ICL7621
FN3403 Rev.5.00 Page 4 of 9
Mar 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 IQ
Each device in the ICL76XX family has a similar IQ 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 IQ control terminal, permitting user selection of
each amplifiers’ quiescent current. The ICL7621 has a fixed IQ
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 IQ settings. This allows
output swings to almost the supply rails for output loads of
1M, 100k, and 10k, 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 transfer 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 FIGURE 2. LEVEL DETECTOR
FIGURE 3. PHOTOCURRENT INTEGRATOR FIGURE 4. TRIANGLE/SQUARE WAVE GENERATOR
ICL76XX
+
-
VIN
VOUT
RL 10k
ICL76XX
+
-
VIN
VOUT
100k
+5 +5
1M
TO CMOS OR
LPTTL LOGIC
VOUT
1µF
ICL76XX
+
-
+
NOTE: Low leakage currents allow integration times up to
several hours.
1M
+
-
1M
DUTY CYCLE
V- V+
680k
1M
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.