Fluke 5720A service manual Calibrating to the External 10 KW Standard

Theory of Operation 2

Analog Section Detailed Circuit Description

2-181. Calibrating to the External 10 KW Standard

Refer to Figure 2-33 for the following discussion. The first step in ohms calibration is to compare the 10 kΩ standard against the 10 kΩ value on the Ohms Cal assembly. This is done by passing a current through both values and measuring the voltage drop across each. The ratio of the voltages is equal to the ratio of the resistances.

The source of current comes from the 2/5/10V source circuit, which contains U1, Q1, Q2, Q3 and Z1. The buffered 13V reference voltage from the DAC assembly (A11) is switched through K9 to a voltage divider made up of resistors in Z1.

To calibrate the 10 kΩ resistance, control lines PB4 and PB5 turn off FETs Q2 and Q3 respectively. In this mode, the output of the divider uses the 15 kΩ and 50 kΩ values to produce an output of 10V. This voltage is buffered by U1 and Q1. The current from this source, 10V OUT HI, is sent through K19 to one side of the 10 kΩ resistance in Z5. 10V SENSE HI is connected to 10V OUT HI by K31A on the Ohms assembly. The other side of the 10 kΩ resistance in Z5 is connected to OHMS OUT HI by K19. This is routed to the Ohms Main assembly where is goes through K29, K15 and K1 to INT OUT HI, which is connected to the OUTPUT HI binding post by relays on the motherboard. From there it goes through the external 10 kΩ standard and back in through the OUTPUT LO binding post and over to the Ohms Cal assembly where it connects to S COM through K24 and K22.

The voltage drops are measured by comparing each voltage to the DAC assembly output in a differential amplifier circuit made of U2 through U5.

Since the DAC and differential amplifier are referenced to R COM, this line is brought out through K27, K23 and K25 to INT SENSE LO. This is connected to the SENSE LO binding post (which is connected to the sense low point of the standard). Line DAC OUT LO is tied to S COM by relay K31. The high side of the DAC, DAC OUT HI and DAC SENSE HI, are tied together and connected by K12 to U2, which is the differential amplifier -input.

The voltage to be measured is connected to U3, which is the differential amplifier +input. The output of U2 and U3 goes to U4 through gain setting resistors in Z1. These set the gain of the differential amplifier to 75. The differential amplifier generates a current through the 1 kΩ (pins 1-2) and 75 kΩ (pins 2-3) resistors in Z1 to R COM. This current on R COM can cause an error during measurement. Op amp U5, configured as an inverting amplifier, generates an equal current of opposite polarity through 76 kΩ resistor R18 to cancel the current from the differential amplifier. The output of U4 goes through K10 to the RCL line. This line is connected to the DAC assembly, where it goes through an amplifier to the adc circuit.

The calibration program uses the adc circuit to measure the differential amplifier output voltage. First the differential amplifier offset voltage is determined. (This offset is different for each input condition.) Readings are taken to determine each offset voltage by checking the output of the differential amplifier at equal voltages on the inputs. This is done by switching K11 to the reset position to connect the differential amplifier +input and -input together, then measuring the output with the DAC’s adc circuit.

Once the offsets are known, the software determines the calibration voltages by adjusting the DAC output until the adc reading is the same as the offset. At that point, the DAC output equals the voltage being measured.

Three readings are required to determine the ratio of the two resistances. For the first reading, K16 connects the differential amplifier +input to the 10 kΩ resistor in Z5. For the second reading, K17 connects the +input to the other side of that resistor. Subtracting the first reading from the second gives the voltage across the internal 10 kΩ. For the third reading, K1 on the Ohms Cal assembly connects the + input to OHMS SENSE HI.

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