MC145076D 查看數據表（PDF） - Motorola => Freescale
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MC145076D Datasheet PDF : 8 Pages
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PIN DESCRIPTIONS
Xin, Xout
Oscillator Inverter Input and Output (Pins 2, 16)
If an external clock is used to drive the MC145076, the
clock should be connected to Xin pin. For maximum perfor-
mance however, it is recommended that these pins be used
in conjunction as a crystal oscillator.
BIAS
Bias Adjust (Pin 3)
For normal device operation, this pin should be connected
to ground through a 4.7 kΩ resistor, which provides nominal
quiescent output current of 2 mA each channel. In addition to
the 4.7 kΩ resistor, a 47 µF capacitor may be connected from
this pin to the VDD supply.
DIV2
Active–High Clock Divider Control Input (Pin 4)
When this pin is at a logic low level, the internal clock will
be equal to the oscillator, (Xin) frequency, and data can be
clocked into the device at an fXin/2 rate. When this pin is at a
logic high level, the internal clock is one–half the Xin oscilla-
tor frequency, and data can be clocked into the device at an
fXin/4 rate.
DIL, DIR
Left/Right Channel Data Inputs (Pins 5,6)
These pins are the left and right digital input data pins from
the single bit–stream sigma–delta DAC. Serial input data to
the MC145076 is clocked in near the rising edge of CLKOUT.
TEST
Active–High Factory Test Mode Input (Pin 7)
This pin is reserved for factory testing, and should be con-
nected to device ground for normal device operation.
CLKOUT
Buffered Divided Clock Output (Pin 9)
This pin provides a buffered clock output to be used as the
clock source for a sigma–delta bit stream generator. The
CLKOUT frequency is one–half the Xin frequency if DIV2 = 0,
and one–fourth the Xin frequency if DIV2 = 1. The serial input
data is clocked in near the rising edge of CLKOUT.
IOR, IOL
Left/Right Channel Current Outputs (Pins 11,14)
These pins are the current sink outputs of the smoothed
single–bit input data.
VDDD, VDDA
Device Supply Pins (Pins 1,8)
These two pins are the positive power supply pins for the
MC145076, nominally 5 V. For proper device operation, it is
recommended that 0.1 µF and 10 µF capacitors be con-
nected from these pins to ground via the shortest possible
path.
GND
Device Ground Pins (Pins 10,12,13,15)
These pins are the ground pins for the device.
FUNCTIONAL DESCRIPTION
Serial bitstream Digital–to–Analog Converters (DACs)
have become commonplace due to their ability to use over–
sampling techniques to shape quantization noise. This noise
shaping ability enables devices to be built that do not require
the component matching of conventional architectures.
The MC145076 bitstream FIR smoothing filter consists of
two shift registers, two sets of Hamming Window weighted
current source summing networks, and a crystal oscillator in-
verting buffer.
The current source summing networks are used to imple-
ment a Hamming Window function within the MC145076.
Each current source tap sinks a constant current that does
not change with the number of bits that are set in the shift
register. Therefore, each tap acts as a separate single–bit
converter with excellent linearity characteristics. The Ham-
ming window was chosen for the FIR filter coefficients be-
cause this allows a slightly better second lobe attenuation
close to the band where the sampling images are the most
troublesome. For a 256 OSR, the MC145076 FIR filter pro-
vides greater than 40 dB of stop band attenuation, with
approximately 50 dB of attenuation at the 8x image frequen-
cies. This results in an output with full scale images of less
than – 70 dB and out–of–band noise better than – 60 dB. For
other OSR rates, the filter response scales linearly.
CRYSTAL OSCILLATOR
Provisions for an on–chip crystal oscillator are provided to
insure that the clock will be as clean as possible internal to
the MC145076 where the digital–to–analog conversion oc-
curs, thus assuring maximum performance. An output clock
buffer is provided for driving additional off–chip digital circuit-
ry such as a digital noise shaper, over–sampling FIR filter, or
DSP. The off–chip digital processing ensures that the digital
switching noise on chip is kept to a minimum.
APPLICATIONS
A smoothing filter is required when using a sigma–delta
DAC to reduce the out–of–band noise, and to prevent the
high frequencies from intermodulating to lower frequencies.
Using the MC145076 with its current sink output is easier
than a voltage output filter because it gives a degree of im-
munity to mutual ground paths between it and the next ampli-
fier.
The circuit shown in Figure 2 is excellent for most applica-
tions. However, differential operation does reduce low level
switching noise that appears as second harmonic distortion
and weak background noise. Although a simple resistor on
each current source output to VDDA may be adequate for
some applications, the OpAmps provide power supply noise
rejection, and, in Figures 2 and 4, also reduce the signal
swing on the current output pin of the part to further improve
distortion.
MC145076
4
MOTOROLA
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