LMC6034IM National Semiconductor, LMC6034IM Datasheet - Page 8

• Manufacturer Part Number: LMC6034IM
• Description: Operational Amplifier (Op-Amp) IC
• Manufacturer: National Semiconductor

Specifications of LMC6034IM

• No. Of Amplifiers: 4
• Slew Rate: 1.1V/µs
• No. Of Pins: 14
• Peak Reflow Compatible (260 C): No
• Leaded Process Compatible: No
• Mounting Type: Surface Mount
• Package / Case: 14-NSOIC
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

www.national.com
Applications Hint
Capacitive load driving capability is enhanced by using a pull
up resistor to V
conducting 500 µA or more will significantly improve capaci-
tive 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 Electrical Characteristics).
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 LMC6034, 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 LMC6034’s inputs and the
terminals of capacitors, diodes, conductors, resistors, relay
terminals, etc. connected to the op-amp’s inputs. See Figure
5. 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 trace-to-pad resistance of 10
mally 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 LMC6034’s
FIGURE 3. Rx, Cx Improve Capacitive Load Tolerance
FIGURE 4. Compensating for Large Capacitive Loads
+
with a Pull Up Resistor
(Figure 4). Typically a pull up resistor
(Continued)
01113422
12
Ω, which is nor-
01113405
8
actual performance. However, if a guard ring is held within
5 mV of the inputs, then even a resistance of 10
cause only 0.05 pA of leakage current, or perhaps a minor
(2:1) degradation of the amplifier’s performance. See Fig-
ures 6, 7, 8 for typical connections of guard rings for stan-
dard 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 9.
FIGURE 5. Example of Guard Ring in P.C. Board
FIGURE 6. Guard Ring Connections
FIGURE 7. Guard Ring Connections
Non-Inverting Amplifier
Inverting Amplifier
Layout
01113407
11
01113408
Ω would
01113406