Single Supply Active Filter Design

Single supply circuits can have reduced complexity, cost, and power consumption, and increased design life.

This page covers the following topics:

1. modifying active filter circuits for single supply operation
2. implementing a virtual ground power source
3. capacitively coupling the input signal
4. resistor divider - required for some filter topologies

[1] Modifying Active Filter Circuits for Single Supply Operation

Most active filter topologies can be converted from dual supply operation to single supply operation using the following transformations:

1. all grounds are shifted to a Virtual Ground (Vgnd) at DC potential Vcc/2
2. all -Vcc op amp negative supplies are shifted to ground

For example, this Fliege band pass filter stage:

is transformed into this configuration:

However, a few topologies, including the Sallen-Key low pass, require instead that a DC bias equal to Vcc/2 be applied to the stage input. Please see section 4 for more information.

Note that 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.

[2] Implementing a Virtual Ground Power Source

There are three reliable ways to create a Virtual Ground power source, using either:

A - the TLE2426 IC
B - a buffered resistor divider
C - a voltage regulator

A - TLE2426 IC

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.

B - Buffered Resistor Divider

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:

OP AMP	Imax (ma)	Package
LMH6642	70	SOT, SOIC
OPA690	190	SOT, SOIC
OPA551	200	DIP, SOIC
BUF634	250	DIP, SOIC
LT1210	1100	TO-220-7

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.

C - Voltage Regulator

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.

[3] Capacitively Coupling the Input Signal

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.

The capacitor might be 0.1-1uF for audio frequencies, and of smaller value at higher frequencies.

[4] A Resistor Divider may Instead be Required

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.

If you are designing with Filter Wiz PRO, the following topologies require a resistor divider network on the stage input:

* - required only if Rd not present

Typical values might be 100K for RA and RB, and 1uF for Cc (if required). However, because these components will interact with the filter circuit, it is definitely preferable to add an op amp buffer.

For multi-stage filters, it is likely unnecessary to capactively couple between filter stages. An exception would be when connecting a stage biased with a Virtual Ground, and a stage biased with an input resistive divider (see section 4).

Requirements:

• Windows 7,Vista,XP
• min. 1024x768

________________________