Track/Hold Circuit; Track/Hold Amplifier - Fluke 8842A Instruction Manual

8842A
Instruction Manual

5-9. TRACK/HOLD CIRCUIT

The Track/Hold (T/H) circuit presents a stable voltage to the A/D Converter during the
input period of the A/D conversion cycle. The circuit also provides a gain of 100 in the
20 mV, 20 and 200 mA ranges, and a gain of 10 in the 200 mV dc, 200 , and 2000 mA
dc ranges.
The T/H circuit consists of the T/H Amplifier (Figure 5-3), T/H capacitor C308, quad
analog switches U301, U302, and U303, and associated components. As shown in Figure
5-3, the T/H Amplifier functions as an op amp, with Q314 supplying additional gain. In
subsequent figures, the T/H Amplifier is represented as a single op amp.
The circuit operates by cycling between the track, settling, hold, and precharge
configurations shown in Figure 5-4. The In-Guard C selects a particular settling and
hold configuration for each function and range, and suppresses the precharge
configuration for certain ranges. This control is achieved by latching function and range
information in U301, U302, and U303.
Basic timing for the T/H circuit is provided by the A/D Converter over clock lines PC,
HD1, TR1, and TR2. (See the timing diagram in Figure 5-5, top.) The T/H cycle is
initiated when the In-Guard C pulls line TR low.
5-8
Figure 5-3. Track/Hold Amplifier
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