4
FN3403.5
March 4, 2010
Application Information
Static Protection
All devices are static protected by the use of input diodes.
However, strong static fields should be avoided, as it is
possible for the strong fields to cause degraded diode
junction characteristics, which may result in increased input
leakage currents.
Latchup Avoidance
Junction-isolated CMOS circuits employ configurations
which produce a parasitic 4-layer (PNPN) structure. The
4-layer structure has characteristics similar to an SCR, and
under certain circumstances may be triggered into a low
impedance state resulting in excessive supply current. To
avoid this condition, no voltage greater than 0.3V beyond the
supply rails may be applied to any pin. In general, the op
amp supplies must be established simultaneously with, or
before any input signals are applied. If this is not possible,
the drive circuits must limit input current flow to 2mA to
prevent latchup.
Choosing the Proper I
Q
Each device in the ICL76XX family has a similar I
Q
setup
scheme, which allows the amplifier to be set to nominal
quiescent currents of 10µA, 100µA or 1mA. These current
settings change only very slightly over the entire supply voltage
range. The ICL7611/12 have an external I
Q
control terminal,
permitting user selection of each amplifiers’ quiescent current.
The ICL7621 has a fixed I
Q
setting of 100µA.
Output Stage and Load Driving Considerations
Each amplifiers’ quiescent current flows primarily in the
output stage. This is approximately 70% of the I
Q
settings.
This allows output swings to almost the supply rails for
output loads of 1MΩ, 100kΩ, and 10kΩ, using the output
stage in a highly linear class A mode. In this mode,
crossover distortion is avoided and the voltage gain is
maximized. However, the output stage can also be operated
in Class AB for higher output currents (see graphs in “Typical
Performance Curves” beginning on page 6). During the
transition from Class A to Class B operation, the output
tra
nsfer characteristic is nonlinear and the voltage gain
decreases.
Frequency Compensation
The ICL76XX are internally compensated, and are stable
for closed loop gains as low as unity with capacitive loads
up to 100pF.
Typical Applications
The user is cautioned that, due to extremely high input
impedances, care must be exercised in layout, construction,
board cleanliness, and supply filtering to avoid hum and
noise pickup.
FIGURE 1. SIMPLE FOLLOWER
ICL76XX
+
-
V
IN
V
OUT
R
L
≥ 10kΩ
FIGURE 2. LEVEL DETECTOR
ICL76XX
+
-
V
IN
V
OUT
100kΩ
+5 +5
1MΩ
TO CMOS OR
LPTTL LOGIC
FIGURE 3.
V
OUT
1µF
ICL76XX
+
-
+
λ
NOTE: Low leakage currents allow integration times up to
several hours.
PHOTOCURRENT INTEGRATOR FIGURE 4.
1MΩ
+
-
1MΩ
DUTY CYCLE
V- V+
680kΩ
1MΩ
WAVEFORM GENERATOR
+
-
1/2
ICL7621
1/2
ICL7621
NOTE: Since the output range swings exactly from rail to rail,
frequency and duty cycle are virtually independent of power supply
variations.
TRIANGLE/SQUARE WAVE GENERATOR
ICL7621
