1.0 Application Notes (Continued)
DS012830-10
(c) Follower
DS012830-8
(a) Inverting Amplifier
DS012830-9
(b) Non-Inverting Amplifier
DS012830-11
(d) Howland Current Pump
FIGURE 9. Guard Ring Connections
1.3.1 CAPACITIVE LOAD TOLERANCE
Like many other op amps, the LMC6035/6 may oscillate
when its applied load appears capacitive. The threshold of
oscillation varies both with load and circuit gain. The configu-
ration most sensitive to oscillation is a unity-gain follower.
See the Typical Performance Characteristics.
The load capacitance interacts with the op amp’s output re-
sistance 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 10, the addition of a small resistor (50Ω–100Ω) in
series with the op amp’s output, and a capacitor (5 pF–10
pF) from inverting input to output pins, returns the phase
DS012830-5
FIGURE 10. Rx, Cx Improve Capacitive Load Tolerance
Capacitive load driving capability is enhanced by using a pull
up resistor to V+ (Figure 11). Typically a pull up resistor con-
ducting 500 µA or more will significantly improve capacitive
load responses. The value of the pull up resistor must be de-
termined based on the current sinking capability of the ampli-
fier 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).
margin to
a
safe value without interfering with
lower-frequency circuit operation. Thus, larger values of ca-
pacitance 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.
1.4 Micro SMD Considerations
Contrary to what might be guessed, the micro SMD package
does not follow the trend of smaller packages having higher
thermal resistance. LMC6035 in micro SMD has thermal re-
sistance of 220˚C/W compared to 230˚C/W in MSOP. Even
±
when driving a 600Ω load and operating from 7.5V sup-
plies, the maximum temperature raise will be under 4.5˚C.
For application information specific to micro SMD, see Appli-
cation note AN-1112.
DS012830-6
FIGURE 11. Compensating for Large
Capacitive Loads with a Pull Up Resistor
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