REV. A

OP471

–10–

Noise Measurement - Current Noise Density

The test circuit shown in Figure 10 can be used to measure current

noise density. The formula relating the voltage output to current

noise density is:

i

e

G

40nV / Hz

R

n

nOUT

S

=

Ê

ËÁ

ˆ

¯˜ -

()

2

2

where:

G = gain of 10,000

W source resistance

Capacative Load Driving and Power Supply Considerations

The OP471 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 OP471.

R2

100k

R3

1.24k

OP471

DUT

R5

8.06k

OP27E

R4

200

SPECTRUM ANALYZER

R1

5

GAIN = 10,000

Figure 10. Current Noise Density Test Circuit

R1

100 *

*

SEE TEXT

R3

50

OP471

C5

0.1 F

*

C4

10 F

+

V–

1000pF

C1

200pF

R2

PLACE SUPPLY DECOUPLING

CAPACITORS AT OP471

C3

0.1 F

C2

10 F

+

V+

Figure 11. Driving Large Capacitive Loads

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

combines with the load capacitance to form a lowpass 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 load capacitances of up

to 1,000 pF when used with the OP471.

In applications where the OP471’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 applications 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 disconnected. 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 applica-

tions, only one diode clamp is required per board or system.

8V/ s

OP471

Figure 12. Pulsed Operation

Unity-Gain Buffer Applications

£ 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.

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

≥ 500 W, the output

£ 20 mA at

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

transition will occur.

capacitance (2.6 pF) creates additional phase shift and reduces

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

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 5 nV/

÷Hz @ 1 kHz (R.T.I.). Gain for each

paralleled amplifier and the entire circuit is 100. The 200

W

resistors limit circulating currents and provide an effective output

resistance of 50

W. The amplifier is stable with a 10 nF capacitive

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