OP481GS datasheet(11/16 Pages) AD

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REV. B

OP281/OP481

–11–

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 4 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.

100

Figure 4. Ringing and Overshoot of the Output

of the Amplifier

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 refer-

ence can be created by using a voltage divider buffered by an

amplifier. Figure 5 shows the schematic for such a circuit.

The two 1 M

W resistors generate the reference voltage while

drawing only 1.5

mA 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

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

reference generator draws less than 5

mA from a 3 V supply source.

OP281

10k

0.022 F

VREF

1.5V TO 6V

1 F

3V TO 12V

100

Figure 5. 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 applica-

tions such as a monitoring system. Figure 6 shows a circuit that

uses the OP281 as a window comparator.

OP281-A

VIN

5.1k

VOUT

5.1k

10k

OP281-B

Figure 6. Using the OP281 as a Window Comparator

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

vided that VH > VL. The output of 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 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 will go to the

positive rail, and D1 will conduct current. This creates a base

voltage that will turn on Q1 and drive VOUT low. The same

condition occurs if VIN falls below VL with A2’s output going

high, and D2 conducting current. Therefore, VOUT will be high

if the input voltage is between VL and VH, and VOUT will be 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.

+ R2

+ R4

The 2 k

W resistor connects the input voltage to 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 equivalent NPN transistor.

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