LMC660

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SNOSBZ3D – APRIL 1998 – REVISED MARCH 2013

CAPACITIVE LOAD TOLERANCE

Like many other op amps, the LMC660 may oscillate when its applied load appears capacitive. The threshold of

oscillation varies both with load and circuit gain. The configuration most sensitive to oscillation is a unity-gain

follower. See Typical Performance Characteristics.

The load capacitance interacts with the op amp's output resistance to create an additional pole. If this pole

frequency is sufficiently low, it will degrade the op amp's phase margin so that the amplifier is no longer stable at

low gains. As shown in Figure 17, the addition of a small resistor (50

Ω to 100Ω) in series with the op amp's

output, and a capacitor (5 pF to 10 pF) from inverting input to output pins, returns the phase margin to a safe

value without interfering with lower-frequency circuit operation. Thus larger values of capacitance can be

tolerated without oscillation. Note that in all cases, the output will ring heavily when the load capacitance is near

the threshold for oscillation.

Figure 17. Rx, Cx Improve Capacitive Load Tolerance

resistor conducting 500

μA or more will significantly improve capacitive load responses. The value of the pull up

resistor must be determined based on the current sinking capability of the amplifier with respect to the desired

output swing. Open loop gain of the amplifier can also be affected by the pull up resistor (see DC Electrical

Characteristics).

Figure 18. Compensating for Large Capacitive Loads with a Pull Up Resistor

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 LMC662,

typically less than 0.04 pA, it is essential to have an excellent layout. Fortunately, the techniques for 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 contamination,

the surface leakage will be appreciable.

To minimize the effect of any surface leakage, lay out a ring of foil completely surrounding the LMC660's inputs

and the terminals of capacitors, diodes, conductors, resistors, relay terminals, etc. connected to the op amp's

inputs. See Figure 19. 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 example, a PC board

Ω, which is normally considered a very large resistance, could leak 5 pA if the

trace were a 5V bus adjacent to the pad of an input. This would cause a 100 times degradation from the

LMC660's actual performance. However, if a guard ring is held within 5 mV of the inputs, then even a resistance

Ω would cause only 0.05 pA of leakage current, or perhaps a minor (2:1) degradation of the amplifier's

performance. See Figure 20a, Figure 20b, and Figure 20c for typical connections of guard rings for standard op

amp configurations. If both inputs are active and at high impedance, the guard can be tied to ground and still

provide some protection; see Figure 20d.

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