AD8597/AD8599

Rev. B | Page 15 of 20

FUNCTIONAL OPERATION

INPUT VOLTAGE RANGE

The AD8597/AD8599 are not rail-to-rail input amplifiers;

therefore, care is required to ensure that both inputs do not

exceed the input voltage range. 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 the input voltage range, the loop opens and 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 may flow through these diodes, causing

permanent damage to the device. If inputs are subject to over-

voltage, insert appropriate series resistors to limit the diode

current to less than 5 mA maximum.

The input stage has two diodes between the input pins to

protect the differential pair. Under high slew rate conditions,

when the op amp is connected as a voltage follower, the diodes

may become forward-biased and the source may try to drive

the output. A small resistor should be placed in the feedback

loop and in the noninverting input. The noise of a 100 Ω

resistor at room temperature is ~1.25 nV/√Hz, which is higher

than the AD8597/AD8599. Thus, there is a tradeoff between

noise performance and protection. If possible, limiting should

be placed earlier in the signal path. For further details, see the

Amplifier Input Protection…Friend or Foe article at

http://www.analog.com/amplifier_input.

Because of the large transistors used to achieve low noise, the

input capacitance may seem rather high. To take advantage of

the low noise performance, impedance around the op amp should

be low, less than 500 Ω. Under these conditions, the pole from

the input capacitance should be greater than 50 MHz, which

does not affect the signal bandwidth.

OUTPUT PHASE REVERSAL

Output phase reversal occurs in some amplifiers when the

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

mode voltage is moved outside the input voltage 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 that causes a radical shifting of

internal voltages that results in the erratic output behavior.

The AD8597/AD8599 amplifiers have been carefully designed

to prevent any output phase reversal if both inputs are main-

tained within the specified input voltage range. If one or both

inputs exceed the input voltage range but remain within the

supply rails, the op amp specifications, such as CMRR, are not

guaranteed, but the output remains close to the correct value.

NOISE AND SOURCE IMPEDANCE CONSIDERATIONS

The AD8597/AD8599 ultralow voltage noise of 1.1 nV/√Hz is

achieved with special input transistors running at high collector

current. Therefore, it is important to consider the total input-

(1)

This equation is plotted for the AD8597/AD8599 in Figure 58.

Because optimum dc performance is obtained with matched

source resistances, this case is considered even though it is clear

from Equation 1 that eliminating the balancing source resistance

amplifier dominates. As source resistance increases, the Johnson

achieved; the current noise component is larger than the

resistor noise.

100

1

10

0.1

10

100

1k

10k

SOURCE RESISTANCE (Ω)

TOTAL NOISE

RESISTOR NOISE

ONLY

Figure 58. Noise vs. Source Resistance