Fluke 5720A service manual 2-153. 220V DC Internal Calibration Network, PA Calibration

5700A/5720A Series II Calibrator

Service Manual

is configured the same as for 220V ac operation, except that the input is the square-wave signal HVCL from the High Voltage Control assembly, rather than OSC OUT HI, through relays K13B and K10A.

2-153. 220V DC Internal Calibration Network

The 220V dc internal calibration network determines the exact gains and offsets of the power amplifier. This circuit uses part of the resistor network HR8 as the input attenuator, and uses op amp U9, and zener diodes VR57 and VR58. Relay K4 connects the output of this circuit to the RCL line.

Zener diodes VR57 and VR58 reduce the power supplies for chopper-stabilized amplifier U9, which is used as a voltage follower.

2-154. PA Calibration

The following paragraphs describe how the Power Amplifier assembly is calibrated during the internal portion of calibration, or calibration check. This process calibrate the offset, gain, and frequency characteristics of the Power Amplifier.

The instrument measures the offset of the main amplifier (220V range). The DAC output is amplified by the Power Amplifier, which is configured for the dc 220V range, and its output connected to the internal cal zero amplifier located on the Switch Matrix assembly. Output of this internal cal zero amplifier is channeled to the DAC’s adc amplifier circuit by the RCL line. Input of the internal cal zero amplifier is first connected to R COM where a checkpoint reading is taken by the DAC’s adc circuit. The output of the Power Amplifier is then connected to the input of the internal cal zero amplifier and the DAC output is adjusted to the checkpoint reading out of the zero amplifier. This adjusted DAC output is a measure of the offset of the Power Amplifier. Also, refer to the Switch Matrix theory of operation.

The 175 kΩ/25 kΩ (internal cal) resistor network located on HR8 is calibrated next (see Figure 2-23). First the offset of the 220V dc internal cal amplifier is measured. This is done by connecting pin 6 of 175 kΩ/25 kΩ resistor network to RCOM by relay K1, and pin 7 to ACOM by U5C and relay K3. ACOM is connected to RCOM through 051 during this step. Output of this internal cal amplifier is connected to the RCL line by relay K4, where it is measured by the adc circuit on the DAC assembly. As before, the DAC’s output needed to obtain the checkpoint for this step represents the offset of the internal calibration amplifier.

Resistor network attenuation is calibrated next. Components U5A, U5D and K3 connect BSRF13 and BRF13 to the 175 kΩ end, while K1 connects R COM to the 25 kΩ end of the resistor network. The resulting 1.625V at the output is connected to the RCL line by relay K4, where it is connected to the +input of the adc amplifier circuit on the DAC assembly. The DAC output is connected to the adc circuit -input, and adjusted until the checkpoint reading is obtained. The exact value of this voltage is now known, so the system software computes the exact attenuator ratio from this known voltage, BSRF13 value, and the previous offset measurement.

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