SM73306 Datasheet, PDF (16/22 Pages) Texas Instruments – SM73306 CMOS Rail-to-Rail Input and Output Operational Amplifier

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FIGURE 3. R_I Input Current Protection for
Voltages Exceeding the Supply Voltages
RAIL-TO-RAIL OUTPUT
The approximate output resistance of the SM73306 is 110Ω
sourcing and 80Ω sinking at V_s = 5V. Using the calculated
output resistance, maximum output voltage swing can be es-
itmated as a function of load.
COMPENSATING FOR INPUT CAPACITANCE
It is quite common to use large values of feedback resistance
for amplifiers with ultra-low input current, like the SM73306.
Although the SM73306 is highly stable over a wide range of
operating conditions, certain precautions must be met to
achieve the desired pulse response when a large feedback
resistor is used. Large feedback resistors with even small
values of input capacitance, due to transducers, photodiodes,
and circuit board parasitics, reduce phase margins.
When high input impedances are demanded, guarding of the
SM73306 is suggested. Guarding input lines will not only re-
duce leakage, but lowers stray input capacitance as well.
(See Printed-Circuit-Board Layout for High Impedance
Work).
The effect of input capacitance can be compensated for by
adding a capacitor, C_f, around the feedback resistors (as in
Figure 1 ) such that:
or
R₁ C_IN ≤ R₂ C_f
Since it is often difficult to know the exact value of C_IN, C_f can
be experimentally adjusted so that the desired pulse re-
sponse is achieved.
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FIGURE 4. Cancelling the Effect of Input Capacitance
CAPACITIVE LOAD TOLERANCE
All rail-to-rail output swing operational amplifiers have voltage
gain in the output stage. A compensation capacitor is normally
included in this integrator stage. The frequency location of the
dominant pole is affected by the resistive load on the amplifier.
Capacitive load driving capability can be optimized by using
an appropriate resistive load in parallel with the capacitive
load (see Typical Curves).
Direct capacitive loading will reduce the phase margin of
many op-amps. A pole in the feedback loop is created by the
combination of the op-amp's output impedance and the ca-
pacitive load. This pole induces phase lag at the unity-gain
crossover frequency of the amplifier resulting in either an os-
cillatory or underdamped pulse response. With a few external
components, op amps can easily indirectly drive capacitive
loads, as shown in Figure 5.
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FIGURE 5. SM73306 Noninverting Amplifier,
Compensated to Handle Capacitive Loads
PRINTED-CIRCUIT-BOARD LAYOUT
FOR HIGH-IMPEDANCE WORK
It is generally recognized that any circuit which must operate
with less than 1000 pA of leakage current requires special
layout of the PC board. When one wishes to take advantage
of the ultra-low bias current of the SM73306, typically
150 fA, it is essential to have an excellent layout. Fortunately,
the techniques of obtaining low leakages are quite simple.
First, the user must not ignore the surface leakage of the PC
board, even though it may sometimes appear acceptably low,
because under conditions of high humidity or dust or contam-
ination, the surface leakage will be appreciable.
To minimize the effect of any surface leakage, lay out a ring
of foil completely surrounding the SM73306's inputs and the
terminals of components connected to the op-amp's inputs,
as in Figure 6. To have a significant effect, guard rings should
be placed on both the top and bottom of the PC board. This
PC foil must then be connected to a voltage which is at the
same voltage as the amplifier inputs, since no leakage current
can flow between two points at the same potential. For ex-
ample, a PC board trace-to-pad resistance of 10¹²Î©, which is
normally considered a very large resistance, could leak 5 pA
if the trace were a 5V bus adjacent to the pad of the input.
This would cause a 33 times degradation from the SM73306's
actual performance. If a guard ring is used and held within 5
mV of the inputs, then the same resistance of 10¹²Î© will only
cause 0.05 pA of leakage current. See Figure 7 for typical
connections of guard rings for standard op-amp configura-
tions.
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