KEI 427 Instruction 电路图.pdf

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1、INSTRUCTION MANUAL MODEL 427 CURRENT AMPLIFIER 0 1975, KEITHLEY INSTRUMENTS, INC. CLEVELAND, OHIO, U.S.A. DOCUMENT NUMBER 29103 RadioFans.CN 收音机爱 好者资料库 CONTENTS MODEL 427 CONTENTS 1. GF,Ng3, WEIGHT: Style M 3%” half.rack, overall bench size4highx8%widex12L/a”deep(100 x217 x310 mm). Net weight, 7 Ibs

2、. (3.0 kg). i” 1074 MODEL 427 GENERAL DESCRIPTION SECTION 1. GENERAL DESCRIPTION l-1. GENERAL; The Model 427 Current Amplifier is a C. Built-in Current Suppression. Small changes in high-speed, feedback-type amplifier with particular the signal level can be measured since large ambient features usef

3、ul for automated semiconductor testing, current levels can be easily suppressed. mass spectrometry, and gas chromatography applications. d. Overload Indication. Accurate measurements are assured since overloads are automatically indicated. l-2. FEATURES. e. Variable Cain. The GAIN Switch is designat

4、ed a. Wide Dynamic Range. Selectable rise times permit in eight gain positions from lo4 to 1011 volts per low-noise operation important when resolving small ampere - therefore gain adjustment is straight forward. current levels. f. Variable Kise Time. Optimum response can be b. High Speed. Typical r

5、esolution is 20 picoamperes out of a 10sSZpere signal with a 100 microsecond selected for each gain setting since a separate RISE TIME switch is provided on the front panel. rise time. 0471 1 GENERAL DESCRIPTION MODEL 427 TABLE 1-l. Front Panel Controls and Terminals Functional Description Paragraph

6、 PUSH Power Switch (S302) GAIN Switch (5201) RISE TIME (5101) SUPPRESSION MAX AMPERES Switch (S303) FINE Control (R333) POLARITY Switch (5304) ZERO ADJ Control (R235) INPUT Receptacle (5202) OUTPUT Receptacle (5102) OVERLOAD Indicator (DS302) Controls power to instrument. sets gain in Volts per ampe

7、re. Sets rise time Fn milliseconds. Sets maximum suppression. Adjusts suppression. Sets polarity of suppression. Adjusts output zero. Input source connection. Output connectFon. Indicates overload condition. 2-4, al 2-4, a2 2-4, a3 2-4, a4 2-4, a5 2-4, a6 2-4, a7 2-3, a 2-3, b 2-5, d TABLE 1-2. Rear

8、 Panel Controls and Terminals Control or Terminal Functional Description Paragraph Line Switch (5301) Power Receptacle (P305) Fuse (F301) Sets instrument for 117V or 23411. Connection to line power. Type 3AG Slow-Blow, 117V l/4 A (w-17) 234V l/S A (w-20) 2-4, b 2-3, c 0lJTPuT Receptacle (5103) Outpu

9、t connection. 2-3, b WARNING Using a Line Power Cord other than the one supplied with your instrument may result in an electrical shock hazard. If the Line Power cord is lost or damaged, replace only with Keithley Part No. CO-7. 0878 MODEL 427 GENERAL DESCRIPTION GAIN SWITC S20 ,-SUPPRESSION-, FINE

10、POLARITY ADJUST SWITCH INPUT 5202 . P ZERO ADJ OVERIDAD Power OUTFW R235 DS302 S302 5102 FIGRF 2. want Panel Controls. I FIGURE 3. Rear Panel Controls. 0471 OPERATION SECTION 2. OPERATION MODEL 427 2-1. MEASUREMENT CONSIDERATIONS. a. Current-Detection Devices. The DleasUrement of small electrical cr

11、rent8 has been the basis for a number of instrumental methods used by the analyst. Ion chambers, high-impedance electrodes, many forms of ch=“metog=aphic detectors, phototubes and multipli- ers are coaronly-used t=ansduce=a which eequire the measurement of small currents. Devices used for this measu

12、rement a=e often called electraaeters. b. In any measure- ment, if e”“=ce noise greatly exceeds that added by the inst=“mentatian, optimization of instrumenteti” is unimportant. When source noise approaches theoret- ical minimum, optimization of instrumentation charac- teristics becomes imperative.

