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Section VIIL
Model 3455A
THEORY OF OPERATION
8-10. INTRODUCTION.
8-11. The following paragraphs contain both a general and
detailed description of the methods and circuits used in the
Model 3455A Multimeter. The general description explains
the basic purpose of each block of the functional block
diagram shown in Figure 8-1. The detailed description
describes
the methods
and pertinent circuitry used
to
accomplish the function of each block of the detailed block
diagram.
8-12. INPUT SWITCHING AND DC ATTENUATOR.
8-13. General.
8-14. The front or rear input terminals of the 3455A are
selected by a two-section
rotary switch located on the
rear panel of the instrument. Reed relays are used to per-
form
all internal input switching where voltages greater
than 17 volts may be encountered.
All other input switch-
ing is done with FET switches.
8-15. Detailed Description.
8-16. Refer to Figure 8-2, Simplified Input Switching Dia-
gram. The front or rear inputs for '"'Volts", "Ohms"
and
"Guard" are selected by rear panel switch $1. Relays K3
and K9 connect the Ohms Converter to the "Ohms Signal"
terminals. Relays K2 and K4 are used to convert the Multi-.
meter from 4-wire to 2-wire ohms measurement capability.
The "Input" terminals are connected to the, dc preampli-
fier input on the .1 V de through 10 V dc and all "Ohms"
ranges by relay K1 and FET switch Q1. Relay K5 connects
the input to the operational attenuator on the 100 and
1000 V de ranges. Output of the Attenuator is connected
to the input of the dc preamplifier by FET switch Q15 on
the 100 V-dce range and by FET switch Q16 on the 1000 V
de range. The AC Converter output is connected to the de
preamp input by FET switch Q3.
8-17. Operational Attenuator.
8-18. The Operational Attenuator provides a fixed attenu-
ation of 10-to-1 on the 100 V de range or 100-to-1 on the
1000 V dc range. Figure 8-3 shows a simplified diagram of
the attenuator. The circuit operates as a conventional oper-
ational amplifier with fractional gains of .1 and .01. A gain
of lis selected by FET switch Q38 when the attenuator is
not in use. The amplifier input is protected from overload
by diodes CR27
and CR28. Output of the amplifier is
limited to approximately plus or minus 17 V dc by protec-
tion diodes CR29 through CR31.
8-19. AUTO CALIBRATION-DC VOLTAGE.
8-20. General.
8-21. The purpose of the Auto Calibration sequence is to
eliminate offset and gain errors which may be present in the
AC
To
CONVERTER
AC.
CONVERTER
KI
ini
oo
naa]
=
OVERLOAD
fp
PROTECTION
=
Figure 8-2. Simplified Input Switching Diagram.
8-2
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