HP 3478A Technical Manual page 99

3478A 3478A
d. d. Oh Ohms ms Lo Low S w Sen ense
path consists of RI01, R102, and S6ADC and S6CDC path consists of RI01, R102, and S6ADC and S6CDC
(S6ADC and S6CDC (S6ADC and S6CDC are in U102) are in U102). . The path is u
connect the LO OHMS SENSE Input Terminal (low ohms connect the LO OHMS SENSE Input Terminal (low ohms
input) to ground (LO INPUT Terminal), when the 3478A input) to ground (LO INPUT Terminal), when the 3478A
is in is in the 4-Wire the 4-Wire Ohms Function. Ohms Function. In th
Function, the low ohms input is the LO INPUT Terminal Function, the low ohms input is the LO INPUT Terminal
(which is connected to ground). (which is connected to ground).
e. e. Cu Curr rren ent I t Inp nput ut Pa Path th (Figure 7-F-2
in- cludes R107 in- cludes R107 and S3DC and S3DC (in U102).
across R107 (which is generated by the current being across R107 (which is generated by the current being
measured) is connected through S3DC to the DC/Ohms measured) is connected through S3DC to the DC/Ohms
Input Amplifier. This path Input Amplifier. This path is used
rent Function. rent Function.
7-F-14. Autozero. 7-F-14. Autozero. The Autozero Function of the 3478A The Autozero Function of the 3478A
is used to measure the offset (called a zero measure- is used to measure the offset (called a zero measure-
ment) of the DC/Ohms Input Amplifier and subtract it ment) of the DC/Ohms Input Amplifier and subtract it
from the input measurement. from the inpu t measurement. As long as
Function is enabled, a zero measurement is taken be- Function is enabled, a zero measurement is taken be-
tween each tween each input measurement. input measurement. The zero measurement
is done in two different ways, dependent on the function is done in two different ways, dependent on the function
selected. selected. The following e The following explains the t
the measurement and the circuitry used. the measurement and the circuitry used.
a. a. Au Autoz tozero ero (in t (in the DC he DC Vol
Wire Ohms Functions). Wire Ohms Functions). In the DC Volts, DC Current,
and 2-Wire Ohms Functions, the zero measurement is and 2-Wire Ohms Functions, the zero measurement is
made with switch MC (measure customer) open, and made with switch MC (measure customer) open, and
switch MZ (measure zero) and S8DC closed (see Figure switch MZ (measure zero) and S8DC closed (see
7-F-2 7-F-2G). G). Switch SSDC Switch SSDC and MZ
Input Amplifier to ground through a 102K ohm resistor. Input Amplifier to ground through a 102K ohm resistor.
After the zero measurement is made, switch S8DC and After the zero measurement is made, switch S8DC and
MZ opens, and switch MC closes for the input measure- MZ opens, and switch MC closes for the input measure-
ment. ment. (Note: MOSFET switches (Note: MOSFET switches MC, MZ, and
in U102.) in U102.)
b. b. Au Autoz tozero ( ero (in the 4 in the 4-Wi
the 4-Wire Ohms Function, the zero measurement is the 4-Wire Ohms Function, the zero measurement is
made with switch MC (and S8DC) open and switch MZ, made with switch MC (and S8DC) open and switch MZ,
S6ADC, S6ADC, and and S6BDC S6BDC closed.
Amplifier is connected to ground through the Ohms Low Amplifier is connected to ground through the Ohms Low
Sense Path (includes an externally connected lead; see Sense Path (includes an externally connected lead; see
Figure 7-F-2H) ) to to the Figure 7-F-2H the LO
measurement is different than in the DC Volts, DC Cur- measurement is different than in the DC Volts, DC Cur-
rent, and 2-Wire Ohms Functions, since the Input rent, and 2-Wire Ohms Functions, since the Input
Amplifier is connected to ground through the externally Amplifier is connected to ground through the externally
connected low ohms sense and lo connected low o hms sense and lo input leads.
offset measurement is made, switch MC and S6CDC offset measurement is made, switch MC and S6CDC
closes and switch MZ, S6ADC, and S6BDC opens. closes and switch MZ, S6ADC, and S6BDC opens.
Switch S6CDC is used to to connect the junction of Switch S6CDC is used to to connect the junction of
switch S6ADC and S6BDC to ground, shunting any switch S6ADC and S6BDC to ground, shunting any
possible voltage possible voltage on on the the junction t
MOSFET switches S6ADC, S6BDC, and S6CDC are in MOSFET switches S6ADC, S6BDC, and S6CDC are in
U102.) U102.)
7-F-15. Pre-Charge 7-F-15. Pre-Charge Stage Stage (Figure
stray capacitance exists in U102 between the input of the stray capacitance exists in U102 between the input of the
DC/Ohms Input DC/Ohms Input Amplifier and Amplifier and ground.
se Pa Path th (Figure 7-F-2E (Figure 7-F-2E). ). This
The path is used to
In the 2-Wire Ohms e 2-Wire Ohms
(Figure 7-F-2F). F). This path This path
(in U102). The voltage drop The voltage drop
is used only in only in the DC Cur the DC Cur- -
As long as the Autoz the Autozero
The zero measurement
xplains the two way wo ways to make s to make
Volts, D ts, DC Cur C Curren rent, an
In the DC Volts, DC Current,
and MZ connects the connects the DC/Ohms DC/Ohms
MC, MZ, and S8DC are S8DC are
-Wire Oh re Ohms Fu ms Funct nction)
closed. The DC/Ohms The DC/Ohms Input
LO INPUT INPUT Terminal. Terminal.
input leads. After the
junction to gr o ground. ound.
