Keithley 486 Instruction Manual page 149

SECTlON 6
Theon~ of
Operation
Analog Converter) through serial-to-parallel registers
U153 and U152. A loaded value of $oooo will result iu a
DAC output tVour) of OV while a value of $FFFF will re-
sult in a VOLT of +lOV. U149 provides the 5V reference for
the DAC.
The output of the DAC is inverted by U143 prior to being
applied to the output amplifier. The inverting output am-
plifier consists of op amp UHO, and N-channel power
FE% QlOl, 4105 and 4106. On the 5OOV range, feedback
resistor R117 provides x65.536 gain. For the 5OV range,
K103 is closed changing amplifier gain to x6.5536. On the
5COV range, this provides an output range of 0 -65536V
at 1OmV per DAC count. On the 50V range, the output
range is 0 -65.536V par DAC count.
UllO is config-ured as a high impedance amplifer that is
used to control the power amplifier (QlOl and QlO5). As
the programmed voltage is increased, drain current
through QlO5 decreases causing an increase in drain cur-
rent through QlOl, which is connected to the +3OOV sup-
ply. As a result of the increased QlOl dram current, the
voltage at the output increases.
With the 2.5ma I-limit selected, relay K102 is configured
as shown in schematic 457-106, page 4. Q103 and R180
limit current to 3.5mA while the constant current source
that is controlled by of QlO6 sinks 1mA leaving a net of
25mA for the load on the V-Source.
When the 25pA I-limit is selected (K102 activated), U119
is used to sense current flow to the load. As load current
increases to -2OpA, the output of U119 will change from
OV to
-9V.
With -9V at the output of Ull9, current will
start to flow through VR102 overriding Ullo's control of
4105. Ul19 now forces the output voltage to limit at a
pointwhere20pAis5owingintheload.Wheneithercur-
rent limit occurs, the transitor of photo-coupler AT101
turns on, placing the I-LIMIT control line at digital low.
When the voltage source is placed in standby, relay KIOO
opens, placing the output in a high impedance (open)
state. Relay K102 controls polarity switching. For posi-
tive
(+) outputs, K102 connects
Vo~r to the red (HI) ba-
nana jack For negative c-j outputs, K102 connects Vom to
the black (LO) banana jack.
6-6
V-Source Power Supplies
The +SOOVB supply is derived by applying 25OVAC to
the voltage doubler circuit that consists of CRlOO, CR105,
Cl03 and C109. Regulation for the supply is provided by
C104, C105, and transistors QlOO and Q102. For safety,
Q104isusedtodischargecapadtorsC103andC109when
instrument power is turned off. When the instrument is
turned off, a discharge circuit consisting of C102, CR102,
R115andR116tumsonQ104forashortperlcdoftimeto
allow the capacitors to discharge to ground. RlOO and
R114 also help to discharge these capacitors. VR106 is
used to protect the Model 487 if it is inadvertently con-
nectedto230Vaclinepowerwhilesetfor115Voperation.
The+l5VSsupplies areusedexclusivelyforthev-Source
circuiiq. Rectification for the fl5VS supplies is per-
formedbyCRl03,whileClC0andClOl providefiltering.
UlOO regulates the +EVB supply, and UlOl regulates the
-15VB supply. Output filtering is performed by Cl10 and
c112.
The +5VS supply is derived from the 15VEl supplies us-
ing the 5V regulator U102. Filtering is provided by Clll.
6.3 ANALOG CONTROL CIRCUITRY
The analog circutts, can be found on drawing number
486-106 (page 3), or on 487-106 (page 3). These circuits
controlvirbaUy all aspects of the analog section of the in-
strument, including the voltage source of the Model 487.
The following information describes the key aspects of
the analog control circutts.
Figure 6-10 shows a simplified block diagram of the ana-
log control circuits. A synchronous serial data transmis-
sion scheme is used to pass digital control information
through opto-isolators U103, U109 and U105 to several
shift registers that store the control information fUl36,
U12.2 and Ul34). The three transmission lines are:
ANALOG DATA: transmits the serial control data
ANALCX CLOCK: contiols the data transmission rate
ANALOG STROBE latches control information into
command registers
For the Model 487, data to the
V-Source
DAC is sent
throughopto+olatorsU125,U131 andU133.Thecontrol
circuit exclusive to the V-Source is located on schematic
487-106, page 4.