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The gain of the ranging amplifier is determined by the follow-

ing formula:

-RF

Av=-

R128

(Note that RF = R142 in parallel with R143, R145 or R146,

depending on which is selected).

For example, for X10 gain, the selected feedback resistor RF if

142, yielding a gain of:

-2MR

Av =

-= -10

2oOkQ

A = x10, x1. x0.1 OR x0.01

Figure 6-11. Simplified Schematic of Ranging

Amplifier

6.4.2 Multiplexer and Buffer Amplifier

The multiplexer selects among the three signals that are part

of the Model 617 measurement cycle. During the cormnon

phase, the multiplexer selects signal common. During the

reference phase, the -2V reference voltage is selected, while

the signal from the ranging amplifier is selected during the

signal phase.

Figure 6-12 shows a simplified schematic of the multiplexer

and buffer amplifier. U145 is the multiplexer IC. which con-

tains CMOS devices that act as analog switches to select

among the three input signals. The multiplexer IC is con-

trolled by digital signals that are generated by the

microprocessor.

Figure 6-13 shows the general signal switching phases for the

three signals. During each phase, an integration is performed

by the A/D converter, and the resulting data is then used by

the microprocessor to calculate the final reading.

CONTROL MULTIPLEXER

\L.

BUFFER

ID ID

RTER RTER

Figure 6-12. Multiplexer and Buffer

Keithley 617 User Manual

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