Switch capacitance filter is the circuit of a kind of large scale integration that makes by capacitance of MOS switch, MOS and MOS operation amplifier filter. Switch capacitance filter can process imitate signal directly, and need not need shift of A/D, D/A in that way like digital filter, simplified circuit design, raised systematic dependability. In addition, because MOS parts of an apparatus is in speed, compositive degree, opposite precision is controlled and the respect such as small power comsumption has distinct advantage, the swift and violent development that is circuit of switch capacitance filter offerred very good condition.

 

 

The essence of SCF circuit is sampling data system, it handles imitate directly successive signal. Compare with digital filter, leave out device of A/D, D/A, this also is one of reasons that SCF can enter application very quickly. Accordingly, although SCF works in disperse region, but still belong to imitate filter.

 

The imagines main cause to cross resistor and switch capacitance at flowing charge of of all kinds SCF is same. This are very natural, technology of wave of active RC filter replaced inductor effectively, switch capacitance technology above all the tentative idea is to try to use switch electric capacity of course (SC) will replace resistor.

 

The fundamental of switch capacitance filter is, the two node of circuit have the capacitor that leads high speed switch secondhand, its effect is equivalent to the join between this two node a resistor.

 

The simulation of time of a kind of disperse that makes by MOS switch, capacitor and operation amplifier filter. Switch capacitance filter applies extensively at the pulse encode modulation of communication system. In applying actually they make it is normally odd an integrated circuit or do with other circuit in same on chip. The proper join that passes outside terminal can obtain different response characteristic. Certain and alone switch capacitance filter can regard general filter as application. Get used to filter wave oneself for example, dog synthesis of filter wave, oscillatory analysis and music of language make peace. But the blame of the parasitism parameter of operation amplifier bandwidth, circuit, switch and operation amplifier is ideal the noise of character and MOS parts of an apparatus, can affect the performance of this kind of filter directly. The working frequency of switch capacitance filter still not tall, its application range is current mostly frequency of frequency of be confined to paragraph. The switch capacitance filter with the simplest fundamental of switch capacitance filter sees a picture 1. When switch K park is left, u1 of signal voltage source charges to capacitor C1 when;K swings to right, capacitor C1 discharges to voltage source U2. When switch works with the frequency Fc of prep above signal, make C1 is changed alternately between the two voltage node in U1 and U2 receive, so the charge that C1 passes between U1, U2 can form I=fcC1(u1-u2) of average electric current, an equivalent resistor was received between the U1 that is equivalent to graph 1a and U2, its value is 1/fcC1. Such, the switch capacitance circuit of graph 1a but equivalent at first-order active low connect filter (graph 1b) , its transfer function is

 

H(jω)= J (fc/ω) (C1/C2);

 

The ω=2πf in type. From go up form is visible, of switch capacitance filter deliver character to depend on ratio C1/C2 and switch frequency Fc. In fact, graph 1b is an integral circuit, accordingly, switch capacitance filter can be used at the corresponding circuit of imitate filter, what wait in order to realize LC filter, active filter is characteristic.

 

The design devises the method of switch capacitance filter, roughly but end is two kinds big. With imitate successive filter is a foundation one kind, through particular commutation concern the network function commutation successive system is function of corresponding network of disperse time system, so that be directly inside disperse time-domain,design accurately. Can decompose network function at this moment for low rank function, pass cascade or feedback structure implementation with module of switch capacitance circuit next. Another kind is each spur track in using signal to shed plan law or impedance imitate law to replace LC circuit with switch capacitance circuit for prototype with LC echelon filter or resistor, inductance, have between component one to one correspondence relation.

 

Jump filter of active of filter of Ou switch capacitance jumps the implementation of Ou circuit, it is the imitate that is based on filter of pair of passive LC echelon. Jump at this moment each spur track of Ou circuit parts corresponding at passive filter prototype each spur track, and its admittance appears with integral function form. If switch capacitance filter will jump the integral function of each spur track uses Ou circuit the switch capacitance integrator of the input that need branch (graph 2) comes true, and plan the influence that enters termination load, can get and 5 rank LC are low circuit of the wave that connect filter (graph 3a) is opposite circuit of answering switch capacitance filter (graph 3b) , and the low sensitivity characteristic that still maintains filter of archetypal and passive LC.

Switch capacitance integrator is opposite in every clock cycle input signal sampling, produce add phase shift to avoid to output signal, affect filter wave badly to answer, must be like graph 3b in that way, make the switch of photograph adjacent integrator to contrary directional cast. Voltage of switch capacitance filter is retrorse filter of switch switch capacitance also is to use LC filter to be archetypal circuit, but with switch capacitance equivalent component replaces imitate component. The charge in when circuit works, asking with “ voltage retrorse switch ” controls capacitance network flows, the annulus that makes place of the component when movement of switch of equivalent component interior is formed does not have charge flow in the road.

 

Implementation “ voltage is retrorse the method of switch ” is very much, graph 4a is to use the voltage that operation amplifier forms to follow implement formal voltage is retrorse switch, graph 4b is its circuit symbol. Its job process is: Close when switch K1, when K2 is opened, follow because of voltage action, the voltage UH on capacitor CH is equal to input voltage Ua, namely UH=ua; and open in switch K1, when K2 closes, the voltage on capacitance CH is added in operation amplifier to input end reversely. Such, because of operation amplifier empty short circuit, inside every switch cycle, the voltage on port is apropos and retrorse. Switch capacitance filter pursues 5a is form by this kind of method 5 rank the electric principle plan that low electrify presses filter of capacitance of retrorse switch switch, graph 5b is its archetypal circuit. With jump filter of Ou switch capacitance is compared, the operation amplifier number that the circuit of this kind of type needs is less, and the low sensitivity characteristic that still can maintain passive LC network, but its switch clock is phasic the relation is more complex.

 

Switch capacitance filter

Switch capacitance filter still has a lot of planting to make way, the filter of wave switch capacitance that if be on foundation of principle of wave number filter,comes true with switch capacitance. The archetypal circuit of this kind of filter can be LC filter, also can be to contain an unit yuan circuit; and acerb to asking to compare optionally tape understands filter baud sex, baud sex realizes usable N passageway and the pectination filter that filter of capacitance of switch of bogus N passageway presents. They mostly also with LC filter or contain an unit yuan circuit is prototype. Because they provide a characteristic each, can use compose to become the filter wave circuit with type diversity, extensive use.

 

The switch in switch capacitance filter is periodic job, its put through the one share that time holds a cycle only. If a few groups of switch work inside a cycle by turns, with respect to the switch capacitance filter that can make time answer use, can save operation amplifier, simplify circuit. Change clock frequency is changeable circuit parameter, gain of the heartbeat in be like, peak value, alternative wait, because this can be formed general-purpose muti_function filter may filter of program switch capacitance.

 

Filter of capacitance of production technology switch can use NMOS or CMOS craft is made. The discharge leakage current of public errand of the connects electric block, amplifier bandwidth that production technology matters to distributinging capacitance, switch, capacitor and voltage node. Make by standard craft, can satisfy the requirement that uses at frequency limits normally. Apply certain improvement technology can expand one pace reduces working frequency Duan Hejin capacitor public errand.