5.0 LAYOUT AND GROUNDING
Functional Description (Continued)
Analog inputs will provide best accuracy when referred to the
A typical application, such as is shown in Figure 6, might se-
lect the input voltage divider to provide 1.9V at the analog in-
puts of the LM80. This is sufficiently high for good resolution
of the voltage, yet leaves headroom for upward excursions
from the supply of about 25%. To simplify the process of re-
sistor selection, set the value of R2 first. Select a value for
R2 or R4 between 10 kΩ and 100 kΩ This is low enough to
avoid errors due to input leakage currents yet high enough to
both protect the inputs under overdrive conditions as well as
minimize loading of the source. Then select R1 or R3 to pro-
vide a 1.9V input as show in Figure 6.
AGND pin or
a supply with low noise. A separate,
low-impedance ground plane for analog ground, which pro-
vides a ground point for the voltage dividers and analog
components, will provide best performance but is not man-
datory. Analog components such as voltage dividers should
be located physically as close as possible to the LM80.
The power supply bypass, the parallel combination of 10 µF
(electrolytic or tantalum) and 0.1 µF (ceramic) bypass ca-
pacitors connected between pin 9 and ground, should also
be located as close as possible to the LM80.
For positive input voltages the equation for calculating R1 is
as follows:
=
R1 [ (VS − VIN) / VIN] R2
For negative input voltages the equation for Calculating R3
is as follows:
R3 = [ (VS − VIN) / (VIN − 5V)] R4
The analog inputs have internal diodes that clamp inputs ex-
ceeding the power supply and ground. Exceeding any ana-
log input has no detrimental effect on other channels. The in-
put diodes will also clamp voltages appearing at the inputs of
an un-powered LM80. External resistors should be included
to limit input currents to the values given in the ABSOLUTE
MAXIMUM RATINGS for Input Current At Any Pin. Inputs
with the attenuator networks will usually meet these require-
ments. If it is possible for inputs without attenuators to be
turned on while LM80 is powered off, additional resistors of
about 10 kΩ should be added in series with the inputs to limit
the input current.
Voltage
Measure-
ments
(VS)
R1 or
R3
R2 or
R4
Voltage
at
Analog Inputs
( ADC code 190)
+1.9V
+2.5V
+3.3V
+5.0V
+12V
23.7 kΩ
22.1 kΩ
24 kΩ
75 kΩ
30 kΩ
+1.9V
14.7 kΩ
30.1 kΩ
35.7 kΩ
16.2 kΩ
+1.9V
160 kΩ
160 kΩ
36 kΩ
+1.9V
−12V
+1.9V
−5V
+1.9V
DS100040-11
FIGURE 6. Input Examples. Resistor values shown in
table provide approximately 1.9V at the analog inputs.
13
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