Operations Manual DA924
9
P a r t I I :
T h e o r y of o p e r a t i o n
The simplified block diagram (figure 1), shows the basic building blocks.
Oven control
The PCM DAC is constructed of custom made laser trimmed thin film resistor networks;
yet any resistor is subject to short term drift due to temperature variations and long term
drift due to component aging. The resistor networks are kept at a constant temperature by a
linearly controlled heating element. (A bang-bang controller, such as a home thermostat is
undesirable because it produces turn on and turn off surges-- thus audible kicks.) Keeping
the resistors at constant temperature overcomes resistor dependency on environmental
temperature variations.
Figure 1
Calibration
A sequence where each resistor is tapped (one at a time) for voltage comparison against a
reference level tells the processor the required adjustments. The voltage difference between
any given network node and the reference is greatly amplified and then fed to a strobing
comparator (see multiplexers for calibration and error amplifier and comparator gain blocks
in the simplified diagram). The processor strobes the comparator and reads its output. The
strobing is repeated 4000 times for the sake of averaging out any error due to amplifier
noise. At the end of a comparator strobing cycle, the processor decides whether to increase
or decrease the specific voltage of the measured node. This is done via the calibration 14 bit
DACs (see diagram). Each calibration DAC is used as a 13 bit device to ensure monotonic
performance. Each DAC is fed to its corresponding node through a large value resistor,
thus a full 10V swing on the calibration DAC can only pull a given node by +/-4mV,
providing an effective adjustment of a part in 5 million per calibration DAC step.
The calibration process takes a long time because the node adjustment is interactive
(adjusting a node causes some misadjustment at all the other nodes). A single calibration
