Poor ground plus good filtering?

25 October 2000 Circuit & System Protection

New developments by Murata have resulted in the new terminal capacitors for noise suppression in the GHz range, combined with a more than 50% reduction in the footprint.

Two well-established families of its 0805 sized filters are now also available as four element arrays in 1206 packages, reducing the occupied area from 10,12 mm2 to 5,0 mm2.

The first of the two ranges offers electrostatic capacitance values from 22 pF to 22 nF rated at 16 to 25 V d.c. Low residual inductance ensures high frequency performance. The package has two ground terminals and is nonpolarised.

Applications for these filters are in PCs and peripherals that tend to emit high levels of noise, compact products such as PDAs, PC cards and mobile telecommunications tools. Murata says that the second range is particularly effective where no stable round is available; a resistive element in the filter absorbs noise and returns it to ground.

A distributed constant circuit contributes a smooth change of impedance, preventing reflection of signals or distortion of waveforms.

Compressing the four into the smaller area gives even better filtering at high frequencies than is achieved by the discrete versions, claims Murata.

This series has capacitance values of 10, 47 or 100 pF, each of which can be matched with a resistance value of 6, 8, 47 or 100 W. All, except for the one highest value (16 V d.c.), are rated at 25 V d.c. These will find homes in interface or clock lines, word processors and the like.

The electrostatic range is identified as the Murata NFA3216D and contains four NFM39Rs; the distributed constant series is the NFA3216G and contains NFM839Rs.

Murata's whole group of products can help designers solve problems. Discrete filters are easy to exchange providing flexibility when testing new circuits for optimum performance. Production can then go ahead using the arrays to get the space and production efficiency benefits. They are also drop-in replacements for the Murata ferrite bead arrays.

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