Fluke 5500A, Service Manual

Page: 51 / 296

Theory of Operation
DDS Assembly (A6)
2
2-5
+
_
DAC
SCOM
C
ref
K
C
x
= (1 + K) • C
ref
NORMAL
HI
NORMAL
LO
C
x
=
-1
om005f.eps
Figure 2-3. Synthesized Capacitance Function
2-4. DDS Assembly (A6)
The DDS (Direct Digital Synthesis) assembly (A6) contains the following blocks:
•
References for all voltage and current functions.
•
Gain determining elements for voltage functions and thermocouple measuring and
sourcing.
• ±
7 V references.
•
Thermocouple sourcing and measuring amplifier.
•
An A/D (Analog-to-Digital) measurement system for monitoring all functions.
•
Self calibration circuitry.
•
Zero calibration circuitry.
•
Precision voltage channel DAC (VDAC).
•
Precision current channel DAC (IDAC).
•
Dual-channel DDS (Direct Digital Synthesizer).
•
Inguard CPU that controls relays and latches throughout the analog assemblies.
These functional blocks, when used with the Voltage (A8) and/or Current (A7)
assemblies, provide single or dual channel ac and dc volts, amps, and watts, offsettable
and nonsinusoidal waveforms, duty cycle, thermocouple measuring and sourcing,
internal calibration and diagnostics, and digital control over all the analog assemblies.
DACS are used to control the level of dc signals and to control the amplitude of ac
signals.