Fluke 8842a MA AC Scaling, Frequency Response Trimming, True RMS AC-to-DC Conversion

8842A

Instruction Manual

U806A, U806B, and a voltage divider (R804 and R805) provide gain which is selected for each range by the analog switches in U804. The configuration for each range is shown in Figure 5-17. (In this figure, the CMOS analog switches are represented by mechanical switches.) When U806A is not used, its non-inverting input is grounded by Q804. When U806B is not used, its non-inverting input is connected to the CURRENT SENSE line.

5-45. mA AC Scaling

The mA AC function uses the same current shunt and protection network which is used for dc current. In the mA AC function, Q802 switches the CURRENT SENSE line to the non-inverting input of U806B, which provides a gain of 10.

5-46. Frequency Response Trimming

The frequency response is trimmed by software calibration using a digitally controlled one-pole low-pass filter (R832 and a combination of C826, C827, C828, and C829). The analog switches in U808 configure the four capacitors to select one of 16 possible RC constants. The input of the digitally controlled filter is buffered by voltage follower U801A. The individual gain stages are also provided with fixed frequency compensation.

5-47. True RMS AC-to-DC Conversion

U801B buffers the input to rms converter U802. U802 computes the rms value of the scaled input voltage as shown in Figure 5-18. Rather than explicitly squaring and averaging the input, U802 uses an implicit method in which feedback is used to perform an equivalent analog computation.

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Figure 5-18. True RMS AC-to-DC Converter

The filter averages the divider output signal. This filter consists of U809A, C813, R815, and the internal 25 kΩ resistor and op amp between pins 8 and 9 of U802. The output is further filtered by a three-pole post-filter comprised of U809B and associated resistors and capacitors. This output is then switched into the Track/Hold Amplifier of the dc front end via U302 pins 15 and 14. The Track/Hold Amplifier is set up for unity gain on all ac ranges.

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