Uk?ad do pomiaru t?umienia napi?cia wspó?bie?nego precyzyjnych komparatorów napi?cia typu LM111

摘要

The comparator open-loop common-mode rejection ratio (CMRR) is a measure of the comparator's ability to reject common signals present in equal measure at both inputs. CMRR according to the definition is the ratio of the differential-mode gain to common-mode gain. Often, manufacturer data sheets only give DC CMRR value and CMRR curve over frequency while no measurement methods are given. The present work deals with measurements of common mode rejection ratio of values of the order 1 · 10~5 ÷1· 10~7 (i.e. (100 + 140) dB) of the voltage comparators type LM111. The present investigation concerns the method of measurement of coefficient CMRR with closed loop feedback of voltage comparator at alternating current. Important relationship between measured output voltage (U_wy) and finite values of parameters of comparator and others parameters of measuring circuit has been discovered by means of detailed analysis (see equations (12) and (17)). This relationship is the basis for quantitative analysis of the error of measuring method. Finally proposed - by author - a new solution of measuring circuit different from others existing up to date offer significant improvements over previous methods and is capable of accurate measurements on high-performance analogue comparators. It is based on special precise inductive voltage transformer with bifilar windings (Tr, Fig. 4). Such the error of measuring method is independent from finite value of differential open loop voltage gain and the output voltage is directly related to CMRR. The unity gain differential amplifier (buffer) which is implemented at input device under test (DUT: KB, Fig. 2÷-4) eliminates differential and common-mode input resistances of DUT on measuring output voltage and thereby CMRR.%W pracy rozwa?ono zagadnienie pomiaru, przy pr?dzie przemiennym i zamkni?tej p?tli sprz??enia zwrotnego, wspó?czynnika t?umienia napi?cia wspó?bie?nego (CMRR) o warto?ci rz?du 1·10~5÷1·10~7(tj. (100 + 140) dB) scalonych komparatorów napi?cia typu LM111. W wyniku przeprowadzo nej analizy teoretycznej w?a?ciwo?ci metrologicznych uk?adu pomiarowe go ustalono istotny zwi?zek mi?dzy stosunkiem mierzonych w uk?adzie napi??, zale?nym od wspó?czynnika CMRR, a innymi parametrami bada nego komparatora napi?cia, w tym zw?aszcza wzmocnieniem napi?cia ró?nicowego, stanowi?cy podstaw? ilo?ciowej oceny b??du metody pomia rowej. Zaproponowano nowe techniczne rozwi?zanie uk?adu pomiarowego zawieraj?ce specjalny transformator pomiarowy napi?cia przemiennego wyposa?ony we wtórne uzwojenia o nawoju bifilarnym, co przyczyni?o si? do wyeliminowania wp?ywu skończonej warto?ci wzmocnienia ró?nico wego badanego komparatora napi?cia na mierzon? warto?? wspó?czynnika CMRR.