LMV301 datasheet(13/19 Pages) NSC

13 / 19 page

Application Hints (Continued)

Capacitive Load Tolerance

Like many other op amps, the LMV301 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. 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. As

shown in

Figure 2, the addition of a small resistor (50

Ω to

100

Ω) in series with the op amp’s output, and a capacitor

(5pF to 10pF) from inverting input to output pins, returns the

phase margin to a safe value without interfering with lower

frequency

circuit

operation.

Thus,

larger

values

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.

Capacitive load driving capability is enhanced by using a pull

up resistor to V

+ (Figure 3). Typically a pull up resistor

conducting

500µA or

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.

PRINTED-CIRCUIT-BOARD LAYOUT

FOR HIGH-IMPEDANCE WORK

It is generally recognized that any circuit which must operate

with less than 100pA of leakage current requires special

layout of the PC board. When one wishes to take advantage

of the low bias current of the LMV301, typically less than

0.182pA, 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 acceptable low, because under conditions of the high

humidity or dust or contamination, the surface leakage will

be appreciable. To minimized the effect of any surface

leakage, lay out a ring of foil completely surrounding the

LMV301’s inputs and the terminals of capacitors, diodes,

conductors, resistors, relay terminals, etc. connected to the

op amp’s inputs. See

Figure 4. To have a significant effect,

guard rings should be placed on both the top and bottom of

the PC board. The 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 trace-to-pad

resistance of 10

Ω, which is normally considered a very

large resistance, could leak 5pA if the trace were a 5V bus

adjacent to the pad of an input. This would cause a 100

times degradation from the LMV301’s actual performance.

However, if a guard ring is held within 5mV of the inputs, then

even a resistance of 10

Ω would cause only 0.05pA of

leakage current, or perhaps a minor (2:1) degradation of the

amplifier performance. See Figure 5a, Figure 5b, Figure 5c

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 5d.

20019305

FIGURE 2. Rx, Cx Improve Capacitive Load Tolerance

20019323

FIGURE 3. Compensating for Large Capacitive Loads

with a Pull Up Resistor

20019377

FIGURE 4. Example, using the LMV301,

of Guard Ring in P.C. Board Layout

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