Theory of Operation
6-7
Figure 6-12
Multiplexer and buffer
Figure 6-13
Multiplexer phases
6.4.3 -2V reference source
Model 6512 measurements are based on comparing the un-
known signal with an internal -2V reference voltage source.
During each measurement cycle, the unknown signal is sam-
pled and then compared with signal common and the -2V
reference values.
Figure 6-14 shows a simplified diagram of the -2V reference
source. VR102 provides a highly stable -6.3V reference,
while U139 and elements of R153 provide a constant current
to minimize zener voltage variations. R146A and R146B di-
vide down the -6.3V value to the final -2V reference voltage.
The output of U139 (-7V) is used as bias for the A/D convert-
er, and as a negative supply for various other components.
Figure 6-14
-2V reference source
6.5
A/D converter
The Model 6512 uses an A/D converter that utilizes both
constant-frequency charge balance and single-slope tech-
niques. This combination gives the instrument both high ac-
curacy and relatively fast conversion times.
A simplified schematic of the A/D converter is shown in Fig-
ure 6-15, along with an associated integrator waveform.
The charge balance phase begins when the input ENABLE/
DISABLE line is set high. This action occurs at the end of a
firmware-generated delay period that allows the signal to set-
tle following signal selection. Once the input is enabled, the
signal from the buffer amplifier is added to the level-shift
current applied through R153H. In this manner, the ±2V bi-
polar signal from the buffer amplifier is converted to a uni-
polar signal that can be integrated.
CONTROL MULTIPLEXER
U138A
TO A/D
CONVERTER
S
U145
SIGNAL
COMMON
-2V
REFERENCE
BUFFER
REFERENCE
PHASE
CALCULATE
A READING
SIGNAL
PHASE
CALCULATE
A READING
COMMON
PHASE
SIGNAL
PHASE
U139
-7V TO A/D
CONVERTER
R153B
R153C
VR102
R146B
R146A
R153A
-2V REFERENCE
6.3V
Summary of Contents for 6512
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