Fluke 863, Service Manual

Page: 37 / 101

Theory of Operation
Input Signal Conditioning
2
2-17
Ohms 2-29.
Ohms measurements are connected at the
Ë
and
â
inputs. A ratiometric
measurement method is used: the same current is sourced through the unknown resistor
and a reference resistor. The Slow A/D Converter first integrates the voltage across the
unknown resistor, then deintegrates using the voltage across the reference resistor.
The source voltage of either 1.3V or 3V from U30 is applied to one of the range resistors
in Z5 (1.00 M
Ω
, 101.01 k
Ω
, 10.01 k
Ω
, or 1.00 k
Ω
) and the 10-M
Ω
resistor of Z5.
Current flows through the appropriate range resistor to S1-7 and S1-8, then through the
protection resistors RT1 and R103 to
Ë
. For the 10-M
Ω
resistor, the current path
is from U30-19 through the 10-M
Ω
resistor to
Ë
.
The 30 M
Ω
range uses the same range resistor as the 3 M
Ω
range.
The 300 nS and 3000 nS ranges use the same range resistors as the 3 M
Ω
and 300 k
Ω
ranges; however, the Slow A/D Converter first integrates with the voltage across the
reference resistor, then deintegrates with the voltage across the unknown.
Table 2-8. Ohms Ratiometric Measurements
Range
Source V from U30 To 10 M Ω Ω Ω Ω (U30-19) and Range
Resistor
Slow A/D Converter
Range
300 Ω 3V 1.00 k Ω (U30-12) 300 mV
3 k
Ω
1.3V 10.01 k
Ω
(U30-13) 300 mV
30 k
Ω
1.3V 101.01 k
Ω
(U30-17) 300 mV
300 k Ω 1.3V 1.00 M Ω (U30-14) 300 mV
3 M
Ω
1.3V (10 M
Ω
only) 300 mV
30 M Ω 3V (10 M Ω only) 3V
300 nS 3V (10 M
Ω
only) 3V
3000 nS 3V 1.00 M
Ω
(U30-14) 3V
Diode Test 2-30.
Diode Tests are connected at the
Ë
and
â
inputs. Auto Diode and Component
Test use a similar measurement method. An AC current (at 3V) is sourced from U1
through U30-12. The current goes through the 1.00-k
Ω
resistor of Z5, through S1-7 and
S1-8, through RT1, and R103 and out
Ë
. The voltage is sensed back through the
component test path an and into the Fast A/D Converter. The voltage is sensed through
R99 and R20, S1-5 and S1-6, and U30-21. U30 routes the signal to the Slow A/D
Converter (3V range used.) Software reads the Fast A/D Converter (U10), decides the
polarity of the diode, and displays the forward voltage.
Manual diode applies the 3V-ohms source voltage from U30 across a 1-k
Ω
resistor of Z5
and out
Ë
. Manual diode then uses the Slow A/D Converter to measure the voltage
at
Ë
.
Capacitance 2-31.
Capacitance and ohms measurements use a common circuit configuration. As the
capacitor is being charged, the Slow A/D Converter integrates the voltage across the
known resistor. When the integrating cycle stops, the charging stops. The Slow A/D
Converter deintegration uses the voltage across the known resistor.