REV. B

OP470

–12–

CAPACITIVE LOAD DRIVING AND POWER

SUPPLY CONSIDERATIONS

The OP470 is unity-gain stable and is capable of driving large

capacitive loads without oscillating. Nonetheless, good supply

bypassing is highly recommended. Proper supply bypassing

reduces problems caused by supply line noise and improves the

capacitive load driving capability of the OP470.

In the standard feedback amplifier, the op amp’s output resistance

combines with the load capacitance to form a low pass filter that

adds phase shift in the feedback network and reduces stability.

A simple circuit to eliminate this effect is shown in Figure 11.

The added components, C1 and R3, decouple the amplifier

from the load capacitance and provide additional stability. The

values of C1 and R3 shown in Figure 11 are for a load capaci-

tance of up to 1000 pF when used with the OP470.

R1

100 *

*

SEE TEXT

R3

50

OP470

C5

0.1 F

*

C4

10 F

+

V–

1000pF

C1

1000pF

R2

PLACE SUPPLY DECOUPLING

CAPACITORS AT OP470

C3

0.1 F

C2

10 F

+

V+

Figure 11. Driving Large Capacitive Loads

In applications where the OP470’s inverting or noninverting

inputs are driven by a low source impedance (under 100

W) or

connected to ground, if V+ is applied before V–, or when V is

disconnected, excessive parasitic currents will flow. Most applica-

tions use dual tracking supplies and with the device supply pins

properly bypassed, power-up will not present a problem. A source

resistance of at least 100

W in series with all inputs (Figure 11)

will limit the parasitic currents to a safe level if V– is discon-

nected. It should be noted that any source resistance, even 100

W,

adds noise to the circuit. Where noise is required to be kept at a

minimum, a germanium or Schottky diode can be used to clamp

the V- pin and eliminate the parasitic current flow instead of

using series limiting resistors. For most applications, only one

diode clamp is required per board or system.

UNITY-GAIN BUFFER APPLICATIONS

When Rf

£ 100 W and the input is driven with a fast, large

signal pulse(> 1 V), the output waveform will look as shown

in Figure 12.

2V/ s

OP470

R1

Figure 12. Pulsed Operation

During the fast feedthrough-like portion of the output, the input

protection diodes effectively short the output to the input, and a

current, limited only by the output short-circuit protection, will

be drawn by the signal generator. With Rf

£ 500 W, the output

is capable of handling the current requirements (IL < 20 mA at

10 V); the amplifier will stay in its active mode and a smooth

transition will occur.

When Rf > 3 k

W, a pole created by Rf and the amplifier’s input

capacitance (2 pF) creates additional phase shift and reduces

phase margin. A small capacitor (20 pF to 50 pF) in parallel

with Rf helps eliminate this problem.

APPLICATIONS

Low Noise Amplifier

A simple method of reducing amplifier noise by paralleling

amplifiers is shown in Figure 13. Amplifier noise, depicted in

Figure 14, is around 2 nV/

÷Hz @ 1 kHz (R.T.I.). Gain for each

paralleled amplifier and the entire circuit is 1000. The 200

W

resistors limit circulating currents and provide an effective out-

put resistance of 50

W. The amplifier is stable with a 10 nF

capacitive load and can supply up to 30 mA of output drive.

R2

50k

1/4

OP470E

+15V

–15V

R3

200

R1

50

R5

50k

1/4

OP470E

R6

200

R4

50

R8

50k

1/4

OP470E

R9

200

R7

50

R11

50k

1/4

OP470E

R12

200

R10

50

Figure 13. Low Noise Amplifier