OP281/OP481

Rev. D | Page 14 of 20

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 may 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 39 shows the step response of a single channel

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 39. Ringing and Overshoot of the Output of the Amplifier

MICROPOWER REFERENCE VOLTAGE GENERATOR

Many single-supply circuits are configured with the circuit biased

to half of the supply voltage. In these cases, a false ground reference

can be created by using a voltage divider buffered by an amplifier.

Figure 40 shows the schematic for such a circuit.

The two 1 MΩ 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 a bypass capacitor to be

connected at the reference output. This bypass capacitor helps

to establish an ac ground for the reference output. The entire

reference generator draws less than 5 μA from a 3 V supply source.

OP281

10kΩ

0.022µF

VREF

1.5V TO 6V

1µF

1µF

1MΩ

3V TO 12V

100Ω

1MΩ

8

4

3

1

2

Figure 40. Single Channel Configured as a Micropower Bias Voltage Generator

WINDOW COMPARATOR

The extremely low power supply current demands of the OPx81

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

applications such as a monitoring system. Figure 41 shows a

circuit that uses the OP281 as a window comparator.

A1

R1

R2

3V

OP281-A

VIN

2kΩ

5.1kΩ

3V

3V

VOUT

Q1

5.1kΩ

VH

D1

10kΩ

A2

R3

R4

3V

3V

VL

D2

OP281-B

Figure 41. Using the OP281 as a Window Comparator

The threshold limits for the window are set by VH and VL,

provided that VH > VL. The output of the first OP281 (A1) will

stay at the negative rail, in this case ground, as long as the input

voltage is less than VH. Similarly, the output of the second

OP281 (A2) will stay at ground as long the input voltage is

higher than VL. As long as VIN remains between VL and VH, the

outputs of both op amps will be 0 V. With no current flowing in

either D1 or D2, the base of Q1 will stay at ground, putting the

transistor in cutoff and forcing VOUT to the positive supply rail.

If the input voltage rises above VH, the output of A2 stays at

ground, but the output of A1 goes to the positive rail and D1

conducts current. This creates a base voltage that turns on Q1

and drives VOUT low. The same condition occurs if VIN falls

below VL with A2’s output going high and D2 conducting

current. Therefore, VOUT is high if the input voltage is between

VL and VH, but low if the input voltage moves outside of that range.

The R1 and R2 voltage divider sets the upper window voltage,

and the R3 and R4 voltage divider sets the lower voltage for the

window. For the window comparator to function properly, VH

must be a greater voltage than VL.

R4

R3

R4

V

R2

R1

R2

V

L

H

+

=

+

=

The 2 kΩ resistor connects the input voltage of the input

terminals to the op amps. This protects the OP281 from

possible excess current flowing into the input stages of the

devices. D1 and D2 are small-signal switching diodes (1N4446

or equivalent), and Q1 is a 2N2222 or an equivalent NPN

transistor.