ada4091-2arz-r2 Analog Devices, Inc., ada4091-2arz-r2 Datasheet - Page 14

• Manufacturer Part Number: ada4091-2arz-r2
• Description: Micropower, Single-supply, Rail-to-rail Input/output Operational Amplifier
• Manufacturer: Analog Devices, Inc.

ADA4091-2
INPUT OVERVOLTAGE PROTECTION
The ADA4091-2 has two different ESD circuits for enhanced
protection as shown in Figure 39. 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 bidi-
rectional Zener diode with a transfer characteristic as shown
in Figure 39.
For a worst-case design analysis, consider two cases. The
ADA4091-2 has the normal ESD structure from the internal op
amp inputs to the supply rails. In addition, it has 42 V DIACs
from the external inputs to the rails as shown in Figure 37.
Therefore, two conditions have to be considered to determine
which one is the limiting factor.
•
•
–1
–2
–3
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 also 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 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.
5
4
3
2
1
0
–50
–40
Figure 38. DIAC Transfer Characteristic
–30
–20
–10
VOLTAGE (V)
0
10
20
30
40
50
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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 is approximately
24 nV/√Hz, and a 5 kΩ resistor has a noise of 9 nV/√Hz. Add-
ing an additional 5 kΩ resistor increases the total noise by less
than 15% root-sum-square (RSS). Therefore, resistor values
should be kept below this value if overall noise performance is
critical.
Note that this is input protection under abnormal conditions
only. The correct amplifier operation is only specified with an
input voltage range as shown in the Specifications section of
this data sheet.
Figure 39. Complete Input Protection Network
D7 R2
D8
D6
D5
D3
R1
D4
D2
D1
V+
V–