Single Supply Active Filter with Virtual Ground

All filter stages utilizing a Virtual Ground will exhibit the Virtual Ground DC potential at their outputs, whereas all stages employing a resistive divider at their input (without a Virtual Ground) will present the divider potential at their output. For supply currents up to 20 mA, the TLE2426 (and the TLE2425 for 5V circuits) serves as a cost-effective and space-efficient precision Virtual Ground. The output impedance is typically less than 1 ohm for frequencies up to 30 kHz when sourcing or sinking 10 mA. The 8-pin version includes an additional Noise Reduction pin, which allows for an external capacitor to bypass the internal divider junction (a 1 µF capacitor is recommended). It is feasible to add capacitors between the output Vgnd and the two power rails to provide higher instantaneous source and sink currents, provided that the average current does not exceed the TLE2426's output current capacity. It should be noted that any such capacitors will introduce poles that could affect the active filter circuit response. This circuit can serve as an alternative to the TLE2426 or in scenarios where the required Virtual Ground current exceeds 20 mA. For small currents (~5 mA), a general-purpose operational amplifier can be utilized, such as the TL071 or uA741. For very high currents, alternative op-amps may be considered. It is important to note that, as with all filter op-amps, both power rails should be bypassed with 0.1 µF and 1 µF capacitors. Star grounding of all true grounds (excluding Virtual Ground) is recommended. Several micropower voltage regulators offer adjustable voltage outputs and a minimum output current of 40 mA, with very low quiescent current. The ADM663 (also known as MAX663) is among the most common and can be configured with only two external resistors, with an optional third resistor for current limiting (Rcl). If current limiting is unnecessary, the SENSE pin should be connected to Vout2. If the DC bias of the signal input to the active filter differs from the Virtual Ground, capacitive coupling must be employed. The only exception occurs when the filter stage already incorporates a capacitor at the input, as seen in many high-pass topologies. Certain filter configurations, such as the Sallen-Key low-pass (refer to section 1), necessitate a resistive divider to DC bias the input. The addition of this network eliminates the need for a Virtual Ground. If Filter Wiz PRO is used for filter design, exporting an .ES1 file to the free application eSketch can facilitate circuit simulation to verify appropriate values for coupling capacitors and divider resistors.

The described circuit architecture effectively integrates Virtual Ground and resistive divider configurations to achieve desired filtering characteristics. The TLE2426 and TLE2425 voltage regulators are pivotal components in this design, providing robust Virtual Ground functionality while maintaining low output impedance, which is critical for high-frequency applications. The ability to bypass the internal divider with an external capacitor enhances noise performance, making it suitable for sensitive applications.

The introduction of capacitors between Vgnd and the power rails allows for increased current handling capabilities, which is essential in dynamic signal processing scenarios. However, careful consideration must be given to the impact of these capacitors on the frequency response of the filter. The recommendation for star grounding emphasizes the importance of minimizing ground loops and ensuring signal integrity in multi-stage filter designs.

Operational amplifiers such as TL071 and uA741 offer versatility for low-current applications, while high-current scenarios necessitate more robust op-amps. The use of micropower voltage regulators like ADM663 simplifies circuit design by allowing for adjustable outputs with minimal external components, thereby enhancing overall system efficiency.

In scenarios where the input signal's DC bias does not align with the Virtual Ground, the necessity for capacitive coupling is underscored, ensuring that unwanted DC offsets do not adversely affect filter performance. The Sallen-Key topology's requirement for a resistive divider highlights the various design considerations that must be navigated when developing active filters.

Utilizing software tools like Filter Wiz PRO in conjunction with eSketch provides an efficient means of validating design parameters, ensuring that the resulting circuit meets the intended specifications for performance and stability. This comprehensive approach to filter design, encompassing component selection, grounding strategies, and simulation, is essential for achieving optimal results in electronic circuit applications.All filter stages using a Virtual Ground will have the Virtual Ground DC potential on their outputs, while all stages having a resistive divider on their input (and no virtual Ground) will have the divider potential on their output. For supply currents of up to 20 ma, the TLE2426 (and the TLE2425 for 5V circuits) provide a low cost and space effective precision virtual ground.

Output impedance is typically less than 1 ohm for frequencies up to 30kHz ( when sourcing or sinking 10 ma). The 8-pin version has an additional Noise Reduction pin, allowing an external capacitor to bypass the internal divider junction (a 1uF capacitor could be used).

It is possible to add capacitors between the output Vgnd and the two rails, in order to provide higher instantaneous source and sink currents, as long as the average current doesn`t exceed the TLE2426 output current capacity. Note that any such capacitors will add poles that could interfere with the active filter circuit response.

This circuit can be used as an alternative to the TLE2426, or whenever the required Virtual Ground current exceeds 20ma. For small currents (~5ma), a general purpose op amp can be used, such as the TL071 or uA741. For very high currents, the following op amps could be considered: Note that as with all filter op amps, both rails should be bypassed with 0.

1uF and 1uF capacitors. Star grounding of all true grounds (Virtual Ground excluded) is recommended. Several micropower voltage regulators provide an adjustable voltage output and at least 40ma output current, with very low quiescent current. Probably the most common is the ADM663 (also MAX663), which can be configured with only 2 external resistors, and an optional third resistor for current limiting (Rcl).

If current limiting is not required, then SENSE should be connected to Vout2. If the DC bias on the signal being input to the active filter is not the same as the Virtual Ground, then capacitive coupling must be used. The only exception is if the filter stage already has a capacitor on the input, as is the case of many high pass topologies.

A few filter topologies, such as the Sallen-Key low pass (see section 1), require a resistive divider to DC bias the input. Addition of this network negates the requirement of a Virtual Ground. If Filter wiz PRO is being used to design the filter, it is helpful to export an. ES1 file to the free application eSketch, and simulate the circuit to ensure proper values of coupling capacitor and divider resistors.

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