Fluke 76 - Ohms Functions

Theory of Operation
Circuit Descriptions 2
2-5
4V
3
AC
CONVERTER
(U2)
R19
11
ACTIVE
FILTER
A/D
COM
V/
W
R2
C1
76
3
2
S1
S1
DC mV
V
1
2
Z1
9.996M
7
Z1
9.996M
12
S1
HIGH
LOW
DC
AC
R1, R8,
R12, RTI
l2f.eps
Figure 2-2. 4V Range Simplified Schematic
Ohms Functions 2-8.
When the 400 Ohm range is selected, internal switches connect the resistor Z1-2 (9.996
MΩ) to resistor Z1-6 (1.0001 kΩ). (See Figure 2-3, 400 Ohm Range Simplified
Schematic.) Then through switch contacts S1 6, 7, and 9, these resistors form a reference
resistor of 1 kΩ.
The source voltage is connected internally at both V0 and V4 of U4. The current is routed
through two parallel resistor’s Z1-6 and Z1-2 (1.0001 kΩ and 9.996 MΩ, into S1 at
contacts 6 and 9. The signal then travels out of S1 at contact 7, through R1, R8, R12 and
RT1 and to the VΩ input. The signal then goes through the unknown resistance, and
back to the COM input. The same current flows through the unknown resistance and the
reference resistor. The voltage dropped across the unknown resistance is sensed from the
VΩ input jack through R2 and S1 (contacts 2 and 3) to OVS of U4.
The a/d converter senses the voltage drop across the 1 kΩ reference resistor through the
low (RRS of U4 through R13) and high (V0 and V4) points. These two voltages are used
by the a/d converter to perform a ratiometric measurement. Since the same current flows
through the reference and unknown resistors, the ratio of the resistance values is the same
as the ratio of the voltage drops across them.
For the 4 kΩ range, the 10.010 kΩ resistor (Z1-5) used in parallel with the 9.996 MΩ
resistor (Z1-2) forms a 10 kΩ reference resistor. For the 40 kΩ range, 101.01 kΩ (Z1-4)
and 9.996 MΩ form a 100 kΩ reference resistor. And for the 400 kΩ range, 1.1111 MΩ
(Z1-3) and 9.996 MΩ provide a 1 MΩ reference resistor. The 4 MΩ and 40 MΩ ranges
use the 9.996 MΩ resistor alone.