AD8682 datasheet(16/20 Pages) AD

16 / 20 page

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.

–V

D7 R2

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

–40

–50

–20

–30

–10

VOLTAGE (V)

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, 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.

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).

100

200

300

400

500

600

700

800

900

1000

VSY (V)

NORMAL OPERATION

ONE COMPARATOR, VOUT HIGH

ONE COMPARATOR, VOUT LOW

Figure 51. Comparator Supply Current

AD8682 Datasheet(PDF) 16 Page - Analog Devices

AD8682 Datasheet(HTML) 16 Page - Analog Devices

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

Datasheet Download

Link URL