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| 型號(hào)： | ST72621J2B1 |
| 廠商： | STMICROELECTRONICS |
| 元件分類： | 微控制器/微處理器 |
| 英文描述： | 8-BIT, MROM, 8 MHz, MICROCONTROLLER, PDIP20 |
| 封裝： | 0.300 INCH, PLASTIC, DIP-20 |
| 文件頁(yè)數(shù)： | 127/136頁(yè) |
| 文件大?。?/td> | 2475K |
| 代理商： | ST72621J2B1 |
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10.7 10-BIT A/D CONVERTER (ADC)
10.7.1 Introduction
The on-chip Analog to Digital Converter (ADC) pe-
ripheral is a 10-bit, successive approximation con-
verter with internal sample and hold circuitry. This
peripheral has up to 8 multiplexed analog input
channels (refer to device pin out description) that
allow the peripheral to convert the analog voltage
levels from up to 8 different sources.
The result of the conversion is stored in a 10-bit
Data Register. The A/D converter is controlled
through a Control/Status Register.
10.7.2 Main Features
■ 10-bit conversion
■ Up to 8 channels with multiplexed input
■ Linear successive approximation
■ Data register (DR) which contains the results
■ Conversion complete status flag
■ Continuous or One-Shot mode
■ On/off bit (to reduce consumption)
The block diagram is shown in Figure 52.
10.7.3 Functional Description
10.7.3.1 Analog Power Supply
Depending on the MCU pin count, the package
may feature separate VDDA and VSSA analog pow-
er supply pins. These pins supply power to the A/D
converter cell and function as the high and low ref-
erence voltages for the conversion. In smaller
packages VDDA and VSSA pins are not available
and the analog supply and reference pads are in-
ternally bonded to the VDD and VSS pins.
Separation of the digital and analog power pins al-
low board designers to improve A/D performance.
Conversion accuracy can be impacted by voltage
drops and noise in the event of heavily loaded or
badly decoupled power supply lines.
10.7.3.2 PCB Design Guidelines
To obtain best results, some general design and
layout rules should be followed when designing
the application PCB to shield the the noise-sensi-
tive, analog physical interface from noise-generat-
ing CMOS logic signals.
– Use separate digital and analog planes. The an-
alog ground plane should be connected to the
digital ground plane via a single point on the
PCB. The analog power plane should be con-
nected to the digital power plane via an RC net-
work.
– Filter power to the analog power planes. The
best solution is to connect a 0.1F capacitor, with
good high frequency characteristics, between
VDDA and VSSA and place it as close as possible
to the VDDA and VSSA pins and connect the ana-
log and digital power supplies in a star network.
Do not use a resistor, as VDDA is used as a refer-
ence voltage by the A/D converter and resist-
ance would cause a voltage drop and a loss of
accuracy.
– Properly place components and route the signal
traces on the PCB to shield the analog inputs.
Analog signals paths should run over the analog
ground plane and be as short as possible. Isolate
analog signal from digital signals that may switch
while the analog inputs are being sampled by the
A/D converter. Do not toggle digital outputs on
the same I/O port as the A/D input being convert-
ed.
10.7.3.3 Digital A/D Conversion Result
The conversion is monotonic, meaning that the re-
sult never decreases if the analog input does not
and never increases if the analog input does not.
If the input voltage (VAIN) is greater than VDDA
(high-level voltage reference) then the conversion
result is FFh in the ADCDRMSB register and 03h
in the ADCDRLSB register (without overflow indi-
cation).
If the input voltage (VAIN) is lower than VSSA (low-
level voltage reference) then the conversion result
in the ADCDRMSB and ADCDRLSB registers is
00 00h.
The A/D converter is linear and the digital result of
the conversion is stored in the ADCDRMSB and
ADCDRLSB registers. The accuracy of the con-
version is described in the Electrical Characteris-
tics Section.
RAIN is the maximum recommended impedance
for an analog input signal. If the impedance is too
high, this will result in a loss of accuracy due to
leakage and sampling not being completed in the
alloted time.
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