(Figure 7-F-21). 7-F-21).
ground. During a zero During a zero
measurement (measure zero in the Autozero Function), measurement (measure zero in the Autozero Function),
This
this capacitor this capacitor is at zero
ment, the input to the 3478A is applied to the Input ment, the input to the 3478A is applied to the Input
sed to
Amplifier and the capacitor is charged to the input Amplifier and the capacitor is charged to the input
voltage. voltage. This charging may
input voltage and a input volt
this, the input of the Input Amplifier is pre-charged to the this, the input of the Input Amplifier is pre-charged to the
input voltage before the input measurement is made. input voltage before the input measurement is made.
This is done by the Pre-Charge Amp and MOSFET This is done by the Pre-Charge Amp and MOSFET
Switch PRE (both in U102). Switch PRE (both in U10
a. a. Aft
PRE closes (switch MC is still open). PRE closes (switch MC is still open).
b. b. The i
Charge Amp using the input paths. Charge Amp using the input paths.
c. c. Sin
a voltage with the same polarity and value as the input a voltage with the same polarity and value as the input
voltage is applied through PRE to the DC/Ohms Input voltage is applied through PRE to the DC/Ohms Input
Amplifier. This pre-charges t Amplifier.
ero
put voltage. put voltage.
d. d. Swi
input measurement is then made. input measurement is then made.
e. e. The s
measure- ment. measure- ment.
7-F-16. DC/Ohms 7-F-16.
t, and 2- d 2-
DC/Ohms Input amplifier, in conjunction with the In- put DC/Ohms Input amplifier, in conjunction with the In- put
Switching Circuitry, is to condition the 3478A's in- put Switching Circuitry, is to condition the 3478A's in- put
signals. The conditioning is signals.
scale IOV dc voltage to the A/D Converter for all DC scale IOV dc voltage to the A/D Converter for all DC
Figure
Volts, DC Current, a Volts, DC
done by configuring the amplifier to a gain of X3.33, done by configuring the amplifier to a gain of X3.33,
X33.3, or X333 X33.3, or X333. . The gains used in
ranges (in the DC Volts, DC Current, and Ohms Func- ranges (in the DC Volts, DC Current, and Ohms Func-
tions) is shown in tions) is shown in Figure 7-F-3.
7-F-17. Ohms Cu 7-F-17.
7-F-18. General. The purpose of the Ohms Current 7-F-18. General.
ion). . In In
Source is to provide a stable current for resistance Source is to provide a stable current for resistance
measurements. measurements. The current
resistance and the resultant voltage drop across the resistance and the resultant voltage drop across the
Input
resistance is measured resistance
directly proportional to the unknown resistance, the directly proportional to the unknown resistance, the
resistance value is determined by the 3478A's Main resistance value is determined by the 3478A's Main
This This
Controller from the voltage reading. Controller from the voltage reading.
7-F-19. 7-F-19.
following circuitry: Voltage Splitter (Voltage Reference), following circuitry: Voltage Splitter (Voltage Reference),
Buffer, Range Resistors, Output FET Control Amp (Gate Buffer, Range Resistors, Output FET Control Amp (Gate
After the
Bias Amplifier), Output MOSFET, and Overvoltage Bias Amplifier), Output MOSFET, and Overvoltage
Protection. In addition Protection.
are used as switches (to select various gain determining are used as switches (to select various gain determining
resistors). All the resistors).
U102 which also controls the FET U102 which also controls the FET
(Note: (Note:
A small A small
7-F-5 7-F-5
is at zero volts. volts. After the z
This charging may temporarily temporarily load down
age and a wrong reading wrong reading is taken.
2). The operation is as The operation is as follows:
After th er the ze e zero mea ro measur sureme
The inpu nput vol t voltag tage is co e is conne
Since th ce the Pr e Pre-C e-Char harge Am ge Amp is a XI g
This pre-charges the stray he stray capacitor to t
Switch tch PRE PRE the then n ope opens ns and
The same t ame takes akes pla place be ce befor
DC/Ohms Input Input Amplifier. Amplifier. The purpose of the
The conditioning is done to a done to apply the
Current, and Ohms full scale nd Ohms full scale inputs.
The gains used in the 3478A'
Figure 7-F-3.
Ohms Current rrent Source Source
The purpose of the Ohms Current
The current is applied is applied to the
is measured. . Since this Since
The Ohms The Ohms Current Current Source
In addition to these, to these, a number of M
All the previously previously mentioned FETs are mentioned FETs are inside
TM 11-6625-3071-14 TM 11-6625-3071-14
After the zero measure- ero measure-
load down the the
is taken. To prevent To prevent
follows:
ement is ma nt is made, s de, swit witch ch
nnecte cted to t d to the Pr he Pre- e-
p is a XI gain a ain ampli mplifie fier, r,
capacitor to the in- he in-
and MC MC clo closes ses. . An An
fore th e the nex e next ze t zero ro
The purpose of the
pply the same full same full
inputs. This is This is
the 3478A's different s different
to the unknown unknown
this voltage voltage drop drop is is
Source consists consists of t of the he
a number of MOSFETS OSFETS
inside