Not Recommended for New Designs

LMF100

SNOSBG9B – JULY 1999 – REVISED JUNE 2015

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10 Power Supply Recommendations

The LMF100 can operate with a single-ended power supply as well as bipolar supplies. Refer to Figure 56

through Figure 58 for methods of generating V+/2 for single-supply operation. In this circumstance, pins VA+ and

VD+ are connected to the positive power supply (4 to 15 V), and VA

− and VD− are connected to ground. The

AGND pin must be tied to V+/2. Furthermore, the half-supply node should be very “clean”, as any noise

appearing on it will be treated as an input to the filter. Ensure liberal bypassing is employed to reject any supply

noise and present a low impedance to the clock frequency. Bypass caps should always be located as close to

the supply pins a practical. Moreover, the regulator or op-amp approaches of generating V+/e is preferred for

very low clock frequency applications. The main power supply voltage should also be clean (preferably

regulated) and bypassed with 0.1-µF nonpolar ceramic capacitor. If there is no bulk cap nearby, a 10-uF

electrolytic tantalum in parallel with the 0.1-µF supply bypass cap should achieve cleaner and optimal transient

response. Select capacitors with low ESR and ESL rating and test them to ensure no ringing occurs. The power

source is preferably a linear supply or regulator. If a switching supply is used ensure it is a clean switcher and

deploy proper bypassing or post regulate with an LDO as necessary.

11 Layout

11.1 Layout Guidelines

The most critical part to the success of a switched capacitor filter design is a properly layout PCB. Because of

the mixed signal circuitry involved, take extra care in the board design for noise abatement, star-grounding, and

shielding techniques. A ground plane must separate digital and analog ground planes if possible, or have

separate paths and join together only at the common return node at the supply source. All component leads and

PCB tracks are kept as short as possible. The filter clock input should be a shielded cable.

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