HP 3478A Technical Manual page 101

3478A 3478A
+ 8V, its inverting terminal must also be + 8V (the in- + 8V, its inverting terminal must also be + 8V (the in-
verting terminal draws lit verting ter minal draws little or n
the resultant voltage drop across the Range Resistors + the resultant voltage drop across the Range Resistors +
4V (or + .4V for the 30M ohm Range). 4V (or + .4V for the 30M ohm Range).
d. d. A cur A curren rent is t t is then g
proportional to the value of the Range Resistor and the proportional to the value of the Range Resistor and the
voltage drop across the resistor. voltage drop across the resistor.
e. e. The cu The curre rrent is a nt is appl
through the Output MOSFET (in U102), High Voltage through the Output MOSFET (in U102), High Voltage
Protection Circuitry, Protection Circuitry, and t and the HI
3478A's 3478A's Ohms Ohms Ranges, Ranges,
Resistors, and selected MOSFET Switches are listed in Resistors, and selected MOSFET Switches are listed in
Table 7-F-1. Table 7-F-1.
7-F-21. Voltage 7-F-21. Voltage Splitter Splitter (U201).
the Voltage Splitter (Reference) Circuitry. the Voltage Splitter (Reference) Circuitry.
a. a. The V The Volta oltage S ge Splitt plitter is
amplifier with an output of + 12V (for the 300 ohm to 3M amplifier with an output of + 12V (for the 300 ohm to 3M
ohm Ranges) or + 8.4V (for the 30M ohm Range). ohm Ranges) or + 8.4V (for the 30M ohm Range).
b. b. The g The gain i ain is det s determ
RR3, RR4 (both in RR3, RR4 (both in U102), and U102), and R201.
value of RR3 plus RR4 is 20K ohm and R201 is 40K value of RR3 plus RR4 is 20K ohm and R201 is 40K
ohm, t ohm, the no he non-inverting n-inverting gain
[20K/40K] + 1 = 1.5). [20K/40K] + 1 = 1.5).
c. c. The + 1 The + 12V ou 2V outp tput is g
to the amplifier input (8 to the amplifier input (8*1.5 =
mined by the + O1V Reference (from the 3478A's mined by the + O1V Reference (from the 3478A's
Reference Circuitry, see paragraph 7-F-49d) and Reference Circuitry, see paragraph 7-F-49d) and voltage
divider RRO, RRI, and divider RRO, RRI, and RR2. RR2. The sum of RRI and RR2 is
200K ohm and RRO is 50K ohm resulting in a voltage 200K ohm and RRO is 50K ohm resulting in a voltage
drop across the resistors of + 8V and + 2V, respective- drop across the resistors of + 8V and + 2V, respective-
ly. ly. The + 8V The + 8V is applied t is applied through FET Sw
U201. U201.
d. d. The + 8. The + 8.4V ou 4V outp tput is ge
+5.6V +5.6V to t to the amplifier he amplifier input (5 input (5.6*1.5=8.4).
also determined by the + O1V Reference (from the also determined by the + O1V Reference (from the
3478A's Reference Circuitry) and voltage divider RRO, 3478A's Reference Circuitry) and voltage divider RRO,
RRI, and RR2 RRI, and RR2. . In this case t In this case the voltage dro
(which is +5.6V developed across 140K ohm) is applied (which is +5.6V developed across 140K ohm) is applied
through FET Switch SIR to U201. through FET Switch SIR to U201.
7-F-22. 7-F-22. Buffer (U20 Buffer (U202) and 2) and Range Resist
RR8). RR8). The Buffer is a n The Buffer is a non-inverting XI on-inverting XI gain amplifier with
its output at either + 12V or + 8.4V its output at either + 12V or + 8.4V (on the 30M
O O h h m m s s
R R a a n n g g e e
3 3 0 0 0 0 - - 3 3 K K
3 3 0 0 K K
3 3 0 0 0 0 K K
3 3 M M
3 3 0 0 M M
tle or no current). o current). This makes This makes
hen gene enerat rated w ed with i ith its va ts value
pplied t ied to the u o the unkn nknown r own resi
he HI INPUT Terminal. INPUT Terminal. The
Ohms Ohms Currents, Currents,
(U201). The following explains The following explains
er is a XI. a XI.5 ga 5 gain no in non-inv n-inverti
ermine ined by fe d by feedb edback re ack resis
R201. Since the Since the total
gain is XI.5 is XI.5. . (The gain is: is: (The gain
ut is gene enerat rated by a ed by appl pplyi ying +8
*1.5 = 12). 12). The +8V is d The +8V is deter-
The sum of RRI and RR2 is
hrough FET Switch SOR to itch SOR to
ut is gener nerate ated by a d by appl
.6*1.5=8.4). The + The + 5.6V is
he voltage drop across RR2 p across RR2
Range Resistors (RR5 ors (RR5 to to
gain amplifier with
(on the 30M
Table 7-F-1. Ohms Curren Table 7-F-1.
