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ADM1024ARU View Datasheet(PDF) - Analog Devices

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ADM1024ARU Datasheet PDF : 32 Pages
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ADM1024
ANALOG-TO-DIGITAL 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 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 III
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 µs, 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 3.
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.
AIN1–AIN2
80k
+12VIN
+5VIN
+2.5VIN
(SEE TEXT)
+VCCP1/
VCCP2
122.2k
22.7k
91.6k
55.2k
36.7k
111.2k
42.7k
97.3k
10pF
35pF
25pF MUX
25pF
50pF
Figure 3. Structure of Analog Inputs
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 4.
R1 AIN(1–2)
VIN
R2
Figure 4. Scaling AIN(1–2)
R1 = (VFS – 2.5)
R2
2.5
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.
To measure a negative input voltage, an attenuator can be used
as shown in Figure 5.
+VOS
R2
R1
VIN
AIN(1–2)
Figure 5. Scaling and Offsetting AIN(1–2) for
Negative Inputs
R1 = |VFS |
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.
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 full-scale input voltage.
+VOS
R2
R1
AIN(1–2)
VIN
R3
Figure 6. Scaling and Offsetting AIN(1–2) for
Bipolar Inputs
R1 = |VFS |
R2 R2
(R3 has no effect as the input voltage at the device pin is zero
when VIN = minus full scale.)
( ) R1 = VFS+ – 2.5
R3
2.5
–12–
REV. B
 

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