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Very Low Voltage Amplifier With Our tlJanks and appreciation to Mr. Wick for his sharp eyes Readers' Notes: and enlightening comments. If you would like copies of the H-P JOURNAL issues referred to, we suggest YOll get in A great many readers took tile trouble to pass on a few kind touch with your local H-P field office. words of appreciation for Dialogue. These all said about the same thing, in many ways, al1d though they made fascinating reading (for us), it would serve no useful purpose to include A professional cartographer, when drawing up a new map, will them here. On the other hand, there were a couple that were have it copyrighted. Then he will add into his map a small but unustUllly interesting, in that they discussed a point that was distinct error, an artifact. glossed over (and was further distinguished by a slide rule error). Any person who steals this map is unlikely to be bright enough Your recent issue of Analog Dialogue concerning noise in to find or correct the error. Wholesale copy of the map operational amplifier circuits! was of great interest. It was an including the artifact is acceptable proof of infringement of exceUent discussion of noise in general and its measurement. copyright. The thief is caught with marked hills in his hot I appreciated the differentiation between the various types hand. of noise. In my NEREM paper on noisel, I deverly (sic) stated that the Since Hewlett-Packard's voltmeters are often used for noise Fudge Factor to use (when trying to measure white noise on measurements, ) was particularly interested in your comments an average-rectified-rcsponding. AC voltmeter, with its calibra­ on page 7 regarding the characterization of noise. You men­ tion specified in terms of RMS value of sines) was +11%. tioned that the indication on an average-responding RMS­ (Ed. The actual figure published in the paper to which Mr. calibrated meter should be multiplied by 1.11 to obtain the Pease refers was J 1.3%) Actually, the correct value, as stated RMS value of the noise measured. in our up-to-date applications article P/N-10 is +12.8%, as a thoughtful interpretation of Bennett's Electrical Noise3, page 44, will prove out. Sub:-dlUdng the ins-uru�ncous v�lut o( currt'nt, /, g;yc> Irrns' 110< RMS currenl v.luc. When performing ItM� rne.surcm<nl • Now, .in reference to your recent Analog Dialogue article • either :L. "nut' RMS" meter mu�t be lhCd, or the rc:.diug of .an (foot of fust column, page 7), tell me there isn't any jam AC :..vC'r... sc ("lIlC w;a,v(: ""RMS" (J.libr:ttcd mt'!cr' �hould be U1ulripliod by > f.ctor of++h- on your face! [.13 Yours for Better Analogs MAll.CII )9(09 Robert A. Pease Philbrick/Nexus Research Dedham, Massachusetts (from Analog Dialogue, March, 1969) Mr. Smith, in his original manuscript, stated: "Most meters The average-responding meter does indicate a voltage that is will read 12% low when measuring noise, but since tllis error 1.11 (or Y2 rr/4) times the average or rectified value. This is is constant, a true RJ,1S meter is not required. " 111 converting derived from the fact that the average value of a sine wave is this figure to a multiplying factor, which is more useful, our 0.637 times the peak and the RMS value is 0.707 times the slipstick slipped. The correct figure is indeed 1.13, and we peak. Thus the meter measures 0.637 and displays 0.707, a confess to having committed a 2% error. But, Mr. Pease, ratio of 1.11. This number is, therefore, a calibration con­ that redness on our face is a blush, not jam! stant that must be applied to calculate the effects of wave­ forms other than sine waves. A further comment: If Mr. Pease is claiming that he has intentionally "rigged" a figure in a paper he has written for For white noise we can show that the average value is presentation at a technical society meeting, for the sole pur­ Y2FiTtimes the RMS value. This is derived by integrating the pose of exposing copyright(?) violations, most readers would probability distribution of random noise. In the actual measur­ wish that it had been an obscure figure, rather than one which ing instrument, the integration is performed by the ballistics of is pretty useful. Somehow, this sort of thing is unfair to the the meter movement. Therefore, the indication on an average­ engineer who simply wants ,'nformation, presented as accur­ responding RMS-calibrated meter be ...j2/rr x ..