參數(shù)資料
型號： XE8000EV104
廠商： Semtech
文件頁數(shù)： 31/156頁
文件大?。?/td> 0K
描述： EVAL BOARD FOR XE8805AMI028LF
標準包裝： 1
類型： MCU
適用于相關產品： XE88LC05AMI028
所含物品： 完全組裝的評估板
Semtech 2006
www.semtech.com
18-5
XE8805/05A
increase the output signal bandwidth or to reduce the PWM switching frequency and therefore the power
consumption of the D/A. Signal distortion is less dependent on the signal value.
Disadvantages:
Using a high order noise shaper together with a PWM modulator with low resolution will use lots of short pulses in
stead of 1 long pulse. The D/A is therefore more sensitive to rise and fall times of the PWM resulting in a slightly
higher non-linearity and temperature dependence. The second order noise shaper also has a reduced resolution
for codes very close to zero or full scale.
18.4.3
D/A setup and resolution
In this section, the resolution that can be obtained with the D/A as a function of settings is calculated. These
calculations are based on the quantization and PWM modulation noise. Noise on the reference, i.e. the supply
voltage is not taken into account. High frequency noise on the supply voltage can be filtered by the output low pass
filter, but in band noise on the reference will show up in the output signal with amplitude that will depend on the
signal value. Therefore, when using the D/A, one should take care to minimize the switching activity on the digital
ports and/or to limit the load on these ports.
18.4.3.1
Noise shaper of order 0
Setting the noise shaper to order 0 (NsOrder=00), reduces the D/A to a regular PWM. Two parameters are setting
the resolution of the D/A: the resolution of the modulator itself and the amount of low pass filtering at the output.
The modulation width
m of the PWM modulator is given by:
CodeLmax
+
= 4
m
The cut-off frequency
c
f
of the low pass filter required to get the resolution is calculated below. The PWM
modulator repetition frequency
PWM
f
can be calculated as a function of the selected modulation width
m , the
frequency of the RC oscillator of the circuit
RC
f
and the selected frequency division set by Fin :
m
RC
PWM
f
2
1
+
=
Fin
To obtain an analog signal with the required solution, the PWM signal has to be low pass filtered. The resolution
that can be obtained depends on the filter order and the ratio between the PWM modulation frequency
PWM
f
and
the filter cut-off frequency
c
f . For a low pass filter of LpOrder, we obtain:
=
c
PWM
f
LpOrder
resolution
2
log
The total resolution of the D/A is then the minimal value of both criteria:
)
,
min(
PWM
resolution
m
resolution
=
In Table 18-5 the required cut-off frequency of the low pass filter is shown for a noise shaper of order 0 as a
function of the desired resolution for both a first and second order low pass filter. The PWM modulation factor m
should be chosen equal to the desired resolution.
Not
Recommended
for
New
Designs
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