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參數(shù)資料
| 型號(hào)： | ADM1024EVB |
| 廠商： | ON Semiconductor |
| 文件頁數(shù)： | 4/30頁 |
| 文件大?。?/td> | 0K |
| 描述： | BOARD EVAL FOR ADM1024 |
| 標(biāo)準(zhǔn)包裝： | 1 |
| 其它名稱： | EVAL-ADM1024EB EVAL-ADM1024EB-ND |
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ADM1024
http://onsemi.com
12
A-to-D Converter
These inputs are multiplexed into the on-chip, successive
approximation, Analog-to-Digital Converter (ADC). This
has a resolution of eight bits. The basic input range is 0 V to
2.5 V, which is the input range of AIN1 and AIN2, but five
of the inputs have built-in attenuators to allow measurement
of 2.5 V, 5.0 V, 12 V, and the processor core voltages VCCP1
and VCCP2 without any external components. To allow for
the tolerance of these supply voltages, the ADC produces an
output of 3/4 full scale (decimal 192) for the nominal input
voltage, and so has adequate headroom to cope with
overvoltages. Table 7 shows the input ranges of the analog
inputs and output codes of the ADC.
When the ADC is running, it samples and converts an input
every 748
ms, except for the external temperature (D1 and
D2) inputs. These have special input signal conditioning and
are averaged over 16 conversions to reduce noise, and a
measurement on one of these inputs takes nominally 9.6 ms.
Input Circuits
The internal structure for the analog inputs is shown in
Figure 13. Each input circuit consists of an input protection
diode, an attenuator, plus a capacitor to form a first-order
low-pass filter that gives the input immunity to high
frequency noise.
Figure 13. Structure of Analog Inputs
10pF
AIN1–AIN2
35pF
25pF
(SEE TEXT)
50pF
MUX
+12VIN
+5.0VIN
+2.5VIN
+VCCP1/VCCP2
97.3k
42.7k
111.2k
36.7k
55.2k
91.6k
22.7k
122.2k
80kW
W
2.5 V Input Precautions
When using the 2.5 V input, the following precautions
should be noted. There is a parasitic diode between Pin 18
and VCC due to the presence of a PMOS current source
(which is used when Pin 18 is configured as a temperature
input). This will become forward biased if Pin 18 is more
than 0.3 V above VCC. Therefore, VCC should never be
powered off with a 2.5 V input connected.
Setting Other Input Ranges
AIN1 and AIN2 can easily be scaled to voltages other than
2.5 V. If the input voltage range is zero to some positive
voltage, all that is required is an input attenuator, as shown
in Figure 14.
Figure 14. Scaling AIN(12)
VIN
R1
R2
AIN(1–2)
Negative and bipolar input ranges can be accommodated
by using a positive reference voltage to offset the input
voltage range so it is always positive.
(eq. 1)
R1
R2 +
Vf
s *
2.5
To measure a negative input voltage, an attenuator can be
used as shown in Figure 15.
Figure 15. Scaling and Offsetting AIN(12)
for Negative Inputs
VIN
R1
AIN(1–2)
R2
+VOS
This is a simple and cheap solution, but the following
point should be noted. Since the input signal is offset but not
inverted, the input range is transposed. An increase in the
magnitude of the 12 V supply (going more negative) will
cause the input voltage to fall and give a lower output code
from the ADC. Conversely, a decrease in the magnitude of
the 12 V supply will cause the ADC code to increase. The
maximum negative voltage corresponds to zero output from
the ADC. This means that the upper and lower limits will be
transposed.
(eq. 2)
R1
R2 +
Vf
s *
VOS
Bipolar input ranges can easily be accommodated. By
making R1 equal to R2 and VOS = 2.5 V, the input range is
2.5 V. Other input ranges can be accommodated by adding
a third resistor to set the positive fullscale input voltage.
Figure 16. Scaling and Offsetting AIN(12)
for Bipolar Inputs
VIN
R1
AIN(1–2)
R2
+VOS
R3
(R3 has no effect as the input voltage at the device pin is zero
when VIN = minus full scale.)
(eq. 3)
R1
R2 +
Vf
s *
R2
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