Datasheet

LMV301

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SNOS968A –MAY 2004–REVISED MAY 2013

Figure 36. Rx, Cx Improve Capacitive Load Tolerance

Capacitive load driving capability is enhanced by using a pull up resistor to V

(Figure 37). Typically a pull up

resistor conducting 500µA 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.

Figure 37. 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 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 38. 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 39, Figure 40, and Figure 41 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 42.

Product Folder Links: LMV301