Fluke 8842a Precision Voltage Reference, Settling Configuration, Hold Configuration

Theory of Operation 5

PRECISION VOLTAGE REFERENCE

5-11. Settling Configuration

The circuit assumes a settling configuration between the track and hold configurations. The circuit assumes the configuration in Figure 5-4B for unity gain and the configuration in Figure 5-4C for gain of 10.

During this time the DC Scaling circuit is still connected to the T/H amp. However, changes in the input do not affect the value to be measured, which is stored on C308.

5-12. Hold Configuration

The X1 hold configuration (Figure 5-4D) is used for all VDC ranges except r1 and for all ohms ranges except r1. The output of U307 is the negative of the input voltage.

The X10 hold configuration (Figure 5-4E) is used for the mA DC function, the 200 mV dc range, and the 200Ω range, and provides a gain of 10.

5-13. Pre-Charge Configuration

The pre-charge configuration (Figure 5-4F) occurs after the hold configuration in VDC ranges r1, r2, and r4, and ohms ranges r1, r2, r3, and r4. U306 is connected as a buffer to charge stray capacitance at the non-inverting input of the T/H Amplifier. The pre-charge configuration is not used in any other ranges.

5-14. PRECISION VOLTAGE REFERENCE

The Precision Voltage Reference (Figure 5-6) provides precise reference voltages of - 7.00000 and +7.00000. The reference element is a reference amplifier (ref amp). The nominal ref amp voltage is 6.5V.

f5-06.wmf

Figure 5-6. Precision Voltage Reference

5-11