Data Sheet

ADA4091-2/ADA4091-4

Rev. H | Page 15 of 20

INPUT OVERVOLTAGE PROTECTION

The ADA4091-2/ADA4091-4 has two different ESD circuits for

enhanced protection, as shown in Figure 48.

D4

D3

D1

+V

–V

D2

D8

D7 R2

D5

R1

D6

Figure 48. Complete Input Protection Network

One circuit is a series resistor of 5 kΩ to the internal inputs and

diodes (D1 and D2 or D5 and D6) from the internal inputs to

the supply rails. The other protection circuit is a circuit with

two DIACs (D3 and D4 or D7 and D8) to the supply rails. A

DIAC can be considered a bidirectional Zener diode with a

transfer characteristic, as shown in Figure 49.

–3

–2

–1

0

1

2

3

4

5

–40

–50

–20

0

20

30

–30

10

–10

40

50

VOLTAGE (V)

Figure 49. DIAC Transfer Characteristic

For a worst-case design analysis, consider two cases. The

ADA4091-2/ADA4091-4 has a normal ESD structure from the

internal operational amplifier inputs to the supply rails. In addition,

it has 42 V DIACs from the external inputs to the rails, as shown in

Figure 47.

Therefore, two conditions need to be considered to determine

which case is the limiting factor.

•

Condition 1. Consider, for example, that when operating

on ±15 V, the inputs can go +42 V above the negative

supply rail. With the −V pin equal to −15 V, +42 V above

this supply (the negative supply) is +27 V.

•

Condition 2. There is a restriction on the input current of

5 mA through a 5 kΩ resistor to the ESD structure to the

positive rail. In Condition 1, +27 V through the 5 kΩ resistor

to +15 V gives a current of 2.4 mA. Thus, the DIAC is the

limiting factor. If the ADA4091-2/ADA4091-4 supply voltages

are changed to ±5 V, then −5 V + 42 V = +37 V. However,

+5 V + (5 kΩ × 5 mA) = 30 V. Thus, the normal resistor

diode structure is the limitation when running on lower

supply voltages.

Additional resistance can be added externally in series with

each input to protect against higher peak voltages; however, the

additional thermal noise of the resistors must be considered.

The flatband voltage noise of the ADA4091-2/ADA4091-4 is

approximately 24 nV/√Hz, and a 5 kΩ resistor has a noise of 9

nV/√Hz. Adding an additional 5 kΩ resistor increases the total

noise by less than 15% root sum square (rss). Therefore, maintain

resistor values below this value (5 kΩ) when overall noise

performance is critical.

Note that this represents input protection under abnormal con-

ditions only. The correct amplifier operation input voltage range

(IVR) is specified in Table 2, Table 3, and Table 4.