ADA4091-4ACPZ-R7 View Datasheet(PDF) - Analog Devices
Part Name
Description
MFG CO.
ADA4091-4ACPZ-R7 Datasheet PDF : 20 Pages
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INPUT OVERVOLTAGE PROTECTION
The ADA4091-x has two different ESD circuits for enhanced
protection, as shown in Figure 48.
+V
D7 R2
D8
D3 R1
D5
D6
D4
D1
D2
–V
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.
5
4
3
2
1
0
–1
–2
–3
–50 –40 –30 –20 –10 0 10 20 30 40 50
VOLTAGE (V)
Figure 49. DIAC Transfer Characteristic
ADA4091-2/ADA4091-4
For a worst-case design analysis, consider two cases. The
ADA4091-x has a 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 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-x 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-x 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.
Rev. E | Page 15 of 20
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