Model 8903B
Service
FROM
TO-S
AMP
L
DIFFERENTIAL-
I
NGLE-ENDED
4
IFlER
~ ~~ ~
PI0
A2
INPUT
AMPL
I
F
I
ER
ASSEMBLY
GAIN AMPLIFIER
I
TO
HIGH-PASS FILTERS
AND WEIGHTING
BP FILTERS
~~ ~
Figure
8-4.
Example Showing Programmable Gain Amplifier Switching
hardware will revert back
to
the measurement mode it was in before the Direct Control Special Function
was
entered. Thus, in this example, unless
8
dB
gain had been previously set,
it
would be removed from the
audio path when any other key
is
pressed. (However, note that there are some Service Special Functions
that will maintain the requested configurations even
if
another key
is
pressed.)
As
it turns out, other 0.72d codes than those shown
in
Table
8-2
will affect the gain
of
the Programmable
Gain Amplifier. For example, 0.723
will
close Ull(3) and Ull(1) simultaneously (gain is now 10.6
dB).
This fact
is
ascertained from the service sheet schematics.
Example
2
A
second example from the
A2
assembly illustrates
data
readback. One of the means of detecting an
overrange of the input circuits is by the Input Overload Detector. The detector connects
to
both the
high and low inputs (directly) and output (through the Input Range Detector) of the Programmable Gain
Amplifier
of
Example
1.
See Figure
8-5.
At
the input
of
the amplifier, the Input Overload Detector senses
the dc level and compares it against both a positive and negative reference.
At
the output of the Input
Range Detector
it
also compares the dc level (and hence the ac level of the
signal)
against a reference.
If
any detected level exceeds
a
reference, the output
of
the detector goes low and resets flip-flop A7U14 on the
A7 Latch Assembly. A7U14 also sets the gain
of
all input circuits up
to
and including the Programmable
Gain Amplifier to minimum gain without intervention of the Controller. A7U15B, when enabled, inverts
the output of A7U14. The output of A7U14
is
across the least-significant bit of the readback data line
of
the Instrument Bus. In the normal measurement cycle, the Controller reads the status of the Input
Overload Detector (by enabling A7U15B and reading its output) and takes corrective action
if
A7U14 has
tripped.
At
this point in the discussion, a more detailed description of the Instrument Bus
data
lines
is
needed.
Data
(d)
is sent out from the
1/0
port of the Controller
to
the Instrument Bus through buffers (TTL
inverters). However, data
is
read back to the
1/0
port directly, bypassing the buffers. An
1/0
port outputs
a low by actively pulling the line to ground.
It
outputs a high by allowing the output to be passively
~~ ~
reu.2OJUNB
7
8-9
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