LMC6462AMN datasheet(10/18 Pages) NSC

10 / 18 page

Application Information (Continued)

2.0 Rail-to-Rail Output

The approximated output resistance of the LMC6462/4 is

180

Ω sourcing, and 130Ω sinking at V

S = 3V, and 110Ω

sourcing and 83

Ω sinking at V

S = 5V. The maximum output

swing can be estimated as a function of load using the calcu-

lated output resistance.

3.0 Capacitive Load Tolerance

The LMC6462/4 can typically drive a 200 pF load with V

S =

5V at unity gain without oscillating. The unity gain follower is

the most sensitive configuration to capacitive load. Direct ca-

pacitive loading reduces the phase margin of op-amps. The

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

pacitive load induces phase lag. This results in either an un-

derdamped pulse response or oscillation.

Capacitive load compensation can be accomplished using

resistive isolation as shown in

Figure 4. If there is a resistive

component of the load in parallel to the capacitive compo-

nent, the isolation resistor and the resistive load create a

voltage divider at the output. This introduces a DC error at

the output.

Figure 5 displays the pulse response of the LMC6462/4 cir-

cuit in

Figure 4.

Another circuit, shown in

Figure 6, is also used to indirectly

drive capacitive loads. This circuit is an improvement to the

circuit shown in

Figure 4 because it provides DC accuracy as

well as AC stability. R1 and C1 serve to counteract the loss

of phase margin by feeding the high frequency component of

the output signal back to the amplifiers inverting input,

thereby preserving phase margin in the overall feedback

loop. The values of R1 and C1 should be experimentally de-

termined by the system designer for the desired pulse re-

sponse. Increased capacitive drive is possible by increasing

the value of the capacitor in the feedback loop.

The pulse response of the circuit shown in

Figure 6 is shown

Figure 7.

4.0 Compensating for Input Capacitance

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

tance with amplifiers that have ultra-low input current, like

the LMC6462/4. Large feedback resistors can react with

small values of input capacitance due to transducers, photo-

diodes, and circuits board parasitics to reduce phase

margins.

DS012051-7

FIGURE 3. Input Current Protection for Voltages

Exceeding the Supply Voltage

DS012051-8

FIGURE 4. Resistive Isolation of

a 300 pF Capacitive Load

DS012051-9

FIGURE 5. Pulse Response of the LMC6462

Circuit Shown in

Figure 4

DS012051-10

FIGURE 6. LMC6462 Non-Inverting Amplifier,

Compensated to Handle a 300 pF Capacitive

and 100 k

Ω Resistive Load

DS012051-11

FIGURE 7. Pulse Response of

LMC6462 Circuit in

Figure 6

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