9

®

OPA111

should completely surround the high impedance input leads

and should be connected to a low impedance point which is

at the signal input potential.

The amplifier case should be connected to any input shield

or guard via pin 8. This insures that the amplifier itself is

fully surrounded by guard potential, minimizing both leak-

age and noise pickup (see Figure 3).

If guarding is not required, pin 8 (case) should be connected

to ground.

FIGURE 4. Voltage Noise Spectral Density vs Source

Resistance.

FIGURE 6. Pyroelectric Infrared Detector.

FIGURE 3. Connection of Input Guard.

NOISE: FET VERSUS BIPOLAR

Low noise circuit design requires careful analysis of all

noise sources. External noise sources can dominate in many

cases, so consider the effect of source resistance on overall

operational amplifier noise performance. At low source

impedances, the lower voltage noise of a bipolar operational

amplifier is superior, but at higher impedances the high

current noise of a bipolar amplifier becomes a serious

liability. Above about 15k

Ω, the OPA111 will have a lower

total noise than an OP-27 (see Figure 4).

BIAS CURRENT CHANGE

VERSUS COMMON-MODE VOLTAGE

The input bias current of most popular BIFET operational

amplifiers are affected by common-mode voltage (Figure 5).

Higher input FET gate-to-drain voltage causes leakage and

ionization (bias) currents to increase. Due to its cascode

input stage, the extremely low bias current of the OPA111 is

not compromised by common-mode voltage.

APPLICATIONS CIRCUITS

Figures 6 through 18 are circuit diagrams of various appli-

cations for the OPA111.

FIGURE 5. Input Bias Currrent vs Common-Mode Voltage.

80

60

40

20

0

–20

–15

–10

–5

0

5

10

15

Common-Mode Voltage (VDC)

OPA111

AD547

LF156/157

LF155

T

manufacturers' published

typical data.

OP-15/16/17

"Perfect Bias Current Cancellation"

OPA111

2

3

6

Output

8

1000pF Polystyrene

1000M

Ω

1000M

Ω

Pyroelectric

Detector

NOTE: Pyroelectric

detectors respond

to rate-of-change

(AC signal) only.

3

2

4

5

6

7

8

1

OPA111

2

3

6

8

In

Out

Inverting

TO-99 Bottom View

OPA111

2

3

6

In

Out

Non-Inverting

8

OPA111

2

3

6

In

Out

Buffer

8

Board layout for input guarding: guard top and bottom of board.

100

1k

10k

100k

1M

10M

1k

100

10

1

BM

OP-27 + Resistor

OPA111 + Resistor

Resistor Noise Only

OPA111 + Resistor

Resistor Noise Only

OP-27 + Resistor

Source Resistance, R

E

O

R

S

E

e

N

S