Note: Avoid resistive loads of less than 500

Ω, as they may cause instability.

Application Hints

Amplifier Topology

The topology chosen for the LMC660, shown in

Figure 1,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 in-

tegrator, 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 forward

(via C

driver. In addition, the output portion of the 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.

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 the Electrical

Characteristics.

Compensating Input Capacitance

The high input resistance of the LMC660 op amps allows the

use of large feedback and source resistor values without los-

ing gain accuracy due to loading. However, the circuit will be

especially sensitive to its layout when these large-value re-

sistors are used.

CMRR vs Frequency

DS008767-30

Open-Loop Frequency

Response

DS008767-31

Frequency Response

vs Capacitive Load

DS008767-32

Non-Inverting Large Signal

Pulse Response

DS008767-33

Stability vs

Capacitive Load

DS008767-34

Stability vs

Capacitive Load

DS008767-35

DS008767-4

FIGURE 1. LMC660 Circuit Topology (Each Amplifier)

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