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參數(shù)資料
| 型號： | ADC7802BP |
| 英文描述： | Autocalibrating, 4-Channel, 12-Bit ANALOG-TO-DIGITAL CONVERTER |
| 中文描述： | Autocalibrating，4通道，12位模擬數(shù)字轉(zhuǎn)換器 |
| 文件頁數(shù)： | 11/13頁 |
| 文件大小： | 102K |
| 代理商： | ADC7802BP |
11
ADC7802
INPUT PROTECTION
The input signal range must not exceed
±
V
REF
or V
A
by more
than 0.3V.
The analog inputs are internally clamped to V
. To prevent
damage to the ADC7802, the current that can flow into the
inputs must be limited to 20mA. One approach is to use an
external resistor in series with the input filter resistor. For
example, a 1k
input resistor allows an overvoltage to 20V
without damage.
REFERENCE INPUTS
A 10
μ
F tantalum capacitor is recommended between V
+
and V
– to insure low source impedance. These capacitors
should be located as close as possible to the ADC7802 to
reduce dynamic errors, since the reference provides packets
of current as the successive approximation steps are carried
out.
V
+ must not exceed V
. Although the accuracy is speci-
fied with V
+ = 5V and V
– = 0V, the converter can
function with V
+ as low as 2.5V and V
– as high as 1V.
As long as there is at least a 2.5V difference between V
+
and V
–, the absolute value of errors does not change
significantly, so that accuracy will typically be within
±
1LSB. (1/2LSB for a 5V span is 610
μ
V, which is 1LSB for
a 2.5V span.)
The power supply to the reference source needs to be consid-
ered during system design to prevent V
+ from exceeding
(or overshooting) V
, particularly at power-on. Also, after
power-on, if the reference is not stable within 42,425 clock
cycles, an additional calibration cycle may be needed.
POWER SUPPLIES
The digital and analog power supply lines to the ADC7802
should be bypassed with 10
μ
F tantalum capacitors as close
to the part as possible. Although ADC7802 has excellent
power supply rejection, even for higher frequencies, linear
regulated power supplies are recommended.
Care should be taken to insure that V
does not come up
before V
, or permanent damage to the part may occur.
Figure 10 shows a good supply approach, powering both V
A
and V
from a clean linear supply, with the 10
resistor
between V
and V
insuring that V
comes up after V
. This
is also a good method to further isolate the ADC7802 from
digital supplies in a system with significant switching cur-
rents that could degrade the accuracy of conversions.
GROUNDING
digital grounds are not connected internally. These points
should have very low impedance to avoid digital noise
feeding back into the analog ground. The V
– pin is used as
the reference point for input signals, so it should be con-
nected directly to AGND to reduce potential noise problems.
EXTERNAL CLOCK OPERATION
The circuitry required to drive the ADC7802 clock from an
external source is shown in Figure 11a. The external clock
must provide a 0.8V max for LOW and a 3.5V min for
HIGH, with rise and fall times that do not exceed 200ns. The
minimum pulse width of the external clock must be 200ns.
Synchronizing the conversion clock to an external system
clock is recommended in microprocessor applications to
prevent beat-frequency problems.
Note that the electrical specification tables are based on
using an external 2MHz clock. Typically, the specified
accuracy is maintained for clock frequencies between 0.5
and 2.2MHz.
INTERNAL CLOCK OPERATION
Figure 11b shows how to use the internal clock generating
circuitry. The clock frequency depends only on the value of
the resistor, as shown in “Internal Clock Frequency vs
R
CLOCK
” in the Typical Performance Curves section.
SFR
AIN0
AIN1
AIN2
AIN3
V +
V –
DGND
V
D7
D6
D5
D4
D3
V
AGND
CAL
A1
A0
CLK
BUSY
HBE
WR
CS
RD
D0
D1
D2
1
2
3
4
5
6
7
8
9
10
11
12
13
14
28
27
26
25
24
23
22
21
20
19
18
17
16
15
D
A
+
10nF
10μF
+
10μF
10nF
+5V
10
5V
REF
+
10nF
10μF
FIGURE 10. Power Supply and Reference Decoupling.
FIGURE 11. Internal Clock Operation.
To ADC7802
Pin 23
CLK
74HC-Compatible
Clock Source
To ADC7802
Pin 23
+5V
R
f
CLOCK
(in Hz) = 10 /R
(a) External Clock Operation
(b) Internal Clock Operation
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