lmc6442imx National Semiconductor Corporation, lmc6442imx Datasheet - Page 12

• Manufacturer Part Number: lmc6442imx
• Description: Dual Micropower Rail-to-rail Output Single Supply Operational Amplifier
• Manufacturer: National Semiconductor Corporation

www.national.com
Applications Information
USING LMC6442 IN UNITY GAIN APPLICATIONS
LMC6442 is optimized for maximum bandwidth and minimal
external components when operating at a minimum closed
loop gain of +2 (or −1). However, it is also possible to
operate the device in a unity gain configuration by adding
external compensation as shown in Figure 1:
Using this compensation technique it is possible to drive
capacitive loads of up to 300 pF without causing oscillations
(see the Typical Performance Characteristics for step re-
sponse plots). This compensation can also be used with
other gain settings in order to improve stability, especially
when driving capacitive loads (for optimum performance, R
and C
USING “T” NETWORK
Compromises need to be made whenever high gain invert-
ing stages need to achieve a high input impedance as well.
This is especially important in low current applications which
tend to deal with high resistance values. Using a traditional
inverting amplifier, gain is inversely proportional to the resis-
tor value tied between the inverting terminal and input while
the input impedance is equal to this value. For example, in
order to build an inverting amplifier with an input impedance
of 10MΩ and a gain of 100, one needs to come up with a
feedback resistor of 1000 MΩ -an expensive task.
An alternate solution is to use a “T” Network in the feedback
path, as shown in Figure 2.
Closed loop gain, A
FIGURE 2. “T” Network Used to Replace High Value
FIGURE 1. A
C
may need to be adjusted).
V
= +1 Operation by adding C
V
is given by:
Resistor
10006435
C
and R
10006436
C
C
12
It must be noted, however, that using this scheme, the
realizable bandwidth would be less than the theoretical
maximum. With feedback factor, β, defined as:
In this case, assuming a GBWP of about 10 KHz, the ex-
pected BW would be around 50 Hz (vs. 100 Hz with the
conventional inverting amplifier).
Looking at the problem from a different view, with R
by A
and then determine R1 based on this selection:
For convenience, Figure 3 shows R1 vs. R
values of R.
DESIGN CONSIDERATIONS FOR CAPACITIVE LOADS
As with many other opamps, the LMC6442 is more stable at
higher closed loop gains when driving a capacitive load.
Figure 4 shows minimum closed loop gain versus load ca-
pacitance, to achieve less than 10% overshoot in the output
small signal response. In addition, the LMC6442 is more
stable when it provides more output current to the load and
when its output voltage does not swing close to V
The LMC6442 is more tolerant to capacitive loads when the
equivalent output load resistance is lowered or when output
voltage is 1V or greater from the V
load drive capability is also improved by adding an isolating
resistor in series with the load and the output of the device.
Figure 5 shows the value of this resistor for various capaci-
tive loads (A
% overshoot.
Referring to the Typical Performance Characteristics plot of
Phase Margin (Worst Case) vs. Supply Voltage, note that
Phase Margin increases as the equivalent output load resis-
tance is lowered. This plot shows the expected Phase Mar-
gin when the device output is very close to V
least stable condition of operation. Comparing this Phase
Margin value to the one read off the Open Loop Gain/Phase
vs. Frequency plot, one can predict the improvement in
FIGURE 3. “T” Network Values for Various Values of R
V
• Rin, one could select a value for R in the “T” Network
V
= −1), while limiting the output to less than 10
BW(−3 dB) ≈ GBWP • β
−
supply. The capacitive
10006422
−
F
, which is the
for different
−
F
.
defined