AD8571/AD8572/AD8574

Rev. B | Page 18 of 24

BROADBAND AND EXTERNAL RESISTOR NOISE CONSIDERATIONS

The total broadband noise output from any amplifier is

primarily a function of three types of noise: input voltage noise

from the amplifier, input current noise from the amplifier, and

Johnson noise from the external resistors used around the

amplifier used. The Johnson noise from a resistor is a function

creates an equivalent voltage noise proportional to the resistors

used around the amplifier. These noise sources are not correlated

with each other and their combined noise sums in a root-

squared-sum fashion. The full equation is given as

2

/

1

2

2

,

]

)

(

4

[

s

n

s

n

TOTAL

n

r

i

kTr

e

e

+

+

=

(15)

where:

T = ambient temperature in Kelvin (K = 273.15 + °C).

dominated by input voltage noise provided the source resistance

is less than 172 kΩ. With source resistance greater than 172 kΩ,

the overall noise of the system is dominated by the Johnson

noise of the resistor itself.

4 GΩ, which is an impractical value of source resistance.

Hertz, and the equivalent rms noise over a certain bandwidth

can be found as

BW

e

e

TOTAL

n

n

×

=

,

(16)

where BW is the bandwidth of interest in Hertz.

OUTPUT OVERDRIVE RECOVERY

The AD857x amplifiers have an excellent overdrive recovery of

only 200 μs from either supply rail. This characteristic is par-

ticularly difficult for autocorrection amplifiers, because the

nulling amplifier requires a substantial amount of time to error

correct the main amplifier back to a valid output. Figure 29 and

Figure 30 show the positive and negative overdrive recovery

time for the AD857x.

The output overdrive recovery for an autocorrection amplifier is

defined as the time it takes for the output to correct to its final

voltage from an overload state. It is measured by placing the

amplifier in a high gain configuration with an input signal that

forces the output voltage to the supply rail. The input voltage is

then stepped down to the linear region of the amplifier, usually

to halfway between the supplies. The time from the input signal

step-down to the output settling to within 100 μV of its final

value is the overdrive recovery time. Many competitors’ auto-

correction amplifiers require a number of auto-zero clock cycles

to recover from output overdrive and some can take several

milliseconds for the output to settle properly.

INPUT OVERVOLTAGE PROTECTION

Although the AD857x is a rail-to-rail input amplifier, care

should be taken to ensure that the potential difference between

the inputs does not exceed 5 V. Under normal operating condi-

tions, the amplifier corrects its output to ensure the two inputs

are at the same voltage. However, if the device is configured as a

comparator, or is under some unusual operating condition, the

input voltages may be forced to different potentials. This could

cause excessive current to flow through internal diodes in the

AD857x used to protect the input stage against overvoltage.

If either input exceeds either supply rail by more than 0.3 V,

large amounts of current 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 become forward-biased.

Without current-limiting, excessive amounts of current can

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 2 mA.

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 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 AD857x amplifier has been carefully designed to prevent

any output phase reversal, provided 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 2 mA to ensure the

output does not reverse its phase.