HP 3478A Technical Manual page 97

3478A 3478A
the amplifier is then measured and stored into memory. the amplifier is then measured and stored into memory.
This reading is then subtracted from the DC Volts, DC This reading is then subtracted from the DC Volts, DC
Current, and Ohms readings that follow Current, and Ohms readings that follow. . If the AC Volts
or AC Current Functions is selected, the zero reading is or AC Current Functions is selected, the zero reading is
taken differently. . The input t taken differently The input to the
shorted to ground instead of the DC/Ohms In- put shorted to ground instead of the DC/Ohms In- put
Amplifier. Amplifier.
7-F-7. 7-F-7. THEORY THEORY OF OF OPERATION OPERATION
7-F-8. General 7-F-8. General
7-F-9. 7-F-9. The following The following paragraphs
description of the operating description of t he operating circuitry
circuitry is explained as follows: circuitry is explained as follows:
a. a. Inp Input C ut Circ ircuit uitry - ry - par
b. b. Ohm Ohms Cur s Curren rent Sou t Source - p
c. c. AC t AC to DC C o DC Conv onvert erter -
d. d. A/D A/D Con Conver verter ter - pa - parag
e. e. 347 3478A Lo 8A Logic C gic Circ ircuit
f. f. Cha Chassi ssis Co s Comm mmon C on Circ
g. g. Iso Isolat lation ion Log Logic - ic - par
h. h. Float Floating ing Common Common Logic
75. 75.
i. i. Po Powe wer Su r Supp ppli lies - es - pa
7-F-10. Input 7-F-10. Input Circuitry Circuitry
7-F-11. General. 7-F-11. General. The purpose of the Input Circuitry is to The purpose of the Input Circuitry is to
condition the dc input signals to the 3478A to provide full condition the dc input signals to the 3478A to provide full
scale 10V dc input voltages to the A/D Converter, for full scale 10V dc input voltages to the A/D Converter, for full
scale inputs to the 3478A (the explanation for ac inputs scale inputs to the 3478A (the explanation for ac inputs
is in paragraph is in paragraph 7-F-25, AC 7-F-25, AC to DC Convert
Circuitry also acts as a buffer between the in- put and Circuitry also acts as a buffer between the in- put and the
A/D Convert A/D Converter. er. The cicuitry The cicuitry can be
explained) as follows: explained) as follows:
Overvoltage Overvoltage
Protection Input Switching Protection Input Switching
Autozero and Pre-Charge Autozero and Pre-Charge
Pre-Charge Stage Pre-Charge Stage
DC/Ohms Input Amplifier DC/Ohms Input Amplifier
7-F-12. 7-F-12. Overvoltage Overvoltage Protection.
three parts: High Voltage Protection, Low Voltage three parts: High Voltage Protection, Low Voltage
Protec- t Protec- tion, and ion, and Current Pr Current Protection.
operate as follows (refer to Schematic I for the operate as follows (refer to Schematic I for the
explanation). explanation).
a. a. High High Voltage Voltage Protection. Protection. This circuit consist of a
630V Surge Voltage Protector (E101) in series with a 630V Surge Voltage Protector (E101) in series with a
220K ohm resistor. 220K ohm re sistor. The circuitry The circuitry is connected
the HI and LO INPUT Terminals and conducts with a the HI and LO INPUT Terminals and conducts with a
peak voltage level of 630V (+ 20%), which provides a low peak voltage level of 630V (+ 20%), which provides a low
impedance path across impedance pat h across the ter
provides a temporary low impedance path and R109 provides a temporary low impedance path and R109
provides current limiting, if E101 conducts continuously. provides current limiting, if E101 conducts continuously.
If the AC Volts
o the A/D Con- A/D Con- verter is
paragraphs give give a a detailed
circuitry in the 3478A in the 3478A. . The
paragr agraph aph 7-F 7-F-10 -10. .
rce - para aragra graph 7- ph 7-F-1 F-17. 7.
er - par paragr agraph aph 7-F 7-F-25 -25. .
ragrap raph 7- h 7-F-3 F-31. 1.
uitry - pa ry - parag ragrap raph 7-F h 7-F-50 -50. .
ircuit uitry - ry - par paragr agraph aph 7-F 7-F-55
paragr agraph aph 7-F 7-F-71 -71. .
Logic Circu Circuitry itry parag paragraph raph 7-F-
para ragr grap aph 7- h 7-F- F-83 83. .
to DC Converter). er). The Input The Input
can be separated (and separated (and is is
Protection. This circuitry has This circuitry has
otection. The two cir The two circuits
This circuit consist of a
is connected between
the terminals. minals. Capacitor C1 Capacitor C104
b. b. Lo
of diodes connected to the individual input nodes of diodes connected to the individual input nodes (part of
the HI the HI INPUT, LO
diodes on each node are connected (internally in U 102) diodes on each node are connected (internally in U 102)
verter is
to + 3.5V to + 3.5V and -3.5V power
node exceeds either + 4.2V or -4.2V, a diode conducts node exceeds either + 4.2V or -4.2V, a diode conducts
and keeps the level on the node to the + 4.2V levels. and keeps the level on the node to the + 4.2V levels.
