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The choice of CMOS imitate switch and representative application

From;    Author:Stand originally
Inchoate imitate switch big multitask makes the power source voltage at ± 20V, guide electrify block is hundreds of ohm, basically use at imitate signal and digital pilot interface, in last few years, the function of compositive imitate switch had very big rise, they can work in voltage of low-down power source, have guide electrify block, miniature encloses dimension and admirable switch character inferiorly. Be used extensively at checking equipment of equipment, communication product, PBX/PABX and multimedia system to wait. A few have low the ideal substitute that guides the imitate switch of electrify block and low job voltage becomes mechanical relay.

The use method of imitate switch is simpler, but logical choice should be made according to effective use in specific applying. The article basically introduces the representative application of the main character of imitate switch and switch of a few kinds of special imitate.

Choose CMOS switch correctly:

1, guide electrify block: Traditional imitate switch is formed by MOSFET of N raceway groove and paralell connection of MOSFET of P raceway groove, can make signal two-way transmit, if will different VIN is worth a corresponding P of raceway groove MOSFET and MOSFET of N raceway groove guide electrify block is shunt-wound, guide below attainable shunt-wound structure electrify block (RON) along with input voltage (VIN) change concerns, if take no account of the influence of voltage of temperature, power source, RON shows linear concern along with VIN, will cause the change that inserts loss, make total harmonic of imitate switch generation lack fidelity (THD) , design personnel place does not hope this, the metabolic volume that how follows RON VIN falls to the smallest also be a design a crucial problem that place of switch of new generation imitate faces

A few kinds of special imitate switch:
1, high frequency T switch
T switch applies to the application of video and 10MHz of other frequency prep above, if the graph is shown 4 times, it by two imitate switch (S1, S3) series connection composition, another switch S2 receives the node in the ground and S1, S3 between, its close the switch of this kind of structure to keep apart prep above individual switch, because switch of parasitism capacitance and every series connection is shunt-wound, disconnect its allow the T of condition switch to rise and increase what the gender strings together faze to follow frequency. Accordingly, affect what the key with high frequency and characteristic switch depends on switch to disconnect condition is not to put through condition.
Guide when T switch when connecting, s1 and S3 close, s2 disconnects; When switch disconnects, s1, S2 disconnects, s3 closes, right now, the input signal that the parasitism capacitance coupling that those want to pass series connection MOSFET carries to output by S3 bypass, disconnect the 10MHz video T below condition switch (MAX4545) close segregation to amount to - 80dB, and standard imitate switch (MAX312) close segregation to spend only - 36dB.
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