參數(shù)資料
型號: EVAL-ADUC831QSZ
廠商: Analog Devices Inc
文件頁數(shù): 27/76頁
文件大?。?/td> 0K
描述: KIT DEV FOR ADUC831 QUICK START
產品培訓模塊: Process Control
標準包裝: 1
系列: QuickStart™ 套件
類型: MCU
適用于相關產品: ADuC831
所含物品: 評估板、電源、纜線、軟件和說明文檔
產品目錄頁面: 739 (CN2011-ZH PDF)
相關產品: ADUC831BCPZ-ND - IC MCU 62K FLASH ADC/DAC 56LFCSP
ADUC831BCPZ-REEL-ND - IC MCU 62K FLASH ADC/DAC 56LFCSP
ADUC831BSZ-REEL-ND - IC MCU 62K FLASH ADC/DAC 52MQFP
ADUC831BSZ-ND - IC ADC/DAC 12BIT W/MCU 52-MQFP
其它名稱: EVAL-ADUC831QS
EVAL-ADUC831QS-ND
REV. 0
ADuC831
–33–
Using the DAC
The on-chip DAC architecture consists of a resistor string DAC
followed by an output buffer amplifier, the functional equivalent
of which is illustrated in Figure 21. Details of the actual DAC
architecture can be found in U.S. Patent Number 5969657
(www.uspto.gov). Features of this architecture include inherent
guaranteed monotonicity and excellent differential linearity.
OUTPUT
BUFFER
HIGH Z
DISABLE
(FROM MCU)
DAC0
R
ADuC831
AVDD
VREF
Figure 21. Resistor String DAC Functional Equivalent
As illustrated in Figure 21, the reference source for each DAC is
user selectable in software. It can be either AVDD or VREF. In
0-to-AVDD mode, the DAC output transfer function spans from
0 V to the voltage at the AVDD pin. In 0-to-VREF mode, the
DAC output transfer function spans from 0 V to the internal
VREF, or if an external reference is applied, the voltage at the
VREF pin. The DAC output buffer amplifier features a true rail-
to-rail output stage implementation. This means that, unloaded,
each output is capable of swinging to within less than 100 mV of
both AVDD and ground. Moreover, the DAC’s linearity specifi-
cation (when driving a 10 k
resistive load to ground) is
guaranteed through the full transfer function except codes 0 to
100, and, in 0-to-AVDD mode only, codes 3945 to 4095. Linear-
ity degradation near ground and VDD is caused by saturation of
the output amplifier, and a general representation of its effects
(neglecting offset and gain error) is illustrated in Figure 22. The
dotted line in Figure 22 indicates the ideal transfer function,
and the solid line represents what the transfer function might
look like with endpoint nonlinearities due to saturation of the
output amplifier. Note that Figure 22 represents a transfer
function in 0-to-VDD mode only. In 0-to-VREF mode (with VREF
< VDD) the lower nonlinearity would be similar, but the upper
portion of the transfer function would follow the “ideal” line
right to the end (VREF in this case, not VDD), showing no signs
of endpoint linearity errors.
VDD
VDD –50mV
VDD –100mV
100mV
50mV
0mV
000H
FFFH
Figure 22. Endpoint Nonlinearities Due to Amplifier
Saturation
The end point nonlinearities conceptually illustrated in Figure
22 get worse as a function of output loading. Most of the
ADuC831’s data sheet specifications assume a 10 k
resistive
load to ground at the DAC output. As the output is forced to
source or sink more current, the nonlinear regions at the top or
bottom (respectively) of Figure 22 become larger. With larger
current demands, this can significantly limit output voltage
swing. Figure 23 and Figure 24 illustrate this behavior. It
should be noted that the upper trace in each of these figures is
only valid for an output range selection of 0-to-AVDD. In 0-to-
VREF mode, DAC loading will not cause highside voltage drops
as long as the reference voltage remains below the upper trace in
the corresponding figure. For example, if AVDD = 3 V and VREF
= 2.5 V, the highside voltage will not be affected by loads less
than 5 mA. But somewhere around 7 mA the upper curve in
Figure 24 drops below 2.5 V (VREF) indicating that at these
higher currents the output will not be capable of reaching VREF.
SOURCE/SINK CURRENT – mA
5
05
10
15
OUTPUT
VOLTAGE
V
4
3
2
1
0
DAC LOADED WITH 0000H
DAC LOADED WITH 0FFFH
Figure 23. Source and Sink Current Capability with
VREF = VDD = 5 V
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