OP181/OP281/OP481

–11–

REV. 0

10

0%

100

90

0.2ms

0.1V

0V

Overdrive Recovery Time

The amount of time it takes for an amplifier to recover from

saturation can be an important consideration when using an

amplifier as a comparator or when outputs can be driven to the

supplies. The overdrive recovery time for the OP181 is 50

µs

with the amplifier running from a 3 volt supply and increases

to 100

µs with a 10 volt supply. Figure 36 shows the result of

the OP181 running from a 3 V supply with its output being

overdriven.

10

0%

100

90

0.2ms

0.1V

SCALE 0.1V

0V

SCALE 1V

0V

Figure 36. Output of the Op Amp Recovering from

Saturation

Capacitive Loading

Most low supply current amplifiers have difficulty driving

capacitive loads due to the higher currents required from the

output stage for such loads. Higher capacitance at the output

will increase the amount of overshoot and ringing in the

amplifier’s step response and could even affect the stability of

the device. However, through careful design of the output stage

and its high phase margin, the OPx81 family can tolerate some

degree of capacitive loading. Figure 37 shows the step response

of an OP181 with a 10 nF capacitor connected at the output.

Notice that the overshoot of the output does not exceed more

than 10% with such a load, even with a supply voltage of only

+3 V.

10

0%

100

90

Figure 37. Ringing and Overshoot of the Output of the

Amplifier

A Micropower Reference Voltage Generator

Many single supply circuits are configured with the circuit

biased to 1/2 of the supply voltage. In these cases, a false-

ground reference can be created by using a voltage divider

buffered by an amplifier. Figure 38 shows the schematic for

such a circuit.

The two 1 M

Ω resistors generate the reference voltage while

drawing only 1.5

µA of current from a 3 V supply. A capacitor

connected from the inverting terminal to the output of the op

amp provides compensation to allow for a bypass capacitor to be

connected at the reference output. This bypass capacitor helps

establish an ac ground for the reference output. The entire

reference generator draws less than 5

µA from a 3 V supply

source.

6

7

4

2

3

OP181

10k

Ω

0.022µF

+1.5V TO +6V

1µF

1µF

1M

Ω

+3V TO +12V

100

Ω

1M

Ω

Figure 38. A Micropower Bias Voltage Generator

A Window Comparator

The extremely low power supply current demands of the OPx81

family make it ideal for use in long life battery powered applica-

tions such as a monitoring system. Figure 39 shows a circuit

that uses the OP281 as a window comparator.