Applications Hints

1.0 LOW VOLTAGE AMPLIFIER TOPOLOGY

The LMC6574/2 incorporates a novel op-amp design topol-

ogy that enables it to maintain rail-to-rail output swing even

when driving a large load. Instead of relying on a push-pull

unity gain output buffer stage, the output stage is taken di-

rectly from the internal integrator, which provides both low

output impedance and large gain. Special feed-forward com-

pensation design techniques are incorporated to maintain

stability over a wider range of operating conditions than tra-

ditional micropower op-amps. These features make the

LMC6574/2 both easier to design with, and provide higher

speed than products typically found in this ultra-low power

class.

2.0 COMPENSATING FOR INPUT CAPACITANCE

It is quite common to use large values of feedback resis-

tance for amplifiers with ultra-low input current, like the

LMC6574/2.

Although the LMC6574/2 is highly stable over a wide range

of operating conditions, a large feedback resistor will react

even with small values of capacitance at the input of the

op-amp to reduce phase margin. The capacitance at the in-

put of the op-amp comes from transducers, photodiodes and

circuit board parasitics.

The effect of input capacitance can be compensated for by

adding a capacitor, C

Figure 1) such that:

or

R

Since it is often difficult to know the exact value of C

be experimentally adjusted so that the desired pulse re-

sponse is achieved. Refer to the LMC660 and LMC662 for a

more detailed discussion on compensating for input capaci-

tance.

When high input impedances are demanded, guarding of the

LMC6574/2 is suggested. Guarding input lines will not only

reduce leakage, but lowers stray input capacitance as well.

(See

Printed-Circuit-Board Layout for High Impedance

Work).

3.0 CAPACITIVE LOAD TOLERANCE

Direct capacitive loading will reduce the phase margin of

many op-amps. A pole in the feedback loop is created by the

combination of the op-amp’s output impedance and the ca-

pacitive load. This pole induces phase lag at the unity-gain

crossover frequency of the amplifier resulting in either an os-

cillatory or underdamped pulse response. With a few exter-

nal components, op amps can easily indirectly drive capaci-

tive loads, as shown in

Figure 2.

In the circuit of

Figure 2, R1 and C1 serve to counteract the

loss of phase margin by feeding the high frequency compo-

Bandwidth vs

Capacitive Load

DS011934-44

Capacitive Load

vs Phase Margin

DS011934-45

Capacitive Load

vs Gain Margin

DS011934-46

DS011934-6

FIGURE 1. Cancelling the Effect of Input Capacitance

DS011934-7

FIGURE 2. LMC6574/2 Noninverting Gain of 10

Amplifier, Compensated to Handle Capacitive Loads

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