AD8628/AD8629

Rev. C | Page 16 of 20

INPUT OVERVOLTAGE PROTECTION

Although the AD8628/AD8629 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 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 of 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 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 have been designed so that

internal settling occurs within two clock cycles after output

saturation happens. 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

enhances performance when they 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 (500

µs/DIV)

0V

0V

CH1 = 50mV/DIV

CH2 = 1V/DIV

Figure 56. Positive Input Overload Recovery for the AD8628

TIME (500

µs/DIV)

0V

0V

CH1 = 50mV/DIV

CH2 = 1V/DIV

Figure 57. Positive Input Overload Recovery for LTC2050

TIME (500

µs/DIV)

0V

0V

CH1 = 50mV/DIV

CH2 = 1V/DIV

Figure 58. Positive Input Overload Recovery for LMC2001