The difference between the high voltage at the input ter- The difference between the high voltage at the input ter-
minals and the voltage at the input nodes is dropped minals and the voltage at the input nodes is dropped
across the resistors which are in series with the diodes across the resistors which are in series with the diodes
and input terminals (the low pass filter). and input terminals (the low pass filter). Figure 7-F-2
shows a typical protection circuit. shows a typical protection circuit.
detailed
c. c. Cu
The
cur- rent shunt (RI01) protects the shunt from excessive cur- rent shunt (RI01) protects the shunt from excessive
in- put curre in- put currents (>3A).
greater than 250V is applied between the A and INPUT greater than 250V is applied between the A and INPUT
LO Terminals. LO Terminals.
7-F-13. 7-F-13. Input
-55. .
consists mostly of Relay and MOSFET switches, with consists mostly of Relay and MOSFET switches, with
most FET most FET switches loca
7-F-
the switches are to provide five signal paths to the Input the switches are to provide five signal paths to the Input
Amplifier and to connect the amplifier for a zero Amplifier and to connect the amplifier for a zero
measurement (done in the Autozero Function; see measurement (done in the Autozero Function; see
paragraph paragraph 7-F-14).
circuits in U102 which receive their control information circuits in U102 which receive their control information
from the A/ from t he A/D Co
various input path,. various input
1 for the explanation. 1 for the explanation.
a. a. Low V
2B). The path consists of K101, R103, R104, R10, and 2B). The path consists of K101, R103, R104, R10, and
the
SIDC (SIDC is in U102). SIDC (SIDC
connect the HI INPUT Terminal (high input voltage) to connect the HI INPUT Terminal (high input voltage) to
the DC/Ohms Input Amplifier. the DC/Ohms
the 30mV through 3V dc volts Ranges and all Ohms the 30mV through 3V dc volts Ranges and all Ohms
Ranges. Ranges.
b. b. Hig
2C). The path consists of RI 10, K102, S2DC (in 102), 2C). The path consists of RI 10, K102, S2DC (in 102),
and a 100:1 divider (9.9M ohm and 100K ohm resistors, and a 100:1 divider (9.9M ohm and 100K ohm resistors,
RD99 and RD98, in U102). RD99 and RD98,
attenuate input voltages by a facter of 100 and to attenuate input voltages by a facter of 100 and to
connect the attenuated voltage to the DC/Ohms Input connect the attenuated voltage to the DC/Ohms Input
Amplifier. The path is used Amplifier.
cuits
volts Ranges. volts Ranges.
c. c. Oh
path consists of RIOS, R106, and S4ADC and S4BDC path consists of RIOS, R106, and S4ADC and S4BDC
(S4ADC and S4BDC are (S4ADC and
the HI OHMS SENSE Terminal (high ohms input) to the the HI OHMS SENSE Terminal (high ohms input) to the
between
DC/Ohms Input Amplifier, only when the 3478A is in the DC/Ohms Input Amplifier, only when the 3478A is in the
4-Wire Ohms Function. 4-Wire Ohms
part of the path) is used to connect the junction of part of the path) is used to connect the junction of
04
S4ADC and S4BDC S4ADC and S4BDC to ground.
in all functions except the 4-Wire Ohms Function and in all functions except the 4-Wire Ohms Function and
shunts any possible voltage on the S4ADC and S4BDC shunts any possible voltage on the S4ADC and S4BDC
junction to ground. junction to ground.
7-F-3 7-F-3
Low Vo w Volt ltag age Pr e Prot otec ecti tion
INPUT, LO INPUT, A I INPUT, A INPUT Paths,
and -3.5V power supplies. supplies. If the voltage
Curr rren ent t Pr Prot otec ecti tion on. . A fuse in series with the A fuse in series with the
nts (>3A). The fuse also opens The fuse also opens if a voltage
Input Switching. Switching. The Input Switching Circuitry The Input Switching Circuitry
switches located in ted in U102.
7-F-14). The switches The switches are
D Controller. ntroller. The following The following explains the
path,. Refer to Refer to Figure 7-F-2 Figure 7-F-2 and Schematic
Low Volt oltage R age Rang ange Inp e Input P
is in U102). The purpose of The purpose of the path
Input Amplifier. The path is
High Vo h Volt ltag age Ra e Rang nge Inp e Input P
in U102). The purpose of the path is to The purpose of the path is to
The path is used only in t only in the 30V and 300V d
Ohms ms Hi High gh Se Sens nse P e Pat ath h (Figure 7-F-2
S4BDC are in U102). in U102). The path con
Function. Switch Switch S4CDC (which
to ground. Switch S4CDC is closed
TM 11-6625-3071-14 TM 11-6625-3071-14
on. . This circuit consists This circuit consists
(part of
NPUT Paths, etc.). etc.). The The
If the voltage on a on a
Figure 7-F-2A A
if a voltage
U102. The purposes The purposes of of
are controlled controlled by by
explains the
and Schematic
ut Path ath (Fi (Figur gure 7-F e 7-F- -
the path is to is to
The path is used only used only in in
ut Pat ath h (Figure 7-F- (Figure 7-F-
he 30V and 300V dc c
(Figure 7-F-2D). D). This This
The path connects nects
S4CDC (which is also is also
Switch S4CDC is closed