AD8551/AD8552/AD8554

Rev. C | Page 18 of 24

100Ω

100kΩ

3.3nF

@ 200Hz

Figure 59. Reducing Autocorrection Clock Noise Using a Feedback Capacitor

FREQUENCY (kHz)

0

0

1

–20

–40

–60

–80

–100

–120

2

3

4

5

678

9

10

Figure 60. Spectral Analysis Using a Feedback Capacitor

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 of the re-

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

2

_

4

S

n

S

n

TOTAL

n

R

i

kTr

e

e

+

+

=

(15)

Where:

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

the input voltage noise, provided the source resistance is less

than 106 kΩ. With source resistance greater than 106 kΩ, the

overall noise of the system is dominated by the Johnson noise of

the resistor itself.

Because the input current noise of the AD855x is very small,

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 AD855x 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 nontrivial 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

times for the AD855x.

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

stepdown to the output settling to within 100 μV of its final

value is the overdrive recovery time.

INPUT OVERVOLTAGE PROTECTION

Although the AD855x is a rail-to-rail input amplifier, exercise

care to ensure that the potential difference between the inputs

does not exceed 5 V. Under normal operating conditions, 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 can cause excessive

current to flow through internal diodes in the AD855x 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 pro-

tection diodes in the amplifier. These diodes connect 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 subjected to overvoltage, appropriate series resistors should

be inserted to limit the diode current to less than 2 mA maximum.