Fluke 8842a instruction manual Signal Conditioning, IEEE-488 Interface Power Supply, VAC Scaling

Theory of Operation 5

TRUE RMS AC (OPTION -09)

5-41. Signal Conditioning

The SAMPLE COMPLETE and EXT TRIG signals (J903 and J904) are conditioned by U909. Diodes CR903, CR904, CR905, and CR906 and resistors R917 and R918 provide protection from excessive voltages. Jumpers E902 and E903 allow selection of the polarity of the EXT TRIG signal. (A polarity selection procedure is given in the Maintenance section.) The 8842A is configured in the factory so that it is triggered on the falling edge of the EXT TRIG signal.

5-42. IEEE-488 Interface Power Supply

The IEEE-488 Interface power supply circuit provides the IEEE-488 Interface PCA with +5V. The circuit consists of rectifying diodes CR908 and CR909, filter capacitor C910, and voltage regulator VR901. Power comes from transformer T605 on the Main PCA. U908 and associated circuitry resets the Out-Guard μC at power-up and following power- line voltage dropouts.

5-43. TRUE RMS AC (OPTION -09)

The True RMS AC circuit (Figure 5-17) performs two primary functions. First, it scales ac input voltages and ac current sense voltages to a range of 0V to 2V ac rms. Second, it converts the scaled ac voltages to an equivalent dc voltage which is then directed to the A/D Converter via the Track/Hold Amplifier. The True RMS AC circuit is trimmed for flat high-frequency response using a variable filter which is set by the High-Frequency AC Calibration procedure.

f5-17.wmf

Figure 5-17. True RMS AC Option Block Diagram

The following paragraphs describe how these functions are performed. Components are laid out on a single printed circuit assembly (PCA). Component reference designators are numbered in the 800 series.

5-44. VAC Scaling

AC voltage inputs are directed from the HI INPUT terminal to the True RMS AC PCA through protection resistor R309 on the Main PCA. In this way, voltage transients greater than 1560V are clamped by MOVs (RV301, RV402, RV403, and RV404) as in the VDC function. With the VAC function selected, K801 is closed. The input voltage is thus applied to C801, which blocks dc inputs.

U807 and resistor network Z801 provide selectable attenuation and 1 MΩ input impedance. In the upper two ranges, K802 is closed and Q806 is off, providing a gain of - 1/500. In the lower three ranges, K802 is open and Q806 is on, shorting Z801-4 to ground; this configuration provides a gain of -1/5. CR801 and CR802 provide protection by clamping the inverting input of U807 to approximately +/-0.6V. Q805 shifts logic levels to control Q806.

5-27