below V_OUT. Although the asymmetry in the window comparator's performance is not of great significance, you could realize improved symmetry by replacing D₂ with a Schottky diode, which has a lower barrier potential. The comparator's trip points, along with the dc gain of the error-amplifier stage (IC_2B)—determined by the ratio R₅/R₄—establish the upper and lower deadband of Figure 1. With the values shown in Figure 1 the circuit triggers to slew in response to negative-step changes in V_IN as small as 0.260V and positive-step changes that are as small as 0.285V. You can realize better sensitivity by reducing the zener voltage of D₃ or by increasing the ratio R₅/R₄ and ensuring that the error-amplifier stage provides adequate roll-off. The roll-off must ensure that noise levels that may exist in a given application cannot trigger the circuit into slew mode.

Another important parameter to consider when choosing component values for the error- amplifier stage is the step response of that circuit, because it directly impacts the overall settling time of the lowpass filter when it encounters step changes in signal. To be conservative, choose values for R₅ and C₂ such that three times the RC time constant they form is well within the settling time desired for the lowpass filter. For example, with R₅=10 MΩ and C₂=0.01 µF, 3τ=0.3 sec. This case represents the approximate worst-case delay to the response to a step change in V_IN that is outside the deadband of the circuit. In practice, however, the delay is much smaller for step changes in V_IN that are larger in magnitude, given the first-order nature of this circuit. Figure 2 illustrates the response of the lowpass-filter design to a 600-mV step change in V_IN. The graphic also illustrates the circuit's significant filtering capabilities on a severe, 30-Hz, 640-mV p-p noise component superimposed on the signal. An ac-coupled view of the filter's performance at steady state illustrates more than 65 dB of attenuation at 30 Hz (Figure 3).