Datasheet

ADA4092-4
Rev. B | Page 16 of 20
INPUT OVERVOLTAGE PROTECTION
The ADA4092-4 has two different ESD circuits for enhanced
protection, as shown in Figure 49.
08803-123
D4
D3 D1
+V
–V
D2
D8
D7
R2
D5
R1
D6
Figure 49. 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 50.
–3
–2
–1
0
1
2
3
4
5
–40–50 –20 0 20 30–30 10–10 40 50
08803-100
VOLTAGE (V)
CURRENT (mA)
Figure 50. DIAC Transfer Characteristic
For a worst-case design analysis, consider two cases. The
ADA4092-4 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 48.
Therefore, consider two conditions to determine which case is
the limiting factor.
1. Consider, for example, that when operating on ±15 V, t he
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.
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
the first condition, +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 ADA4092-4 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 ADA4092-4 is approximately
25 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
conditions only. The correct amplifier operation input voltage
range (IVR) is specified in Table 2, Table 3, and Table 4.
COMPARATOR OPERATION
Although op amps are quite different from comparators,
occasionally an unused section of a dual or a quad op amp
can be pressed into service as a comparator; however, this is not
recommended. For rail-to-rail output op amps, the output stage
is generally a ratioed current mirror with bipolar or MOSFET
transistors. With the part operating open loop, the second stage
increases the current drive to the ratioed mirror to close the loop,
but it cannot, which results in an increase in supply current. With
three of the op amps operating normally and the fourth one in
comparator mode, the supply current increases by about 200 µA
(see Figure 51).
0
100
200
300
400
500
600
700
800
900
1000
0 4 8 12 16 20 24 28 32 36
I
SY
(µA)
V
SY
(V)
NORMAL OPERATION
ONE COMPARATOR, V
OUT
HIGH
ONE COMPARATOR, V
OUT
LOW
08803-051
Figure 51. Comparator Supply Current