Integrate the application skill of the multi-channel analog switch
Integrate the multi-channel analog switch ' Hereafter referred to as the multiple-way switch) It is the daily devices of important technical fields such as the automatic data acquisition, programme-controlled gain amplification,etc., the rigorous and reliability to the system of the quality of its actual usability influences importantly. About the application technology of the multiple-way switch, recommend that bipunctate is insufficient on some literature: First, introduce to device more, and introduce to the device less to rational matching and coordination of the associated circuit; Second, the thing of the sense of principle is introduced more, and the operational thing is introduced less. Studies have suggested: Only choose the kind of the multiple-way switch correctly, pay attention to rational matching and coordination of multiple-way switch and associated circuit, guarantee every circuit unit has appropriate working condition, could give full play to the characteristic of the multiple-way switch, the ones that even remedied some performance index were deficient, achieved the anticipated result. This text studies the multiple-way switch application skill proceed from the angle of employing. The multiple-way switch on the market takes CMOS circuit as the core at present, so the following discussions, besides especially proving, direct against this kind of product.
" break-before-make " With " broken after being open first " Choice
The make-and-break of multiple-way switch on the market switches over ways at present mostly " Break-before-make " ( Break-Before-Make) .
In automatic data acquisition, should choose " break-before-make " Multiple-way switch. Otherwise, two phenomena of passway short circuit will take place, will damage signal source or multiple-way switch when being serious. However, in the gain-programmed amplifiers, if change the feedback resistance of the integrated operational amplifier with multiple-way switch, in order to change the gain to noise temperature ratio of the amplifier, it is unsuitable to choose " break-before-make " Multiple-way switch. Otherwise, the amplifier will present the open loop state. The open loop gain of the amplifier is extremely high, apt to destroy the normal operation of the circuit, even damage the components and parts, should generally avoid.
2 is chosen the appropriate transmission signal input mode
The transmission signal generally has two ways of single-end inputting and differential input, are suitable for different occasions respectively.
The input mode of single-end is shown as in Fig. 1, namely the same signal ground of a head joint of all signal sources once, the signal is connected to simulation of ADC,etc., another end of every signal source connects the multiple-way switch respectively. Vs is a transmission signal in the picture, Vc is a common mode undesired signal in the system.
Fig. 1 (a) There are to need to last number of channels on the connection one,can also common mode inhibitory potencies of security system; The shortcoming is to only suitable for consulting a public electric potential in all transmission signals, and every signal source is put under same noisy environment, otherwise will introduce additional different mode to interfere with.
Fig. 1 (b) The connection is suitable for the measurement publically to systems simulation of all transmission signals, and the signal level is obviously greater than the common-mode interference in the system. Its advantage can get most numbers of channels, the shortcoming is that the system has basically lost inhibitory potencies of the common mode.
The differential input mode is shown as in Fig. 2, namely connect respectively the input terminus of the way switch at most of both ends of all signal sources. Advantage its against common-mode interference ability strong, shortcoming whether actual number of channels have only the intersection of single-end and half of input mode. When the signal and noise of the transmission signal is lower, must use the differential input mode.
3 influence of reducing the on resistance
Multiple-way switch on resistance RON ( So so from 10Ω to 1kΩ about) Than the contact resistance of the mechanical switch ' It is generally the order of magnitude mΩ) Big much, the intersection of signal transmission and precision or the intersection of control and the intersection of gain to noise temperature ratio and amplifying gain to noise temperature ratio Cheng in automatic data acquisition influence more obviously, and RON passway is changed with the changes of the level of mains voltage, range of the transmission signal,etc., therefore its influence is difficult to revise on later stage. Usually try to reduce RON in practice to reduce its influence.
Take CD4051 as examples, test is found [1 ]: RON of CD4051 and inputs the change of the simulation voltage to change with the mains voltage. Act as VDD =5V, VEE =0V hour, RON =280Ω, and with the change sudden change of V1; Act as VDD> 10V, VEE =0V hour, RON =100Ω, and tempolabile with the change of V1. It is obvious, the influence that appropriate VDD which improves CD4051 helps to reduce RON. Must notice: While improving VDD, should improve the input logic level which strobe the control end A, B, C correspondingly. For example: Fetch VDD =12V(VEE =0V) ,Can adopt the mains voltage to pull upward the method to clamp, the resistance-value pulled upward the resistance fetches above 1.5kΩ, make the effective high level which strobes the control end signal not lower than 6V. In this way, guarantee CD4051 ideal is turned on (it is RON small, also realize the conversion (μ P is generally TTL level) of CMOS level and TTL level .
It is obvious, according to the detailed conditions, the appropriate mains voltage of improving the multiple-way switch, reduce a kind of effective measure that its RON influences. In addition, improve the mains voltage appropriately, can also reduce the way bad Δ RON of the on resistance and accelerate the switch speed at the same time.
