Digital Multimeter
Theory of Operation
11
enabled and disabled via control line V5* from microprocessor U2. C15 bypasses the
output of U10.
When U10 is enabled, 5 V is supplied to the voltage reference U8 input. U8 is a 2.5-V
reference with a buffered output and accurately holds the COMMON input 2.5 V above
the -2.5 V power supply or at 0 V. The triangular ground symbol represents this
reference level throughout the Meter schematic. C16, C57, C50 and R109 bypass the
output of U8 keeping the voltage between COMMON and the -2.5 V power supply noise
free and stable. C50 and R109 form a low Q bypass that is directed at electromagnetic
interference (EMI).
Function Selection and Overload Protection
Sections of rotary switch S1 connect the V/Ohms and mA/
µ
A input jacks to the signal
conditioning circuit as required for the respective Meter functions. The Meter schematic
symbols for each of the sections of S1 have the functions that correspond to the closed
positions listed next to them. Table 11 indicates the components that complete normal
operation signal path/paths from input jacks to the required signal conditioning
circuit/circuits for each Meter function.
Table 11. Input Path Components
Function Components
K
VAC
RT1, R1, R2, C1, Z1(9.997 M), C43, R94
V
VDC
RT1, R1, S1(1,3), Z1(9.997 M)
m
L
mV dc
RT1, R1, S1(1,3), Z1(9.997 M), S1(29,5), R85
m
L
- HiZ
mV dc – HiZ
RT1, R1, S1(29,5), R85
d
Temperature
RT1, R1, S1(1,2), R3, S1(29,5), R85, S1(1,3)
Z1(9.997M)
T
Ohms (Below 6 M
Ω
)
RT1, R1, S1(1,2), R3, Z2, S1(4,5), R85
T
Ohms (6 M
Ω
, 50 M
Ω
, nS)
RT1, R1, S1(1,2), R3, S1(1,3), Z1(9.99 7M)
X
Continuity (below 6 M
Ω
)
RT1, R1, S1(1,2), R3, Z2, S1(4,5) R85
X
Continuity (6 M
Ω
, 50 M
Ω
)
RT1, R1, S1(1,2), R3, S1(1,3), Z1(9.997 M)
E
Capacitance
RT1, R1, S1(1,2), R3, Z2, S1(4,5) R85
G
Diode Test
RT1, R1, S1(1,2), R3, Z2, S1(4,5), R85
