Datasheet

®
ACF2101
13
FIGURE 11. Timing Generator.
VOLTAGE INPUT EXAMPLE
Figure 12 illustrates the use of the ACF2101 with a voltage
input. This approach is useful in applications where a con-
stant current source is needed. For example, the ACF2101
can be configured in a bipolar mode by using the current
generated by a voltage reference as an offset current. In the
example in Figure 12, a 10V reference (REF102) is used in
series with a 400kΩ resistor to generate a constant +25µA
input current to the ACF2101. The ACF2101 will operate as
expected in this configuration except in the Hold mode.
When the Hold switch is opened, the input to the ACF2101
becomes high impedance and consequently the Sw In node
will try to go to 10V. The Hold switch is specified to have
a withstand voltage of +0.5V. When the voltage at the Sw In
node exceeds +0.5V the Hold switch will begin to conduct
again. This will not cause damage to the switch, however,
the output will start to unexpectedly integrate again. The
addition of either C
1
or D
1
in the circuit is critical for proper
Hold mode operation. C
1
will divert the charge being gener-
ated by the voltage source in series with the resistor. C
1
is
selected so that the maximum voltage does not exceed 0.4V.
When the Hold switch is closed again, the charge collected
by C
1
is transferred to the integration capacitor. D
1
will
divert the charge being generated by the voltage source and
resistor to ground. When the Hold switch closes again, the
charge stored on the parasitic capacitor of the diode is
transferred to the integration capacitor. D
1
should be se-
lected so that the on voltage of the diode does not exceed
0.4V.
DEMONSTRATION BOARD AND MACROMODEL
Demonstration boards are available to speed prototyping.
The demonstration board, DEM-ACF2101BP-C includes a
programmable timing generator making it easy to do a quick
evaluation.
A Spice-based macromodel is also available. Request
AB-020 for Application Bulletin and Burr-Brown's Spice
Macromodel diskette.
100pF
14 4 10
759
84
13
6
8
1
13
2
12
2
7
6
Hold
54.5kΩ54.5kΩ10kΩ
100pF
100pF
1000pF
1000pF
fs
10kΩ 164kΩ
+V
S
Reset
Select
556
Dual Timer
555
Timer
+15V
Both approximately 6µs
Approximately 18µs
1N914
20kΩ