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楼主 |
发表于 2018-2-3 16:02
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PCM63P的相关功能的说明更详细,可惜我不懂英文,只能根据百度翻译瞎猜
MSB ADJUSTMENT CIRCUITRY
Near optimum performance can be maintained at all signal
levels without using the optional MSB adjust circuitry of the
PCM63P shown in Figure 5. Adjustability is provided for
those cases where slightly better full-scale THD+N is
desired. Use of the MSB adjustments will only affect larger
dynamic signals (between 0dB and –6dB). This improvement
comes from bettering the gain match between the
upper and lower DACs at these signal levels. The change is
realized by small adjustments in the bit-2 weights of each
DAC. Great care should be taken, however, as improper
adjustment will easily result in degraded performance.
In theory, the adjustments would seem very simple to
perform, but in practice they are actually quite complex. The
first step in the theoretical procedure would involve making
each bit-2 weight ideal in relation to its code minus one
value (adjusting each potentiometer for zero differential
nonlinearity error at the bit-2 major carries). This would be
the starting point of each 100kW potentiometer for the next
adjustment. Then, each potentiometer would be adjusted
equally, in opposite directions, to achieve the lowest fullscale
THD+N possible (reversing the direction of rotation for both if no immediate improvement were noted). This
procedure would require the generation of the digital bit-2
major carry code to the input of the PCM63P and a DVM or
oscilloscope capable of reading the output voltage for a one
LSB step (5.72mV) in addition to a distortion analyzer.
A more practical approach would be to forego the minor
correction for the bit-2 major carry adjustment and only
adjust for upper and lower DAC gain matching. The problem
is that just by connecting the MSB circuitry to the
PCM63P, the odds are that the upper and lower bit-2 weights
would be greatly changed from their unadjusted states and
thereby adversely affect the desired gain adjustment. Just
centering the 100kW potentiometers would not necessarily
provide the correct starting point. To guarantee that each
100kW potentiometer would be set to the correct starting or
null point (no current into or out of the MSB adjust pins), the
voltage drop across each corresponding 330kW resistor would
have to measure 0V. A voltage drop of ±1.25mV across
either 330kW resistor would correspond to a ±1LSB change
in the null point from its unadjusted state (1LSB in current
or 3.81nA x 330kW = 1.26mV). Once these starting points
for each potentiometer had been set, each potentiometer
would then be adjusted equally, in opposite directions, to
achieve the lowest full-scale THD+N possible. If no immediate
improvement were noted, the direction of rotation for
both potentiometers would be reversed. One direction of
potentiometer counter-rotations would only make the gain
mismatch and resulting THD+N worse, while the opposite
would gradually improve and then worsen the THD+N after
passing through a no mismatch point. The determination of
the correct starting direction would be arbitrary. This procedure
still requires a good DVM in addition to a distortion
analyzer.
Each user will have to determine if a small improvement in
full-scale THD+N for their application is worth the expense
of performing a proper MSB adjustment. |
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