7
FN1050.6
March 4, 2005
It is well known that MOSFET devices can exhibit slight
changes in characteristics (for example, small changes in
input offset voltage) due to the application of large
differential input voltages that are sustained over long
periods at elevated temperatures.
Both applied voltage and temperature accelerate these
changes. The process is reversible and offset voltage shifts
of the opposite polarity reverse the offset. In typical linear
applications, where the differential voltage is small and
symmetrical, these incremental changes are of about the
same magnitude as those encountered in an operational
amplifier employing a bipolar transistor input stage.
Typical Applications
On/Off Touch Switch
The on/off touch switch shown in Figure 5 uses the
CA3240E to sense small currents flowing between two
contact points on a touch plate consisting of a PC board
metallization “grid”. When the “on” plate is touched, current
flows between the two halves of the grid causing a positive
shift in the output voltage (Terminal 7) of the CA3240E.
These positive transitions are fed into the CA3059, which is
used as a latching circuit and zero-crossing TRIAC driver.
When a positive pulse occurs at Terminal 7 of the CA3240E,
the TRIAC is turned on and held on by the CA3059 and its
associated positive feedback circuitry (51kΩ resistor and
36kΩ/42kΩ voltage divider). When the positive pulse occurs
at Terminal 1 (CA3240E), the TRIAC is turned off and held
off in a similar manner. Note that power for the CA3240E is
supplied by the CA3059 internal power supply.
The advantage of using the CA3240E in this circuit is that it
can sense the small currents associated with skin
conduction while allowing sufficiently high circuit impedance
to provide protection against electrical shock.
Dual Level Detector (Window Comparator)
Figure 6 illustrates a simple dual liquid level detector using
the CA3240E as the sensing amplifier. This circuit operates
on the principle that most liquids contain enough ions in
solution to sustain a small amount of current flow between
two electrodes submersed in the liquid. The current, induced
by an 0.5V potential applied between two halves of a PC
board grid, is converted to a voltage level by the CA3240E in
a circuit similar to that of the on/off touch switch shown in
Figure 5. The changes in voltage for both the upper and
lower level sensors are processed by the CA3140 to activate
an LED whenever the liquid level is above the upper sensor
or below the lower sensor.
Constant-Voltage/Constant-Current Power Supply
The constant-voltage/constant-current power supply shown
in Figure 7 uses the CA3240E as a voltage-error and
current-sensing amplifier. The CA3240E is ideal for this
application because its input common-mode voltage range
includes ground, allowing the supply to adjust from 20mV to
25V without requiring a negative supply voltage. Also, the
ground reference capability of the CA3240E allows it to
sense the voltage across the 1Ω current-sensing resistor in
the negative output lead of the power supply. The CA3086
transistor array functions as a reference for both constant-
voltage and constant-current limiting. The 2N6385 power
Darlington is used as the pass element and may be required
to dissipate as much as 40W. Figure 8 shows the transient
response of the supply during a 100mA to 1A load transition.
Precision Differential Amplifier
Figure 9 shows the CA3240E in the classical precision
differential amplifier circuit. The CA3240E is ideally suited for
biomedical applications because of its extremely high input
impedance. To insure patient safety, an extremely high
electrode series resistance is required to limit any current
that might result in patient discomfort in the event of a fault
condition. In this case, 10MΩ resistors have been used to
limit the current to less than 2μA without affecting the
performance of the circuit. Figure 10 shows a typical
electrocardiogram waveform obtained with this circuit.
V
S
= ±15V
TEMPERATURE (
o
C)
-60 -40 -20 0 20 40 60 80 100 120 140
100
10
INPUT CURRENT (pA)
1K
10K
FIGURE 4. INPUT CURRENT vs TEMPERATURE
CA3240, CA3240A
