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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. Input

voltage noise, or en, is strictly a function of the amplifier used.

The Johnson noise from a resistor is a function of the resistance

and the temperature. Input current noise, or in, creates an equiva-

lent 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:

kTr

i r

n TOTAL

n S

()









(15)

Where, en = The input voltage noise of the amplifier,

in = The input current noise of the amplifier,

rS = Source resistance connected to the noninverting

terminal,

k = Boltzmann’s constant (1.38

-23 J/K)

T = Ambient temperature in Kelvin (K = 273.15 +

°C)

The input voltage noise density, en of the AD855x is 42 nV/

√Hz,

and the input noise, in, is 2 fA/

√Hz. The e

n, TOTAL will be domi-

nated by input voltage noise provided the source resistance is less

than 106 k

Ω. With source resistance greater than 106 kΩ, the

overall noise of the system will be dominated by the Johnson

noise of the resistor itself.

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

does not become a dominant term unless rS is greater than 4 G

Ω,

which is an impractical value of source resistance.

The total noise, en, TOTAL, is expressed in volts per square-root

Hertz, and the equivalent rms noise over a certain bandwidth

can be found as:

n TOTAL

=×

(16)

Where BW is the bandwidth of interest in Hertz.

For a complete treatise on circuit noise analysis, please refer to the

1995 Linear Design Seminar book available from Analog Devices.

Output Overdrive Recovery

The AD855x amplifiers have an excellent overdrive recovery of

only 200

µs from either supply rail. This characteristic is particu-

larly difficult for autocorrection amplifiers, as the nulling amplifier

requires a nontrivial amount of time to error correct the main am-

plifier back to a valid output. Figure 23 and Figure 24 show the

positive and negative overdrive recovery time 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 half-way 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. Most competitors’ auto-

correction amplifiers require a number of autozero clock cycles

to recover from output overdrive and some can take several

milliseconds for the output to settle properly.

Input Overvoltage Protection

Although the AD855x is a rail-to-rail input amplifier, care should

be taken to ensure that the potential difference between the in-

puts does not exceed +5 V. Under normal operating conditions,

the amplifier will correct its output to ensure the two inputs are at

the same voltage. However, if the device is configured as a com-

parator, 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 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 will 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 will become forward-biased. Without current limit-

ing, 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 2 mA maximum.

Output Phase Reversal

Output phase reversal occurs in some amplifiers when the input

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

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

these amplifiers will 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 which results in

the erratic output behavior.

The AD855x 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. This will ensure

the output will not reverse its phase.

Capacitive Load Drive

The AD855x has excellent capacitive load driving capabilities

and can safely drive up to 10 nF from a single +5 V supply.

Although the device is stable, capacitive loading will limit the

bandwidth of the amplifier. Capacitive loads will also increase

the amount of overshoot and ringing at the output. An R-C

snubber network, Figure 54, can be used to compensate the

amplifier against capacitive load ringing and overshoot.

+5V

VOUT

VIN

200mV p-p

AD855x

4.7nF

0.47 F

Figure 54. Snubber Network Configuration for Driving

Capacitive Loads

Although the snubber will not recover the loss of amplifier band-

width from the load capacitance, it will allow the amplifier to drive

larger values of capacitance while maintaining a minimum of

overshoot and ringing. Figure 55 shows the output of an AD855x

driving a 1 nF capacitor with and without a snubber network.

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