Datasheet

5
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 and ICL7641/7642 have
fixed I
Q
settings:
ICL7621 (Dual) - I
Q
= 100µA
ICL7641 (Quad) - I
Q
= 1mA
ICL7642 (Quad) - I
Q
= 10µA
NOTE: The output current available is a function of the
quiescent current setting. For maximum peak-to-peak output
voltage swings into low impedance loads, I
Q
of 1mA should
be selected.
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 under
Typical Operating Characteristics). 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.
Operation At V
SUPPLY
= ± 1V
Operation at V
SUPPLY
= ±1V is guaranteed for the
ICL7642C only.
Output swings to within a few millivolts of the supply rails are
achievable for R
L
≥ 1MΩ. Guaranteed input CMVR is ±0.6V
minimum and typically +0.9V to -0.7V at V
SUPPLY
= ±1V. For
applications where greater common mode range is
desirable, refer to the ICL7612 data sheet.
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.
Note that in no case is I
Q
shown. The value of I
Q
must be
chosen by the designer with regard to frequency response
and power dissipation.
FIGURE 1. SIMPLE FOLLOWER FIGURE 2. LEVEL DETECTOR
FIGURE 3. PHOTOCURRENT INTEGRATOR FIGURE 4. TRIANGLE/SQUARE WAVE GENERATOR
ICL76XX
+
-
V
IN
V
OUT
R
L
≥ 10kΩ
ICL76XX
+
-
V
IN
V
OUT
100kΩ
+5 +5
1MΩ
TO CMOS OR
LPTTL LOGIC
V
OUT
1µF
ICL76XX
+
-
+
λ
NOTE: Low leakage currents allow integration times up to
several hours.
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, fre-
quency and duty cycle are virtually independent of power supply
variations.
ICL7621, ICL7641, ICL7642