HP 3465B Operating And Service Manual

Model 3465B Section IVThe reference supply is + 10 V for all ranges except the20 M range. For this range the reference supply is + 1 V.The full-scale output of the ohms converter is 2 V dc. Onthe 20 M range with a Rx of 20 MFi (full-scale), an outputof 2 V dc is needed. From the formula for the ohmsoutput, it can be seen that 10" would have to equal100 Mf2. Since the range of 10" is 10 kfi to 10 Mf2, a 10"of 10Mf2 combined with a reference supply of IVprovides the desired 1 V dc full-scale ohms converteroutput.4-15. AC—DC Converter. The ac—dc converter is an averageresponding ac converter. It measures the average value of asine wave and multiplies this by a fixed scale factor toconvert it to an rms value. The output of the converter is adc voltage equal to the rms value of the sine wave.4-16. Figure 4-3 is a block diagram of the ac -dc converter.The blocks consist of an impedance converter, an acconverter and a filter. The impedance converter has a highinput impedance to prevent loading of the input signal. Italso provides the gain necessary to drive the ac converter.An impedance converter gain of unity, 9.964 or 10 isselected by the function and range switching. The gain of9.964 is used with the ac current function and the gain of10 is used with the 200 mV, .2 mA, 200 and 20 V,20 mA,20 kfl ranges.4-17. The ac converter amplifies the signal from theimpedance converter by the scale factor. This converts theaverage value of the sine wave to the rms value. Half-waverectification of the sine wave is also performed by the acconverter. This rectified signal is filtered to provide theproportional dc output which is applied to the analog—to-digital converter.4-18. Analog-to-Digital (A-D) Converter.4-19. The A—D converter block is comprised of an inputamplifier, reference supply, integrator, slope amplifier,comparator and auto-zero circuit. It makes an analog—to-digital conversion using the dual-slope integrating technique. Four control state signals from the logic section (10,IZ, II and 12) regulate the measurement sequence. 10 andIZ regulate the input amplifier and auto-zero switchingrespectively while 11 and 12 select the reference supplyrequired during the run-down interval.4-20. Input Amplifier. The first stage of the A—D converter is the input amplifier. During the run-up interval ofthe measurement sequence, control state signal 10 switchesthe output of the signal conditioning block to the inputamplifier. The output of the signal conditioning block is adc voltage which varies between 20 mV and 2 V forfull-scale inputs, depending on the function and rangeselected. The gain of the input amplifier is adjusted by thefunction and range switching to provide an output of 2 Vdc for any full-scale input signal. See Input Amplifier GainTable on Figure 7-3.4-21. Reference Supply. The A—D converter uses a mono-polar reference supply of -tlOV. A reference voltage isapplied to the integrator during the run-down interval todischarge the integrating capacitor. Since the discharge rateis constant, the time required for the integrator to reach azero reference is proportional to the input signal. This timeperiod is the run-down interval and is processed todetermine the display. A positive and negative referencevoltage is required since the input signal can be eitherpolarity. A detailed discussion of the operation of themonopolar reference supply can be found in the detailedtheory.4-22. Integrator. The integrator output is a result of acurrent summation at the integrator summing junction(inverting input). A positive current summation (currentflowing into the integrator input) will cause the integratorto ramp negative. A negative current summation (currentflowing out of the integrator input) will cause the integrator to ramp positive. The integrator sums currents from theinput amplifier, reference supply, - 7 V supply and theauto-zero loop during designated times.4-23. Slope Amplifier. Following the integrator is a X4000amplifier. This amplifier is divided into two stages; the firstwith a gain of 40 and the second with a gain of 100. Theslope amplifier amplifies the integrator output to provide amore vertical crossing of this output with the referencelevel. This provides greater accuracy of the voltage—to-time conversion during the run-down interval.4-24. Comparator. The comparator provides two logicoutputs; a high output of 0 V or a low output of - 7 V. Thecomparator output is high when the integrator output isgreater than the reference level. The comparator is lowwhen the integrator output is less than the reference level.ACINPUT-SIGNALIMPEDANCECONVERTERX!X 9.964orXIOACCONVERTERFILTERDC VOLTAGE TO►ANAUDG-TO-DIGITAL CONVERTERFigure 4-3. Block Diagram, AC—to—DC Converter.4-3