Fluke 8842a instruction manual VDC Protection, MA DC Scaling, Analog Filter

Theory of Operation 5

DC SCALING

In the 20 mV, 200 mV and and 2V ranges, the input voltage is applied directly to the T/H Amplifier via Q310, Q311, and U301B. In the 20 mV range, the T/H Amplifier has a gain of 100; in the 200 mV range, the T/H Amplifier has a gain of 10; in all other dc voltage ranges, the T/H Amplifier has a gain of 1.

In the 20V range, the input voltage is buffered by unity-gain amplifier U306, and divided by 10 by Z301. To allow U306 to handle +/-20V inputs, its power supplies are "bootstrapped" by Q305 and Q306, so that the output voltage of U306 determines the midpoint of its supply voltages. The positive supply is approximately 6.2V above the input and the negative supply is approximately 6.2V below.

In the 200V and 1000V ranges, K301 is de-energized and the input voltage is divided by 100 by Z302. In the 200V range, the reduced input voltage is then applied directly to the T/H Amplifier as in the 2V range. In the 1000V range, the reduced input voltage is buffered by U306 and divided by 10 as in the 20V range.

5-6. VDC Protection

Input protection for the VDC function is provided by a 1K, fusible resistor (R309), four metal-oxide varistors (MOVs) (RV301, RV402, RV403, and RV404), and additional protection resistors and clamp circuits.

WARNING

TO AVOID INJURY OR EQUIPMENT DAMAGE, USE EXACT

REPLACEMENT PARTS FOR ALL PROTECTION COMPONENTS.

In all dc voltage ranges, voltage transients greater than 1560V are clamped by the MOVs. Extreme overvoltage conditions cause R309 to fail open-circuit.

R309 is followed either by a 99 kΩ, 10W resistor network (Z304) in the 20 mV, 200 mV, 2V, and 20V ranges, or by 10 MΩ (Z302) to ground in the 200V and 1000V ranges. Z304 provides current limiting in extreme overvoltage conditions in the 20 mV, 200 mV, 2V, and 20V ranges. The non-inverting input of U306 is clamped to +/-25V by Q307 and Q308.

5-7. mA DC Scaling

In the mA DC function, the unknown current causes a voltage drop across current shunt R319. This voltage drop is then measured as in the VDC function. The DC Scaling circuit is configured as shown by the simplified switch table in Figure 5-2.

5-8. Analog Filter

The three-pole, low-pass analog filter (U304) has a Bessel response with corner frequency at 7 Hz, giving approximately 50 dB of rejection at 50 Hz. The filter is used for the slow reading rate and is used in the VDC ranges and lowest three ohms ranges. The filter is also used in the 20mV DC, 20Ω, and 200 mA DC ranges when in the medium reading rate. The filter is switched into the input signal path by Q304 (Figure 5- 2). In some ranges and functions, additional filtering is provided by Q317 and C314.

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