Fluke 5720A service manual Calibration of the 2.2V Range, Switch Matrix Operation 220 mV DC Range

Theory of Operation 2

Analog Section Detailed Circuit Description

2-93. Calibration of the 2.2V Range

Refer to Figure 2-12 for the following discussion. Calibration of the 2.2V range involves determining its offset and gain constants.

To calibrate the offset:

1.The gain of the internal cal zero amplifier is set to 130 by turning on Q6 via PB4.

2.A checkpoint reading is taken, which represents 0V at the input of the internal call zero amplifier. In this configuration, PBS turns on FET Q7 connecting RCOM to the internal cal zero amplifier input, K29 is open (set position), and the output of the cal zero amplifier is connected to RCL via relays K31 and K32.

3.The input of the internal cal zero amplifier is connected to the output of the 2.2V range. The 2.2V range is adjusted until the adc reads the same as the checkpoint reading within the given tolerance. In this configuration, 2.2V OUT and 2.2V SENSE are tied together by relay K28 and the output (NIT SENSE HI) of the range is channeled into the internal cal zero amplifier input (sheet 1) through relay K29.

To calibrate the gain of the 2.2V range, the 13V buffered reference (BRF13 and BSRF13) from the DAC assembly (A11) is connected to the input of the 2.2V range by FETs Q9 and Q10 on sheet 3 of the schematic. Since this network divides the voltage by five, a voltage of 2.6V is obtained at the 2.2V OUT and 2.2V SENSE points. These points are tied together by relay K28. This voltage is then channeled to the RCL line by relays K31 and K32 where it is connected to the +input of the adc amplifier on the DAC assembly (A11). The DAC output is connected to the -input of the adc amplifier and is adjusted until a null is achieved. At this point, the DAC voltage represents the output voltage of the 2.2V range. Gain is determined since the output, input, and offset of the 2.2V range are now known.

2-94. Switch Matrix Operation: 220 mV DC Range

Refer to Figure 2-13 for the following discussion. The 220 mV range is an extension of the dc 2.2V range. The 2.2V range is divided by ten to produce the 220 mV range. This 10:1 divider (on sheet 2 of the Switch Matrix Schematic) is part of the resistor network on the 4HR1 assembly.

The 220 mV range is passive with an output resistance of 50Ω. This range is generated by connecting the 2.2V OUT and 2.2V SENSE to pin 3 of the 10:1 divider by relays K16 and K6. Lines PA COM and R COM are connected to pin 2 of the 10:1 divider by relays K11 and K21. This divided output from pin 1 is called AC/DC mV on the schematic. This portion of the resistive attenuator is also used for generating the ac 2.2 mV and 22 mV ranges. The AC/DC mV signal is then connected to INT SENSE HI through relay K17A on sheet 1 of the Switch Matrix schematic. Line 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 R COM and PA COM via K11 and K21. A single line is run out to the OUTPUT LO binding post by relay K33 on the Switch Matrix.

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