AD8628/AD8629/AD8630

Rev. I | Page 16 of 24

INPUT OVERVOLTAGE PROTECTION

Although the AD8628/AD8629/AD8630 are rail-to-rail input

amplifiers, care should be taken to ensure that the potential

difference between the inputs does not exceed the supply voltage.

Under normal negative feedback operating conditions, the

amplifier corrects its output to ensure that the two inputs are at

the same voltage. However, if either input exceeds either supply

rail by more than 0.3 V, large currents begin to flow through the

ESD protection diodes in the amplifier.

These diodes are connected between the inputs and each supply

rail to protect the input transistors against an electrostatic discharge

event, and they are normally reverse-biased. However, if the input

voltage exceeds the supply voltage, these ESD diodes can become

forward-biased. Without current limiting, excessive amounts

of current could flow through these diodes, causing permanent

damage to the device. If inputs are subject to overvoltage,

appropriate series resistors should be inserted to limit the diode

current to less than 5 mA maximum.

OUTPUT PHASE REVERSAL

Output phase reversal occurs in some amplifiers when the input

common-mode voltage range is exceeded. As common-mode

voltage is moved outside the common-mode range, the outputs of

these amplifiers can suddenly jump in the opposite direction to

the supply rail. This is the result of the differential input pair

shutting down, causing a radical shifting of internal voltages

that results in the erratic output behavior.

The AD8628/AD8629/AD8630 amplifiers have been carefully

designed to prevent any output phase reversal, provided that

both inputs are maintained within the supply voltages. If one or

both inputs could exceed either supply voltage, a resistor should

be placed in series with the input to limit the current to less than

5 mA. This ensures that the output does not reverse its phase.

OVERLOAD RECOVERY TIME

Many auto-zero amplifiers are plagued by a long overload recovery

time, often in ms, due to the complicated settling behavior of

the internal nulling loops after saturation of the outputs. The

AD8628/AD8629/AD8630 have been designed so that internal

settling occurs within two clock cycles after output saturation

occurs. This results in a much shorter recovery time, less

than 10 μs, when compared to other auto-zero amplifiers. The

wide bandwidth of the AD8628/AD8629/AD8630 enhances

performance when the parts are used to drive loads that inject

transients into the outputs. This is a common situation when an

amplifier is used to drive the input of switched capacitor ADCs.

TIME (500µs/DIV)

0V

0V

CH1 = 50mV/DIV

CH2 = 1V/DIV

Figure 56. Positive Input Overload Recovery for the AD8628

TIME (500µs/DIV)

0V

0V

CH1 = 50mV/DIV

CH2 = 1V/DIV

Figure 57. Positive Input Overload Recovery for Competitor A

TIME (500µs/DIV)

0V

0V

CH1 = 50mV/DIV

CH2 = 1V/DIV

Figure 58. Positive Input Overload Recovery for Competitor B