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Constant-Current Source
The constant-current source provides the stable current necessary to generate the precise voltage drop
across the load. The design of the current source compensates for all series resistance (within
compliance voltage limits) to overcome the effects of test lead and contact resistances.
Reference Generator
The reference generator is shown on the drawing 4100-074 sheet 2. The precise voltages required by
the A-to-D converter and the current source are provided by the zener reference IC106. IC106 and its
associated components produce a nominal 6.95V. This voltage is attenuated by the resistors R111
through R115 and RV101 to provide the required 100mVDC reference voltage to the A-to-D converter.
RV102 is used to adjust the reference voltage to the current source. In the ATC mode (4100ATC and
4150ATC only) the resistor formed by R114 and R119 is replaced by the temperature compensator to
provide the required corrections for temperature variations.
Reference Inverter Stage
The reference inverter is shown on drawing 4100-074 sheet 1. IC11 and its associated components
form an amplifier stage having a gain of -.144. This stage is used to convert the +6.95 VDC reference
voltage to the negative 1 volt reference required by the current source.
Differential Amplifier
The differential amplifier is shown on drawing number 4100-074 sheet 1. IC12 and its associated
components form a unity gain differential amplifier. The output of the Reference Inverter stage (V
ref
)
and the output of the Output Amplifier (V
out
) form the inputs to this amplifier. The output voltage from
this amplifier is thus given by (V
out
-V
ref
).
Output Amplifier
The output amplifier is shown of drawing number 4100-074 sheet 1. IC13, Q18, Q20 and the range
resistors R60 through R76 combine to form the output amplifier of the current source. The range
resistors determine the value of the output current. The voltage drop across these resistors (V
out
) is used
as an input to the Differential Amplifier to provide error correction and to compensate for varying loads.
Output Voltage Clamp
This additional PCB is only fitted in Models 4165 and 4165-1344 and is shown on schematic 4165-070.
The operation of this circuit is as follows:
1)
IC2, IC3, R3 and R4 form a reference circuit providing -2.5V.
2) R5
through
R9 provide divided voltages from this -2.5V reference with outputs at -2.0V and
-50mV (for 4165) or -2.0V and -20mV (for 4165-1344). The selected output of this divider is
connected to one input of the comparator formed by IC1, the other input of which is the voltage
