11

®

OPA627, 637

INPUT PROTECTION

The inputs of the OPA627/637 are protected for voltages

between +V

exceed these limits, the amplifier should be protected. The

diode clamps shown in Figure 7a will prevent the input

voltage from exceeding one forward diode voltage drop

beyond the power supplies—well within the safe limits. If

the input source can deliver current in excess of the maxi-

mum forward current of the protection diodes, use a series

resistor, R

to limit the current. Be aware that adding

resistance to the input will increase noise. The 4nV/

√Hz

theoretical thermal noise of a 1k

Ω resistor will add to the

4.5nV/

√Hz noise of the OPA627/637 (by the square-root of

the sum of the squares), producing a total noise of 6nV/

√Hz.

Resistors below 100

Ω add negligible noise.

Leakage current in the protection diodes can increase the

total input bias current of the circuit. The specified maxi-

mum leakage current for commonly used diodes such as the

1N4148 is approximately 25nA—more than a thousand

times larger than the input bias current of the OPA627/637.

Leakage current of these diodes is typically much lower and

may be adequate in many applications. Light falling on the

junction of the protection diodes can dramatically increase

leakage current, so common glass-packaged diodes should

be shielded from ambient light. Very low leakage can be

achieved by using a diode-connected FET as shown. The

2N4117A is specified at 1pA and its metal case shields the

junction from light.

Sometimes input protection is required on I/V converters of

inverting amplifiers (Figure 7b). Although in normal opera-

tion, the voltage at the summing junction will be near zero

(equal to the offset voltage of the amplifier), large input

transients may cause this node to exceed 2V beyond the

power supplies. In this case, the summing junction should

be protected with diode clamps connected to ground. Even

with the low voltage present at the summing junction,

common signal diodes may have excessive leakage current.

Since the reverse voltage on these diodes is clamped, a

diode-connected signal transistor can be used as an inexpen-

sive low leakage diode (Figure 7b).

FIGURE 7. Input Protection Circuits.

–

+

–V

S

+V

S

Optional R

S

V

O

D: IN4148 — 25nA Leakage

2N4117A — 1pA Leakage

(a)

=

–

+

I

IN

V

O

D

D

D

(b)

D

D: 2N3904

=

NC

Siliconix

OPA627

OPA627

FPO

When used as a unity-gain buffer, large common-mode input voltage steps

produce transient variations in input-stage currents. This causes the rising

edge to be slower and falling edges to be faster than nominal slew rates

observed in higher-gain circuits.

(A)

(B)

SMALL SIGNAL RESPONSE

LARGE SIGNAL RESPONSE

FIGURE 8. OPA627 Dynamic Performance, G = +1.

–

+

OPA627

G = 1