O O h h m m s s
C C u u r r r r e e n n t t
1 1 m m A A
. . 1 1 m m A A
. . 0 0 1 1 m m A A
1 1 u u A A
. . 1 1 u u A A
ohm Range o ohm Ran
MOSFET Switch pairs S6R and S7R, SS8R and S9R, MOSFET Switch pairs S6R and S7R, SS8R and S9R,
SIOR SIOR and and SIIR,
Resistors are used to determine the different ohms cur- Resistors are used to determine the different ohms cur-
lue
rent va rent values. lues.
switch and resistor is selected for the different ohms switch and resistor is selected for the different ohms
ranges. ranges.
esisto stor r 7-F-23. The Output 7-F-23.
the Output MOSFET (in U102) form a feedback circuit the Output MOSFET (in U102) form a feedback circuit
The used to boost the output impedance of the Output used to boost the output impedance of the Output
Range Range MOSFET. The FET MOSFET.
and in conjunction with the open loop gain of U203 and and in conjunction with the open loop gain of U203 and
the Range Resistors, results in a high output impedance the Range Resistors, results in a high output impedance
of the Ohms Current Source. of the Ohms
is necessary to prevent measurement nonlinearities. is necessary to prevent measurement nonlinearities.
erting ng 7-F-24. 7-F-24.
circuitry is used to protect the Ohms Current Source circuitry is used to protect the Ohms Current Source
from high voltages inadvertently applied to the 3478A's from high voltages inadvertently applied to the 3478A's
sistor tors s Input Ter- minals (when the Multimeter is in the Ohms Input Ter- minals (when the Multimeter is in the Ohms
total
Function). The following explains Function).
a. a. If a If a large
3478A's HI INPUT Terminal, the voltage is applied 3478A's HI INPUT Terminal, the voltage is applied
through R204 an through R204 and L201 to the ca
ng +8V V the cathode voltage of CR201 is higher than the anode the cathode voltage of CR201 is higher than the anode
eter- voltage, voltage, the diode
large input voltage from damaging the large input voltage from damaging the current source.
voltage b. b. If a la If a large n
3478A's HI INPUT Terminal, the voltage is dropped 3478A's HI INPUT Terminal, the voltage is dropped
across CR201, R203, and R202 to diode connected FET across CR201, R203, and R202 to diode connected FET
Q205. Q205. This makes Q205 conduct a This makes Q205 conduct and in turn
base of base of Q202 at
along with negative collector to base voltage forces along with negative collector to base voltage forces
Q201, Q202, Q203, and Q201, Q202,
pplyi ying ng Q201 and Q203 conducts no more current than the Q201 and Q203 conducts no more current than the
5.6V is circuit which generates the ohms current (on a given circuit which generates the ohms current (on a given
range). The cir- range).
cannot tell the dif- ference between a valid unknown cannot tell the dif- ference between a valid unknown
resistance and the large negative resistance and t
the large input voltage is dropped across R202 and the large input voltage is dropped across R202 and
R203. R203.
7-F-25. AC to 7-F-25.
7-F-26. The purpose of 7-F-26.
convert the 3478A's ac inputs (volts or current) to dc convert the 3478A's ac inputs (volts or current) to dc
volts. The converter out volts. The converter output is + 3.
scale ac inputs with the output applied to the A/D Con- scale ac inputs with the output applied to the A/D Con-
Ohms Current and t and Ranges
R R a a n n g g e e R R e e s s i i s s t t o o r r
R R e e s s i i s s t t o o r r V V o o l l t t a a g g e e
4 4 K K ( ( R R R R 5 5 ) )
4 4 0 0 K K ( ( R R R R 6 6 ) )
4 4 0 0 0 0 K K ( ( R R R R 7 7 ) )
4 4 M M ( ( R R R R 8 8 ) )
4 4 M M ( ( R R R R 8 8 ) )
7-F-7 7-F-7
ge only). nly). The gain The gain is selecte
SIIR, or S1 or S12R a 2R and S1 nd S13R.
Refer to Refer to Table 7-F-I Table 7-F-I to det
The Output FET Control FET Control Amplifier (U203)
The FET is normally is normally biased into
Current Source. A high out A high output impedance
Overvoltage Protection Overvoltage Protection Circuitry.
The following explains the circuitry
large posit positive ive volt voltage
d L201 to the cathode of CR201.
the diode is reverse b is reverse biased. iased. This prevents th
rge nega egativ tive vol e volta tage is ap
Q202 at -.7V (one -.7V (one diode drop) diode drop). . This voltage
Q203, and Q204 to t Q204 to turn on.
The cir- cuit that cuit that generates generates the ohms cu
he large negative input voltage.
AC to DC Converter DC Converter
The purpose of the AC to the AC to DC Converter
put is + 3.00000V dc for all full-
Ranges
F F E E T T S S w w i i t t c c h h e e s s
4 4 V V S S O O R R . . S S 2 2 R R , , S S 3 3 R R , , S S 6 6 R R , , S S 7 7 R R . . S S 1 1 4 4 R R
4 4 V V S S O O R R , , S S 2 2 R R , , S S 3 3 R R . . S S S S R R , , S S 9 9 R R , , S S 9 9 R R , , S S 1 1 4 4 R R
4 4 V V S S O O R R , , S S 2 2 R R , , S S 4 4 R R , , S S 1 1 O O R R . . S S 1 1 1 1 R R
4 4 V V S S O O R R , , S S 2 2 R R , , 1 1 2 2 R R , , S S 1 1 3 3 R R
. . 4 4 V V S S 1 1 R R , , S S 5 5 R R , , S S 1 1 2 2 R R , , S S 1 1 3 3 R R
TM 11-6625-3071-14 TM 11-6625-3071-14
is selected by d by either either
3R. The Range e The Rang
to determine which ermine which
Amplifier (U203) and and
biased into saturation, saturation,
put impedance
Circuitry. The The
the circuitry operation. operation.
age is ap is applied plied to t to the he
thode of CR201. Since Since
This prevents the e
current source.
ge is appli plied to th ed to the e
nd in turn clamps the clamps the
This voltage
urn on. Transistors Transistors
the ohms current rrent
input voltage. Most of Most of
DC Converter is to is to
00000V dc for all full-
E E n n a a b b l l e e d d