ADA4177-1 Datasheet, PDF (25/31 Pages) Analog Devices – OVP and EMI Protected, Precision, Low Noise and Bias Current Op Amps

Data Sheet
APPLICATIONS INFORMATION
ACTIVE OVERVOLTAGE PROTECTION
The ADA4177-1/ADA4177-2/ADA4177-4 use active overvoltage
protection to protect the device from damage when the inputs
are driven to a voltage up to 32 V above the positive supply
voltage or 32 V below the negative supply voltage. The
ADA4177-1/ADA4177-2/ADA4177-4 not only protect the
input from damage, but they also reduce the input noise.
Common Protection Methods
Add an External Series Input Resistor
When an op amp does not have input overvoltage protection,
moving the input voltage above or below the supply voltage can
cause excessive input current, which can damage the op amp. To
avoid this, add a series resistor at the input. To protect the op
amp from a 30 V transient beyond either rail, limit the input
current to 5 mA, and add a 6 kΩ series resistor to the input.
However, a trade-off of adding the series resister is that it adds
thermal noise. The 6 kΩ series resistor exhibits 10 nV/√Hz of
thermal noise, which adds quadrature thermal noise from the
resistor with the op amp noise.
N = _TOTAL
N _OP
2
AMP
+
N _RESISTOR²
where:
N_{OP AMP} is the op amp noise.
N_RESISTOR is the thermal noise generated by the resistor.
When the additional thermal noise from the series resistor is
added to the thermal noise (8 nV/√Hz) of the ADA4177-1/
ADA4177-2/ADA4177-4, the 6 kΩ series resistor brings the
total thermal noise to 12 nV/√Hz, which is a 70% increase in
thermal noise. Figure 85 shows how noise from the additional
source resistance adds to the total noise at the amplifier input;
the higher the source resistance, the higher the total noise.
Because the ADA4177-1/ADA4177-2/ADA4177-4 have inte-
grated input protection for overvoltage conditions, the noise
trade-off is avoided.
20
18
TOTAL NOISE
16
14
12
RESISTOR NOISE
10
8
ADA4177-2/ADA4177-4 NOISE
6
4
2
0
0
5000 10000 15000 20000 25000 30000
TOTAL SOURCE RESISTANCE
Figure 85. Equivalent Thermal Noise vs. Total Source Resistance
ADA4177-1/ADA4177-2/ADA4177-4
Add External Clamping Diodes
Precision op amps have a low offset voltage (V_OS) and a high
common-mode rejection ratio (CMRR). Both of these
characteristics simplify system calibration and minimize
dynamic error. To maintain these specifications in the presence
of electrostatic discharge (ESD) events, bipolar op amps often
have internal clamp diodes and small limiting resistors in series
with their inputs; however, these do not address fault conditions
where the inputs exceed the rails. In these cases, the system
designer commonly adds clamping diodes (D1 and D2) along with
a series resistor (R_OVP), as shown in Figure 86.
V+
R_F
R_OVP
D2
V_IN
D1
V_OUT
V–
Figure 86. Common Scheme for Protecting Precision Amplifier Inputs from
Overvoltage Conditions
If the signal source at V_IN is driven to one diode voltage beyond
the op amp supplies, the fault current is limited by R_OVP. Schottky
diodes have a low forward knee voltage of 200 mV less than a
typical small signal diode. Therefore, all overvoltage currents
are shunted through the external diodes (D1 and D2). The reverse
leakage current for a typical Schottky diode is extremely variable
with the reverse voltage level. Therefore, as the noninverting
input of the op amp swings, the D1 and D2 leakage currents do
not match, and the differences pass through R_OVP, creating a
voltage drop. The voltage drop on R_OVP appears as a variation in
V_OS, which can drastically reduce the CMRR performance.
Because the ADA4177-1/ADA4177-2/ADA4177-4 have integrated
input protection during overvoltage conditions, the degradation
in performance is avoided.
Input Protection Circuit
The ADA4177-1/ADA4177-2/ADA4177-4 inputs provide
overvoltage protection without the trade-offs encountered in
the common design methods. The conceptual schematic of the
input is shown in Figure 87.
V+
J1B
J2B
V_IN1
J1A
V_IN2
J2A
V–
Figure 87. Conceptual Schematic of the Inputs of the
ADA4177-1/ADA4177-2/ADA4177-4
Rev. C | Page 25 of 31