13、TO determine the cate- gory into which this meas”=ement falls. the researcher needs t” be familiar with the characteristics which impoee theoretical and practical limitations on his me*surement . Most researchers a=e familiar with the theoretical limitations present in voltage measueements The noise

14、 inceeases with 8”“=ce realstance, and the familiar equation for the mean-square noise voltage is q = 4kTRAf Eq. 1 whe=e k is the Boltzma” Constant, T ia the absolute temperature of the s”“=ce resistance R, and Af is the noise bendwidth( 3 times the 3 dB bandwidth for a single RC rolloff.) In the ca

15、se of cureent measure- ments it Is more appropriate to consider the noise current generated by the l”“=ce and load resistances. The mean squaee noise c”=ent generated by a resistor is given by Eq. 2. FIGURE 4. In the shunt method c”=re”t is measured by the voltage drop ac=“ee a resistor. Prom this e

16、quation it is irmnediately apparent that the m%QB”rement of emall current =equi=es large values of R, i.e., high impedance levels. Howwee, thfe gives difficulties for meas”=ements requiring wide bandwidths because of the RC time constant associated with a high-megohm resistor and even a few picofara

17、ds of cir- cuit capacitance. Figure 4 shows a c”=ent s”“=ce generating a voltage across a parallel RC. The fre- quency response of this current measurement is limited by the RC time constant. Figure 5 shows this response end the -3 dB p”int “CC”=B at a frequency Lor* noise and high speed, therefore,

18、 a=e contradictory requirements. TO optimize a current-measuring system, techniques must be used which obtain high speed “sing high-impedance devices. C. Hiah Speed Methods. 1. High epeed can, af c”“=se, be obtained in . shunt-type meae”=eme”t by “sing a low value for the shunt resist”=. As pointed

19、“t above, such a srmll resistor value int=ad”ces excessive noise into the meQ8”rement. 2. A second method to achieve bandwidth is to keep R large, to accept the frequency roll-off starting at F”, and t” change the frequency eesponae of the voltage amplifier a8 ehown in Figure 6a. The combined effect

20、s of the RC time c”“sta”c folloved by this amplifier is shown in Pigure 6b and it is seen that the frequency response of the c”=ent measurement has been extended to Pl. The frequency at which the amplifier gain sta=ts to increase must be exactly equal t” the frequency F” determined by the RC time c”

21、nstant in order for this approach t” result in a flat frequency respanse. Therefore, FO LOG FREQUENCY FIGURE 5. The frequency respanse of the shunt method 1s limited by omnipresent ahunt capacitance. I 0471 MODEL 427 OPERATION this oethad is ueeful only far application* where the shunt capacitance C

22、 is constant. Aaide from thin drawback this is * 1eSitimste approach which is being wed in low-noise, high-speed current- meesuring applicatians. In addition to current noise in the *hunt and in the amplifier input stage, B maJor source of noise in this system *ri* from the voltage-noise generator s

23、saociated with thb in- put atage (reflected a* current noise in the shunt resistor) caused by the high-frequency peeking in the following stages of amplification. More will be said about this in the discussion on noise behavior. 3. A third method used for speeding up * current measurement asas guard

24、ing techniques to eliminate the effects of capacit*nces. Unfortunately only certain type* of capacitance*, such * cable cap=- itances, can be conveniently eliminated in this manner. To eli,r,inate the effect of parasitic c*p*c- itences associated with the *ource itself became* very cumbersome and m*

25、y not be feasible in many in- stsnces. The major *ourc* of noise in this *“*tern *re identical to those mentioned in the second system. 4. A fourth circuit configuration combines the capability of low-noise and high-speed performance with tolerance for varying input C and eliminate* need for separat

26、e guard by making the ground plane *n effective guard. This is the current-feedback technique. This technique gives * typical improve- ment of 3 over shunt technique*. Again, the major sources of noise are identical to those mentioned in the second system. d. Noise in Current Measurements. Noise for

27、ms * b*aic limitation in *nv hinh-speed current-measurinn system. The shunt *y the simplest curren; measurement but does not give low-noise performance. A properly designed feedback *y*tem gives superior noise - bandwidth performance. Noise in these two systems will be discussed next. 1. Noise Behav

28、ior of the Shunt System. High speed end low noise *r* contradictory requirements in any current meesurement because *orw capacitance is always present. The theoretical performance limftetion of the shunt *yetem c*n be calculated * FO F LOG FREWENCY FIGURE 6. ny tailoring the frequency response of th

29、e amplifier (Pig. 6a) the frequency response of the shunt method c*n be extended. The rms thermal noise current (in) generated by * resistance R is given by Eq. 4 The equivalent noise bandwidth (.f) of * parallel SC combination is Af = 1/(4RC) snd the eignal hand- width (3 dB bandwidth) F, = 1/(2nRC

30、). For practical purposes peak-to-peak noise is taken 88 5 times the ml* value. The peak-to-peak noise current can now be written a* i UPP = 2 x 10-9 F, F Eq. 5 In practice, e typical value for shunt cape.cit*nce is 100 picofarads. With this value the following rule-of-thumb is obtained. The lowest

31、ratio of detectable current divided by signal bandwidth using *hunt-techniques is 2-10-14 ampere/Hertz for B peak- to-peak signal-to-noise ratio equal to 1. A coroll- ary far this rule-of-thumb expresses the noise cur- rent in term* of obtainable risetime (lo-SO% rise- time tr = 2.2 RC). The lowest

