AC COUPLED CIRCUITS

The two power modes makes the LMV422 ideal for AC

coupled circuit where the circuit needs to be kept active to

maintain a quiescent charge on the coupling capacitors with

minimum power consumption.

Figure 4 shows a schematic of an inverting and non-inverting

AC coupled amplifiers using the LMV422 with the PS pins

controlled by I/O ports of a microcontroller.

The advantage of the low power active mode for AC coupled

amplifiers is the elimination of the time needed to re-

establish a quiescent operating point when the amplifier is

switched to a full power mode. When amplifiers without a low

power active mode are used in low power applications, there

are two ways to minimize power consumption. The first is

turning off the amplifiers by switching off power to the op

amps using a transistor switch. The second is using an

amplifier with a shut down pin. Both of these methods have

the problem of allowing the coupling capacitors, C1, C2, C3,

C4, and C5, to discharge the quiescent DC voltage stored on

them when placed in the shut down state. When the ampli-

fiers are turned on again, the quiescent DC voltages must

reestablish themselves. During this time, the amplifier’s out-

put is not usable because the output signal is a mixture of the

amplified input signal and the charging voltage on the cou-

pling capacitors. The settling time can range from a several

milliseconds to several seconds depending on the resistor

and capacitor values.

When the LMV422 is placed into the low power mode the

power consumption is minimal but the amplifier is active to

maintain the quiescent DC voltage on the coupling capaci-

tors and the transition back to the operational high power

mode is fast, within few hundred nanoseconds. The active

low power mode of the LMV422 separates the two critical

aspects of a low power AC amplifier design. The values of

the gain resistors, bias resistors, and coupling capacitors

can be chosen independently of the turn on and stabilization

time.

RESISTIVE LOAD

The LMV422 has a minimum current drive of 3 mA in full

power mode. The minimum resistive load should be 10 k

Ω

The current drive in the low power mode is 140 uA, the

minimum resistive load should be 100 k

Ω.

20109825

Figure 3a

20109826

Figure 3b

FIGURE 3.

20109834

FIGURE 4.

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