INPUT PROTECTION
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).
The inputs of the OPA627/637 are protected for voltages
between +VS + 2V and –VS – 2V. If the input voltage can
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, RS, to limit the current. Be aware that adding
resistance to the input will increase noise. The 4nV/√Hz
theoretical thermal noise of a 1kΩ 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 100Ω add negligible noise.
+VS
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.
–
VO
D
+
OPA627
D
D: IN4148 — 25nA Leakage
2N4117A — 1pA Leakage
Siliconix
Optional RS
–VS
=
(a)
IIN
–
+
VO
D
D
OPA627
D: 2N3904
=
(b)
NC
FIGURE 7. Input Protection Circuits.
SMALL SIGNAL RESPONSE
LARGE SIGNAL RESPONSE
(A)
(B)
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.
G = 1
–
+
OPA627
FIGURE 8. OPA627 Dynamic Performance, G = +1.
®
11
OPA627, 637
TI [ TEXAS INSTRUMENTS ]
