LMC660

www.ti.com

SNOSBZ3D – APRIL 1998 – REVISED MARCH 2013

APPLICATION INFORMATION

AMPLIFIER TOPOLOGY

The topology chosen for the LMC660, shown in Figure 15, is unconventional (compared to general-purpose op

amps) in that the traditional unity-gain buffer output stage is not used; instead, the output is taken directly from

the output of the integrator, to allow rail-to-rail output swing. Since the buffer traditionally delivers the power to

the load, while maintaining high op amp gain and stability, and must withstand shorts to either rail, these tasks

now fall to the integrator.

As a result of these demands, the integrator is a compound affair with an embedded gain stage that is doubly fed

integrator is a push-pull configuration for delivering heavy loads. While sinking current the whole amplifier path

consists of three gain stages with one stage fed forward, whereas while sourcing the path contains four gain

stages with two fed forward.

Figure 15. LMC660 Circuit Topology (Each Amplifier)

The large signal voltage gain while sourcing is comparable to traditional bipolar op amps, even with a 600

Ω load.

The gain while sinking is higher than most CMOS op amps, due to the additional gain stage; however, under

heavy load (600

Ω) the gain will be reduced as indicated in DC Electrical Characteristics. Avoid resistive loads of

less than 500

Ω, as they may cause instability.

COMPENSATING INPUT CAPACITANCE

The high input resistance of the LMC660 op amps allows the use of large feedback and source resistor values

without losing gain accuracy due to loading. However, the circuit will be especially sensitive to its layout when

these large-value resistors are used.

Every amplifier has some capacitance between each input and AC ground, and also some differential

capacitance between the inputs. When the feedback network around an amplifier is resistive, this input

capacitance (along with any additional capacitance due to circuit board traces, the socket, etc.) and the feedback

resistors create a pole in the feedback path. In the following General Operational Amplifier circuit, Figure 16 the

frequency of this pole is:

(1)

configurations.

When the feedback resistors are smaller than a few k

Ω, the frequency of the feedback pole will be quite high,

effect on stability, as it will add only a small amount of phase shift.

However, if the feedback pole is less than approximately 6 to 10 times the “ideal”

−3 dB frequency, a feedback

also be stated in terms of the amplifier's low-frequency noise gain: To maintain stability a feedback capacitor will

probably be needed if:

Copyright © 1998–2013, Texas Instruments Incorporated

Submit Documentation Feedback

7

Product Folder Links: LMC660