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 LMC6034 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 be-

tween 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 2 the frequency of this pole is

where C

cluding amplifier input capcitance and any stray capacitance

from the IC socket (if one is used), circuit board traces, etc.,

and R

mula, as well as all formulae derived below, apply to invert-

ing and non-inverting op-amp configurations.

When the feedback resistors are smaller than a few k

Ω, the

frequency of the feedback pole will be quite high, since C

generally less than 10 pF. If the frequency of the feedback

pole is much higher than the “ideal” closed-loop bandwidth

(the nominal closed-loop bandwidth in the absence of C

the pole will have a negligible 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 capacitor,

C

ing input of the op amp. This condition can also be stated in

terms of the amplifier’s low-frequency noise gain: To main-

tain stability a feedback capacitor will probably be needed if

where

is the amplifier’s low-frequency noise gain and GBW is the

amplifier’s

gain

bandwidth

product.

An

amplifier’s

low-frequency noise gain is represented by the formula

regardless of whether the amplifier is being used in inverting

or non-inverting mode. Note that a feedback capacitor is

more likely to be needed when the noise gain is low and/or

the feedback resistor is large.

If the above condition is met (indicating a feedback capacitor

will probably be needed), and the noise gain is large enough

that:

the following value of feedback capacitor is recommended:

If

the feedback capacitor should be:

Note that these capacitor values are usually significantly

smaller than those given by the older, more conservative for-

mula:

Using the smaller capacitors will give much higher band-

width with little degradation of transient response. It may be

necessary in any of the above cases to use a somewhat

larger feedback capacitor to allow for unexpected stray ca-

DS011134-3

FIGURE 1. LMC6034 Circuit Topology (Each Amplifier)

DS011134-4

FIGURE 2. General Operational Amplifier Circuit

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