32、product of detectable current and risetime using shunt technique* is 7 x lo-l5 ampere seconds. In this derivation it has been assumed that the voltage amplifier does not contri- bute noise to the measurement. 2. Noise Behavior of the Feedback System. There are three *ource* of noise in the feedback

33、system that have to be looked at closely. The firat two, input-stage shot noise and current noise from the mea*urinS resistor, are rather straight-forward. The third, voltage noise from the input device of the amplifier, cau*e* *ome peculiar difficulties in the measurement. Any resistor connected to

34、 the input injects white current noise (Eq. 4). In the circuit of Figure 7 the only resistor that is connected to the input is the feedback resistor R. As in the shunt system, R must be made large for lowest noi*e. Beceuse this noise is white, the total contribution can be calculated by equ.,ting Af

35、 to the equivalent noise bandwidth of the system. The second *ource of noise is the current noise from the amplifier input. This component is essentially the shot noise asaoci- ared with the gate leakage current (io) of the input device. Its rms value equals . . . FO F, LOG FREQUENCY FIGURE 6b. Exte

36、nded frequency response. 0471 OPERATION MODEL 427 T; = J-zTp- where e is the electronic charge. The contribution of this noise generator is also white. N*t only do these two noise sources generate white current noise, the noise in a given bandwidth is also independent of the input capecitence C. The

37、 mejor source of noise in e feedback current meesurement is the noise contribution aseocisted with the voltage noise of the input amplifier. The voltage noise ten be rep- resented by a VOltage noise generator (0,) et the emplifier input es shown in Figure 8. This wise generator itself is assumed to

38、be white. However, its total noise contribution to the current-measuring system is not white. Inspection of Figure 6 will reveal that et low frequencies P large em*u*t of feed- beck ie applied around the voltage noise source en). However, the SC combination ettenuetes the high- frequency components

39、of V,t so that no feedback is present et high frequencies. Thus, the noise con- tribution to the output voltage V,t from the valt- age noise source a* is no longer independent of frequency. The noise is “colored” and increases in intensity for ell frequencies higher than F,. The resulting noise spec

40、trum is shown in Figure 9b. The tote1 system noise is related to the are* under this curve. Because the logarithm of frequency is plotted on the horizontal axis, the eree under the curve et higher frequencies represents e signifi- cantly larger amount of noise then e similar eree *t low frequencies.

41、 For comparison, Figure 9a show the frequency response of the current measuring system. Figure 9e ia identical to Figure 6b. It is interesting et this point to compare this noise spectrum with the frequency response of the voltage amplifier in Figure 4 es shown in Figure 6a. A volt- age noise eouec.

42、e et the input of the amplifier would generate a noise spectrum according to the amplifier frequency response as shown in Figure 6a. The noise spectrum of such e system, then, is identical to the noise spectrum of the feedback system as given in Figure 9h. This illustrates the well-known fact that s

43、ignal-to-noise performance of a measurement cen*t be improved by feedback techniques. At the high-frequency end the voltege noise is limited by the frequency FA which is the high-frequency roll- off point of the operational amplifier. It should FIGURE 7. Beslc circuit configuration for the feed- bac

44、k method. he noted that even though the useful bandwidth of the system extends only t* Fl, there era noise com- ponents of higher frequency present. To obtain best widebend-noise performance, these high-frequency noise components have to be removed. This ca* be achieved by adding a low-pass filter s

45、ection follow- ing the feedback input stage. If the band-pass of thin low-pass filter is made adjuetable this filter can nerve the dual purpose of removing high-frequency noise end of limiting the signal bandwidth of the system. 2-2. THEORY OF OPERATION. 8. Current Feedback Technique. The basic circ

46、uit configuration used in the current-feedback technique is shown in Figure 7. In this configuration the current-measuring resistor R is placed in the feedback loop of e* inverting emplifier with a gain of A*. The frequency response obtained with this circuit is iden- tical to thet s+nvn in Figure 6

47、b. F* agein is the frequency associated with the RC time constant: F, = SE Eq. 6 The frequency response of the syetem is extended t* a frequency fl where F , = AoF, Eq. 7 Note that the frequency rerponse is automatically flat without heving to match break points. However, the total bandwidth of the

48、system (Fl) is still limited by the value of the ahunt capacitance C across the input. This improved frequency response of the feed- back technique avoids the use of low values for R which could generate exceesive current noise. . b. Refinements of the Feedback System. A major difficulty of the feed

49、back system ariees from shunt capacitence esaociated with the high-megohm resiaeor R in the feedheck path. If the shunt cepacitence acroes the resistor is CFr then the bandwidth (FF) of the system is determined by the time COnstent RCF: FIGURE 8. The voltage noise associeted with the am- plifier input device is en important eourc of noise in the high-speed feedback syatew 6 0471 MODEL 427 OPERATION FIGURE 9. The bendwid