Fluke 5720A service manual Sense Current Cancellation Circuit, Linearity Control Circuit

Theory of Operation 2

Analog Section Detailed Circuit Description

In the 22V range, Q20 is off and Q21 is on, which switches in the 40 kΩ feedback resistors located on the HR6 assembly. Precise ratio matching of these resistors provides high accuracy in the 22V range.

FET Q15 is on in the 22V range so that the output of the dc amplifier is half the output of the DAC. This is necessary so that the output of the dc amplifier is approximately the same as its inputs, which allows the bootstrap circuit to work.

The output buffer (U5) provides drive for the DAC output. It is used in a feedback loop with the DC Amplifier Hybrid so that the dc accuracy is dependent upon the dc amplifier, and the output drive capability is dependent on the output buffer.

The output buffer is current-limited to a short circuit current of about 60 mA. The short- circuit protection circuitry works as follows:

1.The supply current is sensed by R23. When the output current of U5 reaches approximately 50 mA, the voltage across R23 is large enough to turn on Q10.

2.As Q10 turns on, the voltage across R20 increases, and pulls down the supply voltage at pin 4 of U5.

3.In order to prevent the supply of U5 from dropping below the input, Q8 saturates turning on Q11 which shorts the input to FR1 COM.

4.When the short is removed, R22 and C41 cause Q11 to turn off slowly, which prevents a large overshoot at the DAC output.

2-107. Sense Current Cancellation Circuit

This circuit uses op amp U1A and four resistors on the HR6 assembly. This circuit supplies the sense current of equal, but opposite, polarity to the feedback resistors in the 22V range. This eliminates current in the sense lead during external sensing.

2-108. Linearity Control Circuit

The linearity control circuitry contains the series linearity control circuit and the shunt linearity control circuit, as outlined on the schematic. These linearity control circuits eliminate filter current in the series switch (Q5) and the shunt switch (Q6). This is necessary because Q5 and Q6 have finite resistance (3 to 5Ω) and a small mismatch in the resistances can cause a linearity error.

The series linearity control circuit uses op amp U38, resistor network Z2, and a single 19.996 kΩ resistor on the HR5 assembly. This circuit eliminates filter current in the series switch Q5.

When the series switch (FET Q5) is on, it connects the 13V reference to the first channel input of the filter, and FET Q4 is also turned on. This causes U38 to supply the current to the filter through the 19.996 kΩ resistor in HR5 and Q4, which makes the resistance from TP2 to TP5 look like near 0 ohms.

The shunt linearity control circuit uses op amp U2B, FET Q22, three 80 kΩ resistors on the HR6 assembly, and one resistor in the HR5 assembly.

Op amp U2B is configured as an amplifier with an inverting gain of 1 in the 11V range, and an inverting gain of 0.5 in the 22V range. This gain is determined by FET Q22 and the three 80 kΩ resistors in the HR6 assembly.

When the shunt switch (FET Q6) is on, connecting the input of the filter to REFCOM, the current from the filter flows through the two 40 kΩ resistor (pin 7 to pin 8) on the HR6 assembly to the output of U2B. This cancels out the current that would flow through Q6 which makes it look like 0Ω.

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