4 dispels and shakes the error caused
And mechanical switch similar, multiple-way switch have course of shaking too while switching over in the passway, will present the transient phenomenon. If gather the outcoming signal of the multiple-way switch at this moment, may introduce very great error. Such as [2 ]: Three pieces of analog quantity that some computer automatic data acquisition and processing system is gathered: The water pump rotational speed, flowrate, pressure. Correspondent TTL level of three pieces of analog quantity is: 1.5454V, 1.5698V, 2.9394V. The acquisition system is from 10 times to these three analog quantity consecutive collection separately of passway 1, 2, 3, gather the result and lie between 1.8554- 1.8603, 1.5625- 1.5673, 1.62207- 1.62695, 1, 3 among them, the error of passway is very large. Discover the error because system switch over, stabilize, come down, gather whom data cause in multi-channel switch on and off.
There are two kinds of to dispel commonsense methods shaking: First, realize with circuit of the hardware ' Hardware method) ,The instant using RC electric-wave filter is eliminated and shaken; Another kind solve with the delayed method of the software ' Software method) . In the system with μ P, software method
Show the advantage even more than the hardware method. In the example as above, only when the original QuickBASIC data acquisition procedure adds a do loop to prolong appropriately, gather the result and lie between 1.5454- 1.5478, 1.5698- 1.5722, 2.9394- 2.9418, gather the precision to obviously improve, it is normal to gather the result.
5 improves and switches over the speed
There are relations at switch speeds and its autogenous structure, situations of the working condition and connecting circuit of the multiple-way switch. Should pay attention to the following several points in practice:
Average propagation delay time tpd of all multiple-way switches reduces with rising of VDD. Take CD4051 as examples [3 ], act as VDD =5V hour, tpd =720ns; Act as VDD =10V hour, tpd =320ns; Act as VDD =15V hour, tpd =240ns. It is obvious, the appropriate mains voltage of improving the multiple-way switch, can accelerate its speed of switch.
The signal source internal resistance Rs of the transmission signal has important influence on switching time of the multiple-way switch. Analyze that shows: Under the invariable situation of other conditions, it is proportional to Rs that switching time is similar to, namely the smaller Rs is, the sharper the movements of the switch are. So, the signal source (some sensors are so) to high internal resistance ,Should spend the impedance transformer ' Such as resistance follower) ,Insert the multiple-way switch after hindering the resistance low. In addition, reducing Rs can also reduce the error that the shutoff leakage current of the multiple-way switch causes at the same time.
When there are more numbers of channels of required signal of systems, should adopt 3 houses of Fig. Two-step connection way shown. In Fig. 3, suppose the system needs 32 signal channels altogether, divide these 32 passways into 4 groups, every group connects it to 4 second switch separately, the signal is by second switched output. Set up, for C0, output total electric capacity from 32Co to about 12Co by each output capacitance of switch, the time constant of the circuit is reduced, the switch speed is improved. In addition this kind link way can also make to shut off leakage current drop 31Iz to about 10Iz (set up each the intersection of shutoff and leakage current of switch for Iz) always multiple-way switch ,Thus reduce and shut off the error that the leakage current causes. To two kinds of above-mentioned function, the more prominent many results of the number of channels are. Certainly, the the intersection of connection and the intersection of way and required switch count relative more than a bit, strobe control stand opposite each other complicated a bit too, used mainly in more occasions of the number of channels of signals.
The multiple-way switch on the market is seen more with the products in companies such as RCA, AD, SILICONIX, MOTOROLA, MAXIN,etc. at present, various in style, the characteristic, price differ greatly (see the relevant products data books of relevant companies) . While choosing and using the multiple-way switch, the focal point that considers is to meet the systematic requirements for precision and transfer rate of signal transmission, must also pay attention to the following two points at the same time: First, understand the characteristic of the multiple-way switch in an all-round way, otherwise may present the question different to expect. For example: CMOS multiple-way switch is broken during power dump, but the structure FET multiple-way switch is put through during power dump. If has not noticed this, may damage relevant chips because of make-and-break of the power.
Second, the multiple-way switch is matched only with the associated circuit rationally, coordinate the work, could give full play to its characteristic, even the one that remedied some characteristics was deficient. Otherwise, the one-sided high performance of pursuing the multiple-way switch, neglect matching and coordination with the associated circuit, not only will cause the waste of cost and performance index but also often accept the anticipated result.
In addition, limited by chip kind or application, often there are surplus passways in practice. Because the internal circuit of the multiple-way switch is connected each other, the surplus passway may produce the undesired signal, should deal with appropriately if necessary. Such as [4 ]: Find, all input ends of the surplus passway must be earthed while testing the multiple-way switch CC4097 and CC4067, otherwise produce the undesired signal.
The basic application technology about multiple-way switch, there are introductions on a lot of literature, this text refuses to state.
