ADC7802BN Burr-Brown Corporation, ADC7802BN Datasheet - Page 11

• Manufacturer Part Number: ADC7802BN
• Description: Autocalibrating, 4-channel, 12-bit Analog-to-digital Converter
• Manufacturer: Burr-Brown Corporation

INPUT PROTECTION
The input signal range must not exceed V
than 0.3V.
The analog inputs are internally clamped to V
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 F tantalum capacitor is recommended between V
and V
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
fied with V
function with V
As long as there is at least a 2.5V difference between V
and V
significantly, so that accuracy will typically be within
a 2.5V span.)
The power supply to the reference source needs to be consid-
ered during system design to prevent V
(or overshooting) V
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 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
before V
Figure 10 shows a good supply approach, powering both V
and V
between V
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
To maximize accuracy of the ADC7802, the analog and
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
the reference point for input signals, so it should be con-
nected directly to AGND to reduce potential noise problems.
1LSB. (1/2LSB for a 5V span is 610 V, which is 1LSB for
REF
+ must not exceed V
REF
D
REF
from a clean linear supply, with the 10
– to insure low source impedance. These capacitors
A
–, the absolute value of errors does not change
, or permanent damage to the part may occur.
A
REF
and V
+ = 5V and V
REF
D
+ as low as 2.5V and V
insuring that V
A
, particularly at power-on. Also, after
A
. Although the accuracy is speci-
REF
– = 0V, the converter can
D
comes up after V
D
REF
does not come up
REF
+ from exceeding
REF
REF
– as high as 1V.
– pin is used as
or V
A
. To prevent
A
by more
resistor
A
. This
REF
REF
+
+
A
11
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
FIGURE 10. Power Supply and Reference Decoupling.
FIGURE 11. Internal Clock Operation.
CLOCK
10µF
10µF
+5V
REF
” in the Typical Performance Curves section.
5V
+
+
74HC-Compatible
Clock Source
10nF
10nF
R
(a) External Clock Operation
(b) Internal Clock Operation
10
11
12
13
14
1
2
3
4
5
6
7
8
9
To ADC7802
Pin 23
SFR
AIN0
AIN1
AIN2
AIN3
V
V
DGND
V
D7
D6
D5
D4
D3
REF
REF
D
+
–
CLK
10
ADC7802
AGND
BUSY
HBE
CAL
CLK
WR
CS
RD
D0
D1
D2
A1
A0
V
A
To ADC7802
Pin 23
f
CLOCK
28
27
26
25
24
23
22
21
20
19
18
17
16
15
10nF
(in Hz) = 10 /R
11
10µF
+
+5V
®