LMC6442AIMX NSC [National Semiconductor], LMC6442AIMX Datasheet - Page 11

• Manufacturer Part Number: LMC6442AIMX
• Description: Dual Micropower Rail-to-Rail Output Single Supply Operational Amplifier
• Manufacturer: NSC [National Semiconductor]

Typical Performance Characteristics
specified (Continued)
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 1000MΩ -an expensive task.
An alternate solution is to use a “T” Network in the feedback
path, as shown in Fig. 2.
FIGURE 1. A
c
may need to be adjusted).
V
(A
= +1 Operation by adding C
V
Small Signal Step
= + 1) For Various C
Response
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L
c
and R
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c
c
V
11
S
= 5V, Single Supply, T
Closed loop gain, A
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:
FIGURE 2. “T” Network Used to Replace High Value
V
• Rin, one could select a value for R in the “T” Network
(A
BW(−3 dB) ≈ GBWP • β
Large Signal Step
V
V
A
= +1) (C
is given by:
= 25˚C unless otherwise
Response
Resistor
L
= 200pF)
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www.national.com
10006436
F
defined