REV. A

OP27

–12–

3. Sudden motion in the vicinity of the device can also

“feedthrough” to increase the observed noise.

4. The test time to measure 0.1 Hz to 10 Hz noise should not

exceed 10 seconds. As shown in the noise-tester frequency

response curve, the 0.1 Hz corner is defined by only one

zero. The test time of 10 seconds acts as an additional zero

to eliminate noise contributions from the frequency band

below 0.1 Hz.

5. A noise-voltage-density test is recommended when measuring

noise on a large number of units. A 10 Hz noise-voltage-

density measurement will correlate well with a 0.1 Hz to 10 Hz

peak-to-peak noise reading, since both results are determined

by the white noise and the location of the 1/f corner frequency.

UNITY-GAIN BUFFER APPLICATIONS

≤ 100 Ω and the input is driven with a fast, large signal

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

pulsed operation diagram (Figure 3).

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 Ω, the output is

≤ 20 mA at 10 V);

the amplifier will stay in its active mode and a smooth transition

will occur.

Ω, a pole will be created with R

input capacitance (8 pF) that creates additional phase shift and

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

+

–

OP27

2.8V/ s

Figure 3. Pulsed Operation

COMMENTS ON NOISE

The OP27 is a very low-noise monolithic op amp. The outstanding

input voltage noise characteristics of the OP27 are achieved mainly

by operating the input stage at a high quiescent current. The input

bias and offset currents, which would normally increase, are held

to reasonable values by the input bias-current cancellation circuit.

±40 nA and 35 nA at 25°C

respectively. This is particularly important when the input has a

high source resistance. In addition, many audio amplifier design-

of previous designs have made direct coupling difficult, if not

impossible, to use.

Voltage noise is inversely proportional to the square root of bias

current, but current noise is proportional to the square root of

bias current. The OP27’s noise advantage disappears when high

source-resistors are used. Figures 4, 5, and 6 compare OP27’s

observed total noise with the noise performance of other devices

in different circuit applications.

Total Noise

Voltage Noise

Current Noise

R

sistor Noise

S

=

×

()





























2

2

2

12

Re

/

Figure 4 shows noise versus source-resistance at 1000 Hz. The

same plot applies to wideband noise. To use this plot, multiply

the vertical scale by the square root of the bandwidth.

10

50

10k

5

500

1k

5k

1

100

50

100

50k

OP07

5534

OP27/37

REGISTER

NOISE ONLY

OP08/108

1

2

Figure 4. Noise vs. Source Resistance (Including Resistor

Noise) at 1000 Hz

Ω, the OP27’s low voltage noise is maintained. With

Ω, total noise increases, but is dominated by the resis-

tor noise rather than current or voltage noise. lt is only beyond

Ω that current noise starts to dominate. The argument

can be made that current noise is not important for applica-

tions with low to moderate source resistances. The crossover

between the OP27, OP07, and OP08 noise occurs in the 15 k

Ω to

40 k

Ω region.

Figure 5 shows the 0.1 Hz to 10 Hz peak-to-peak noise. Here

the picture is less favorable; resistor noise is negligible and current

noise becomes important because it is inversely proportional to

the square root of frequency. The crossover with the OP07

occurs in the 3 k

Ω to 5 kΩ range depending on whether bal-

anced or unbalanced source resistors are used (at 3 k

100

50

10k

50

500

1k

5k

10

1k

500

100

50k

OP07

5534

OP27/37

REGISTER

NOISE ONLY

OP08/108

1

2

Figure 5. Peak-to-Peak Noise (0.1 Hz to 10 Hz) as Source

Resistance (Includes Resistor Noise)