DA2002
Theory of Operation
9
Part II: Theory of Operation
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.