Analog Circuitry; Input Circuits A And B; Input Circuit C (Option 2.4Ci) - Keithley 776 Instruction Manual

Theory of Operation
A simplified block diagram of the complete Model 776
is given in Figure 6-3. The input signal is applied
through the input amplifiers to switching circuits which
in turn routes the signals to the correct counting circuits.
If channel C is installed, a signal from the Channel C
input terminal will pass through the channel C input
amplifier to the internal selectors. The CPU. working
under software control, then converts the signals into a
form suitable for displaying on the front panel or for the
IEEE bus.
6-3. Analog
Circuitry
The following paragraphs contain a description of the
input
circuits,
measurement logic circuits, frequency
multiplier and of the power supply. These circuits may
be found on schematic diagrams located at the end of
this manual.
6-3-l. Input Circuits A and B
NOTE
Channels A and B are identical in
terms of components and operation.
Therefore. the following description.
which reviews only Channel A cir-
cuits applies to channel B as well.
Note that letter designations for corn-
ponents in Channel A are different
for similar components in Channel
B.
The signal which is applied to the Channel A input
terminal is preconditioned in front of the amplifier cir-
cuit by means of relays and electronic components as
follows:
Coupling:
Coupling is controlled by a relay Kl, and
capacitor Cl. When instrument is DC coupled, Kl con-
tacts are closed. When instrument is AC coupled. Kl
contacts are open and Cl blocks the DC components of
the input signal.
Attenuation:
Attenuation is controlled by relay K3,
resistor network R3 and R39, and capacitor network C3
and C6. When K2 shorts between contacts 1 and 7. R3
and C6 are shorted and there is no attenuation. Actuat-
ing K2 shorts between contacts 7 and 14 thereby intro-
duces a x-10 voltage attenuation.
Filtering:
The low pass filter is controlled by relay
K4. Contacts on K4 are normally closed; shoning R6.
When the relay contacts open, the impedance of R16,
and the capacitance of Q2 act as a low pass filter.
Termination:
Termination is controlled by relay K2.
Relay is normally open, leaving a 1 Ma impedance
(R4+R5) in front of the input amplifier. When the relay
contacts are closed, the input impedance is shunted by
RI and R2 (5051).
Input Protection:
Front panel input conditioning
arc capable of handling signals within the specified
dynamic
range
of the Model 776. Protection of the input
circuit from over-voltage signals (up to the specified
limits) are done by R4, clipping diodes within UlO, C8,
R7. CR 2 and Q2.
Amplifier:
The amplifier comprises aband split/dif-
ferential amplifier. This section consists of a high fre-
quency amplifier - Q2, and low frequency differential
amplifier - U 10 and QS. The two bands are summed at
the junction of R12 and RlO, and then buffered by 43.
The diode network - CR5 to CR8 limit the amplitude of
the signal which is then applied to the comparator Ul2a.
Ul2a operates as a Schmidt trigger amplifier which
translates, for the following stages, the various input
wave forms to an appropriate ECL logic levels.
Trigger Level Control:
The trigger level control
circuit generates a DC voltage which is directly propor-
tional to the required input threshold point. This voltage
is then applied through R22 to the negative input of the
differential amplifier which was previously discussed.
U7 and U6 are a serial to parallel converters which
control the D to A converter - US, U9a, U9b, and their
associated components. The output of U9b generates a
dc voltage in the range of -5 V to +5 V. This voltage is
then applied. in parallel. to the rear panel TRIGGER
LEVEL A. and through the voltage dividerR2 I and R35
to the negative input of the differential amplifier.
6-3-2. Input Circuit C
The signal which is applied to the Channel C input
terminal is AC coupled through Cl to a 6 dB attenuator
R18 through R22. and through a limiting diode network
CR 1 through CR6. to the input of the first stage ampli-
ficr U 1. The amplifier section comprises three stage
amplifiers U 1 through U3 and their associated compo-
nents. Limiting diodes, CRS through CR13, prevent
saturation of the following stages. The signal from the
U3 is routed to a divide by 128 circuit - U4. The output
from this divider is ac coupled and dc leveled by R16
and R17. and then routed through Jl to the main board
assembly.
U4 is a self oscillating circuit, that is, if no proper
signal is available at its input, the output will
oscillate
at an uncontrollable frequency. To prevent this oscilla-
6-5