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Using an External Pull-Up
In general, an external pull-up is required only when you want to set
the output “high” value greater than
TTL levels. For example, to use a
+12V external power supply, the value of the external pull-up resistor is
calculated as follows:
V
cc
= 12 Vdc
I
max
= I
out low
x safety factor = 1 mA x 0.5 = 0.5 mA
R =
Vcc
I
max
=
12
0.0005
= 24 kΩ
The value of the logic “high” level with the external 24 kΩ pull-up
resistor is calculated as follows:
V
high
= V
cc
x
R
external
R
external
+ R
internal
= 12 x
24 k Ω
24 k Ω + 10 k Ω
= 8.47 Vdc
Driving External Switches
You can use two digital output channels to control an external switch.
For example, you can drive the Agilent 876X series of microwave
switches using an external power supply and two digital output
channels. The state of the 2-to-1 multiplexer is changed by setting
the appropriate output bit low (0).
Agilent 876X Microwave Switch
Digital Output Channel
24 Volt
Power
Supply
–
+
Chapter 8 Tutorial
Multifunction Module
394