Section 4-Theory Of Operation; Introduction; Blockdiagrams; Powersupply - Keithley 220 Instruction Manual

SECTION 4
THEORY OF OPERATION
4.1 INTRODUCTION
This section contains circuit descriptions for the Model 220.
The information is arranged to provide a circuit description
of individual functional circuit blocks. To facilitate
understanding, the descriptions are keyed to accompany
simplified block diagrams and schematics. Detailed
schematics of the Model 220 are located in Section 6.
4.2 BLOCK DIAGRAMS
The circuitry of the Model 220 is represented by the two
simplified block diagrams in Figures 4-l and 4-2. Figure 4-1
shows a simplified block diagram of the Model 220's analog
circuitry (power supply, range circuitn/, amplifiers etc.).
Figure 4-2 shows a simplified block diagram of the Model
220's digital circuitry (microprocessor, RAM, ROM, VIA
etc.).
4.3 POWER SUPPLY
To facilitate understanding of the following discussion refer
to schematic diagram 220-106 (sheet 2 of 21. The power
supply is a conventional AC to DC power converter. Trans-
former TlOl, has three separate secondaries that are fed in-
to three separate bridge rectifiers CRlOl, CR108 and the
bridge configuration of CR102 through CR105. The output
of CR101 is fed into regulator VRlOl and is filtered by Cl07
and Cl03 producing the +5V digital supply. The output of
CR108 is fed into VR102 and VR103 and is filtered by Cl19
through Cl22 to produce the positive and negative 15V sup-
plies. The output of CR102 through CR105 is filtered by
Cl17 and Cl18 to produce the positive and negative 125V
supplies. R118 and R119 are bleeder resistors to prevent
charge retention after AC power is removed.
A tap off of the primary of the transformer TlOl supplies the
nominal 115VAC to the fan.
4.4 ANALOG BOARD
To facilitate understanding of the following discussion refer
to schematic diagram 220-128 (sheet 1 and 2).
The heart of the analog board is the high voltage electro-
meter op amp which is centered on U319. The performance
of U319 is bootstrapped up to the voltage levels supplying
0318 and Q319 bY Q315-Q319, 0313-0318 and their
associated circuitry. 0301, 0302, R344, R348, R349, C313
and C320 establish frequency stability for U319. R343 is an
input voltage offset adjustment for U319.
A constant current source can be derived by a series voltage
source and resistance from the output (analog com-
monlguardl to the amplifier input. The amplifier input to the
common of the high voltage supplies of Q318/Q319 (output
common) comprises a current source. Range resistors
R358, R361. R362. R378, R375, R376, R380 and R381
along with their associated calibration potentiometers, com-
prise the series resistance section of the current source.
These resistances connect to the input node of the amplifier
through relays K301-K305. When several resistances are
connected through a common relay, JFET switches 0303
through Q310 are used to distinguish which resistor is being
used. Several JFETs are organized in pairs for voltage sen-
sing at the resistor to compensate for the voltage drop in the
current carrying JFET.
U315 serves as a sense amplifier with Q311, Q312, R344 and
R345 as a high current buffer for the higher current ranges.
U314, U318A and U320 8, C, D, E and F drive the range
relays. U3188, U317 and U313 A, 8, C and D drive the JFET
switches. U313 and U317 are voltage comparators with
open collector outputs, Cbmbined with R350, this circuitry
provides the voltage drive for the switching JFETs.
The voltage source section centers around U311, the 12 bit
digital to analog converter (DACI. Associated circuitry R301
through R307 and R316, are used to adjust offset and
positive gain. Gain is set on the 1mA range and therefore
R376 does not have an adjustment. Following this circuitry
is a n?twork providing a +/- operator to the output of
U311. U312 provides the active portion while switches on
U306 along with resistors R317, R318, R320 and R323 pro-
vide a selectable gain of + 1 or -1.
R318 provides a gain adjust f-1mAl for this operator. The
output is fed to the sense amplifier U315. R392 is an offset
voltage adjustment potentiometer for U312 and U315.
Serial to parallel shift/store registers U301, U302 and U303
provide digital control from the serial data link. Data is input-
ted via the clock and latch lines. Latch selects the data in
mode as either recirculated data out flatch=logic 01 or the
overcompliance (V-limit) information (latch = logic 1). This
selection is performed by U304A and 8, U3168 and C and
U305A.
Gates U3058, C and D form a flip-flop for controlling the tri-
state mode of the outputs of U301, U302 and U303. This
circuitry, along with R351, R352, CR307, C305, etc.,
prevents erroneous current source outputs both on acquisi-
tion and loss of AC line power.
4-1