Fluke 825A - Input Attenuator; Operation; Recorder Output

825А
3-2. INPUT ATTENUATOR
a.
Inthe
VTVM
mode,
the
resistance
of
the
input
attenuator
and
thus
the
input
resistance
of
the
voltmeter
is
50
megohms
(R301
through
R310).
For
full
scale
deflection,
1
mv
must
be
applied
to
the
null
detector.
The
necessary
reduction
is
provided
by
four
positions
on
the
input
attenuator
that
are
selected
by
range
switch
section
S2C.
~
.
In
the
differential
e
the
resistance
of
the
TM
tems
is
10
megohms
(R305
through
R310)
for
the
10,
1,
0.1,
and
0.
01
null
ranges
and
1
megohm
(R306
through
R310)
for
the
0.
001
volt
null
range.
However,
this
is
not
the
input
resistance
of
the
voltmeter.
The
input
resistance
is
determined
by
dividing
the
terminal
voltage
of
the
unknown
by
the
amount
of
current
drawn
from
the
unknown.
The
current
drawn
from
the
unknown
is
equal
to
the
difference
between
the
unknown
terminal
voltage
and
the
internally
known
voltage
divided
by
the
resistance
of
the
input
attenuator.
Тһе
equation
for
input
resistance
can
be
hence
written
as:
E Ey В Ес (Ra + В,
Rin = = та 5 Seu в) -Rg where:
Rin = ие resistance of voltmeter
Ey
=
Eg
-
lu
Rg
=
terminal
voltage
of
unknown
I, = current drawn from unknown
Eg = source voltage of unknown
Rs = source resistance of unknown
В. = resistance of input attenuator
E = voltage indicated by voltage readout dials
| | „г absolute value (magnitude only)
Thus, the input resistance is infinite at null when E is
equal to E, and Ес.
c. For full scale deflection of 10, 1, 0.1, and 0.01
volts, the voltage difference (unknown voltage minus re-
ference voltage) must be reduced by an attenuator. This
reduction is provided by four positions on the input at-
tenuator that are selected by null switch section S3D.
On the 0. 001 volt null range, the voltage across the
input attenuator is fed directly to the null detector.
3-3. NULL DETECTOR
a. GENERAL, The null detector is a fixed gain device
containing four resistance-capacitance coupled voltage
amplifier stages with a high amount of negative current
feedback. With high negative current feedback, the out-
put current is approximately equal to the signal voltage
divided by the impedance of the feedback network regard-
less of the amplifier characteristics or even the load
impedance. Тһе high negative feedback also makes the
amplifier relatively insensitive to the gain changes in
individual tubes due to aging and replacement. Тһе out-
put current from the null detector is indicated on a meter
that has taut-band suspension. This suspension does
away with all friction associated with meter pivot sticki-
ness. Thus, any tendency for the meter pointer to stick
at one point of the scale and then jump to another point is
completely eliminated.
3-2
b. OPERATION. At the input to the null detector,
R201,
C201,
R202,
and
C202
form
a
double
section
low
pass filter that reduces any AC component present on the
DC
voltage
being
measured.
Тһе
difference
between
the
voltage appearing at the output of the filter and the voltage
developed across the feedback network is converted to an
alternating voltage by G1, a 94 cycle chopper This
chopped voltage is amplified by V202A, V202B, ала
V203A before passing through cathode follower V203B.
During half the chopper cycle the output of the amplifier
is clamped to approximate null detector common poten-
tial by G1 while during the other half the output is filter-
ed by C212 to provide a DC current for the meter. When
the
chopper
provides
connection
between
contacts
1
and
9,
a
voltage
is
developed
across
feedback
network
R220,
R221, and R222 that is proportional to the meter (output)
current. This feedback voltage reduces the filter output
voltage with respect to the four stage amplifier. The
impedance of the feedback network (R220, R221, and
R222) is adjustable between 8. 82 and 9. 83 ohms. Since
the output current is approximately equal to the signal
voltage divided by the impedance of the feedback network,
a 1 mv signal voltage indicates an output current of
101. 7 to 113. 4 ua. However, there is a loss due to
finite amplifier gain and filtering that leaves the output
current around 100 ua which can be set accurately by
means of the feedback network. Thus, current feedback
makes the output current essentially proportional to the
signal voltage. For full scale deflection, a 1 mv signal
voltage will cause 100 ua to flow through the meter.
c. EFFECT OF AC COMPONENTS, The only AC vol-
tage component that will reduce the accuracy of the 825A
is one that either saturates the chopper-amplifier or one
that beats with the chopper frequency. Since the voltage
required for saturation is greater than that required for
beating, the null detector is most sensitive to an AC
component with a frequency that is a submultiple or a low
multiple of the chopper frequency. However, this is
easy to detect because the meter will beat at the differ-
ence frequency The low pass filter at the input of the
chopper-amplifier will attenuate any AC component.
The magnitude of the AC voltage appearing at the output
of the filter depends on both its amplitude and frequency
before filtering. For all practical purposes, one should
never encounter any trouble above a few hundred cycles.
Below this, the filter may not attenuate the AC component
enough. However, this is not as bad as it appears. A
60 cycle AC voltage that is 10% of the input voltage will
cause an error of approximately 0. 01% which is well
within specifications. If AC components that affect
accuracy are ever encountered, additional filtering as set
forth in paragraph 2-9 will eliminate the problem.
4. ADJUSTMENTS. Variable resistor R232 in the null
detector power supply provides.a means of adjusting the
output current of the amplifier to zero when there is no
input signal. The gain of the amplifier is adjusted by
means of R222 in the feedback circuit.
e. RECORDER OUTPUT. The recorder output is
picked off divider string R225, R1, and R226. Output
level control R1 provides a means of adjusting the out-
put voltage up to a maximum of approximately 20 milli-
volts at full scale deflection. The voltage at the output
terminals is proportional to the meter reading.