ADA4177-2ARZ-R7 datasheet(26/33 Pages) AD

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ADA4177-1/ADA4177-2/ADA4177-4

Data Sheet

Rev. E | Page 26 of 33

APPLICATIONS INFORMATION

ACTIVE OVERVOLTAGE PROTECTION

The ADA4177-1/ADA4177-2/ADA4177-4 use active overvoltage

protection to protect the devices 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 its added

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.

RESISTOR

AMP

TOTAL

where:

NTOTAL is the total noise.

NOP AMP is the op amp noise.

NRESISTOR 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 integrated input protection for

overvoltage conditions, the noise trade-off is avoided.

TOTAL SOURCE RESISTANCE

30000

25000

20000

15000

10000

5000

TOTAL NOISE

RESISTOR NOISE

ADA4177-1/ADA4177-2/ADA4177-4 NOISE

Figure 85. Equivalent Thermal Noise vs. Total Source Resistance

Add External Clamping Diodes

Precision op amps have a low VOS 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 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 (ROVP), shown in Figure 86.

ROVP

VIN

VOUT

V–

V+

Figure 86. Common Scheme for Protecting Precision Amplifier Inputs from

Overvoltage Conditions

If the signal source at VIN is driven to one diode voltage beyond

the op amp supplies, the fault current is limited by ROVP. 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 ROVP, creating a

voltage drop. The voltage drop on ROVP appears as a variation in

VOS, 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.

J1B

J1A

VIN1

J2B

J2A

VIN2

V+

V–

Figure 87. Conceptual Schematic of the Inputs of the

ADA4177-1/ADA4177-2/ADA4177-4

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