Fluke 5720A service manual Switch Matrix Operation 220 mV AC Range, Calibration of the mV Ranges

5700A/5720A Series II Calibrator

Service Manual

2-95. Switch Matrix Operation: 220 mV AC Range

Refer to Figure 2-14 for the following discussion. As previously mentioned, the ac 220 mV range uses the same resistor network as the dc 220 mV range. In generating the ac 220 mV range the Oscillator assembly is set to the 2.2V range. Lines OSC OUT and OSC SENSE are connected to pin 3 of the 10:1 divider by relays K5 and K6. Lines OS COM and OSC SENSE LO are connected to pin 2 by relays K11 and K9 respectively.

This divided output from pin 1 is referred to as AC/DC mV on the schematic. The AC/DC mV is then connected to INT SENSE HI through relay K17 on sheet 1 of the Switch Matrix schematic. Signal INT SENSE HI is connected to the OUTPUT HI binding post through relays K2 and K3 on the motherboard. Sensing for the LO occurs by connecting OS COM and OSC SENSE LO via K11 and K9. A single line is run out to the OUTPUT LO binding post by relay K33 on the Switch Matrix.

2-96. Switch Matrix Operation: 2.2 mV and 22 mV AC Ranges

Refer to Figure 2-14 for the following discussion. The ac 2.2 mV and 22 mV ranges use the 100:1 divider and the 10:1 divider of resistor network on 4HR1 for a total division of 1000:1. Switch Matrix operation for these two ranges is the same. For the 2.2 mV range, the Oscillator assembly is set to the 2.2V range. For the 22 mV range, the Oscillator assembly is set to the 22V range.

Signals OS COM and OSC SENSE LO are connected to pin 6 of the 100:1 divider and pin 2 of the 10:1 divider by relays K11 and K9. OSC OUT and OSC SENSE are connected to the input (pin 7) of the 100:1 divider by relay K3 (A and B). The output of this 100:1 divider is then connected to the input of the 10:1 divider (pin 3) by relay K7. At the output of the 10:1 divider (called AC/DC mV on the schematic) there is a total division of 1000:1. Connection to the binding posts is done in the same manner as in the ac 220 mV range. In all cases, the output impedance of the millivolt ranges is 50Ω.

2-97. Calibration of the mV Ranges

Calibration of the mV ranges involves determining the resistor ratios of the 10:1 divider and the 1000:1 divider (100:1 and 10:1 dividers cascaded). In addition, an offset calibration is performed on the 10:1 divider to remove thermal EMF error for the 220 mV dc range.

Refer to Figure 2-15 for the following discussion. The 10:1 divider offset is calibrated by configuring the Switch Matrix for the 220 mV dc range, except with the output of the range (AC/DC mV) switched into the input of the internal cal amplifier through relays K17 and K29. The Calibration procedure is the same as described for the 2.2V range offset calibration except that during a checkpoint reading, control line PC4 turns on Q11 which connects SWM SENSE LO to the input of the zero amplifier, representing 0V for the 220 mV range.

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