Datasheet

ICL7652
Advanced LinCMOSTM PRECISION CHOPPER-STABILIZED
OPERATIONAL AMPLIFIER
TYPICAL APPLICATION DATA
capacitor selection and placement
The two important factors to consider when selecting external capacitors CXA and CXB are leakage and
dielectric absorption. Both factors can cause system degradation that negate the performance advantages
realized by using the ICL7652.
Degradation from capacitor leakage becomes more apparent with increasing temperatures. Low-leakage
capacitors and standoffs are recommended for operation at T A = 125°C. In addition, guardbands around the
capacitor connections on both sides of the printed circuit board are recommended to alleviate problems
caused by surface leakage on circuit boards.
Capacitors with high dielectric absorption tend to take several seconds to settle upon application of power,
which directly affects input offset voltage. In applications needing fast settling of input offset voltage, it is
recommended that high-quality film capacitors, such as mylar, polystyrene, or polypropylene, be used. In
other applications, however, a ceramic or other low-grade capacitor may suffice.
Unlike many choppers available today, the ICL7652 is designed to function with values of CXA and CXB in
the range of 0.1 ~F to 1 ~F without degradation to input offset voltage or input noise voltage. These capacitors
should be located as close as possible to the CXA and CXB pins and returned to either the VDD- pin or the
C RETURN pin. Note that on many choppers, connecting these capacitors to the VDD- pin causes
degradation in noise performance, a problem that is eliminated on the ICL7652.
overload recovery/output clamp
When large differential input voltage conditions are applied to the ICL7652, the nulling loop attempts to prevent
the output from saturating by driving CXA and CXB to internally-clamped voltage levels. Once the overdrive
condition is removed, a period of time is required to allow the built-up charge to dissipate. This time period
is defined as overload recovery time (see Figure 25). Typical overload recovery time for the ICL7652 is
significantly faster than that of competitive products.
thermoelectric effects
T o take advantage of the extremely low offset
voltage temperature coefficient of the ICL7652,
care must be taken to compensate for the
thermoelectric effects present when two dissimilar
metals are brought into contact with each other
(such as device leads being soldered to a printed
circuit board). It is not uncommon for dissimilar
metal junctions to produce thermoelectric
voltages in the range of several microvolts per
degree Celsius (orders of magnitude greater than
the O.O1-!lV/OC typical of the ICL7652).
To help minimize thermoelectric effects, careful
attention should be paid to component selection
and circuit board layout. Avoid the use of
nonsoldered connections (such as sockets,
relays, switches, etc.) in the input signal path.
Cancel thermoelectric effects by duplicating the
number of components and junctions in each
device input. The use of low-thermoelectric-
coefficient components, such as wire-wound
resistors, is also beneficial.
FIGURE 25. OVERLOAD RECOVERY
TEXAS ~
INSTRUMENTS
POST OFFICE BOX 655303 .DALLAS. TEXAS 75265
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