[2rr/4, will ately as possible. It also tends to impeach a source whom we which calculates to be 0.886, or -1.05dB. Thus the average­ know ordinarily to be of high technical integrity, a former responding meter will indicate 0.886 of the true RMS noise colleague for whom we have great admiration. Ours was an voltage, and its reading must be multiplied by 1.13 to obtain hcmest mistake. the correct value. You may wish to refer to the Hewlett-Packard Journal, ERRATUM. In Analog Dialogue, Vol. 3, No. 1, page 4, 1955) Volume 6, Numbers 8,9,10 (April, May, June, for a Figure lb, associated with "An op amp and a chopper give further discussion of waveform errors in average-responding precision ramp generator," as reprinted from Electronic voltmeters. Design, there is an obvious error: The 2Nl131 transistor should be a PNP type. Thank you for publishing Analog Dialogue. We enjoy reading it very much. l Don A. Wick Analog Dialogue, Volume 3, No. 1, March 1969, "Noise and Customer Service Manager Operational Amplifier Circuits" 2 "How to Characterize and Measure Noise in Operational Amplifiers" Hewlett-Packard by R. A. Pease, NEREM 1968 Loveland, Colorado 3E/ectrical Noise, W. R. Bennctt, McGraw·HilI 15 ngineer'snotebook���������� . inch niodel47.o1 adopts th� charge-dispensing approach to v-:f conversion, and has a unit price of $59 in small Voltage-to�frequency module quantities. The module provides- output pulses of a bout 30 mi­ serves diverse applications croseconds width at a repetition rate. that's directly pro­ by Rob�rt Alien Pease portional to the analog voltage level. As Fig. 1 shows, Teledyne Phi/brick, Dedham, Mass. the amplifier functions as a zero Grossing detector with a + 13-:-volt quiescent output. When a� input signal passes through the input resistor, the charge dispensing an4 re­ With. the addition of several external components, a set circllit senses zero crossing and connects a large� versatile new vo.1tage-to-frequency converter can be value discharge capacitor. This capac:itor quickly drives made to yield a broad range of communications and in� a precision timing capacitor below zero, cutting off the strumentation functions at reasonable cost. The l�cubic- amplifier and thus resetting it rapidly to a + 13-v out- put. .' The. negative-going pulse at the amplifier output is normally. inverted in. a lTL:-compatibJe circuit.. Typical input and output waveforms are shown in Fig. 2. +15 V Three of the more interesting of circuit functions real­ izable with the v-fconyerter are: inexpensive digital r---4�-COM voltmeters, ramp generators, and analog�to-digital. con­ INPUT verters, An inexp�nsive DVM can be cons�ructed simply �15 V by adding a counter/disphiy to the output of the v-f converter, taking advantage of the 4701's linearity, �"""'--+-'-+- . OUTPUT which is within 0.015%. Programable, very linear ramp generators based on TRIM v-f techniques (Fig. 3) overcome a major difficulty of conventional designs-they do not suffer from charge 1. Converter. Frequency-to-voltage module follows charge-dis­ leakage of the timing capacitor under temperature ex:­ pensing design approach. Output is buffered and TIL-compatible. tremes. By driving a .ripple counter with an R-2R ladder connected to its output,. the 470 L converter generates a highly linear: 10-v ramp that operates at frequencies to +10 V 80 hertz. Output.im pedance is 51 kQ ±5%. 0-500 J.iS INPUT -1 t _____ +2 V An analog-to-digital converter is constnicted from a �- --- - - --- --- ----- -----/ -- --- T�I� OV 4701 and several inexpensive components. In Fig. 4, the n n n UfUlILOGIC 1 converter output feeds a counter which is reset by a 1- OUTDIIT LOGIC 0 WAVEFORM I ' 1 . I I ' �500w-� � f-100,us CALJL -j T I- ��� TX 128 -1 t- ,-_--, . 10 V 2. Proper timing. Typical waveforms show timi8 relationships be­ .JUUL� . g - . 100 kD. 0 .'- � tween input and output for input levels of 2 and 10 volts. I � '. \ . 51 kD. lOO kD. VOLTAGE RAMP J TRANSFER CHAHACTERISTICSAND TYPICAL LINEARITY ERRORS V:-F OUTPUT INPUT a: u>­ lOO k' CONVERTER UJ z I I I­ 51kD. UJ Z 10k = :::J f- FOUT 10kHz (E1N/l0 V) � :::J � d Cl UJ­ ik 3,12 � a:N LL� � ...;J 0... I- lOO 0... :::J � " 0... a: I- 10 I­ :::J � <:T ·0 1 Cl � � 9 t---'\./V'v.... +.001 +.01 +.1 +1 +10 +100 Cl INPUT VOLTAGE (V) NOTES: u « +0.1 ,----,r:---,---,.----....--,.-----.1. T IS CONSTANT FOR u a:;:u a: 1 0 �f\J\.r.... O...;J CONSTANT INPUT a: GUARANTEED LIMITS AT 25°C a:u« + 0 05 VOLTAGE UJ Cl) . b-:z.:;�:z-7l'r-r777TT7b--rT7777-\r-r.,..__T_7_7T:�'/r� 2. R1 = FLIP-FLOP OUTPUT >-...;J I-...;J IMPEDANCE COMPEN· 11 1---"./V"u".J\./'�/'\/'\,� ___ -:::J a:u... SATION «LL �o �� -0.1 '-----,-L........---"--_---""--__L-_ ----.J .p0l .01 .1 1 10 lOO 3.
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