AD8571/AD8572/AD8574

–13–

REV. 0

FREQUENCY – kHz

0

160

0

10

1

2345

6

789

20

40

60

80

100

120

140

Figure 50a. Spectral Analysis of AD857x Output in

Unity Gain Configuration

FREQUENCY – kHz

0

0

10

1

2345

6

789

20

40

60

80

100

120

Figure 50b. Spectral Analysis of AD857x Output with

60 dB Gain

Figure 51 shows the spectral output of an AD8572 configured

in a high gain (60 dB) with a 1 mV input signal applied. Note

the absence of any IMD products in the spectrum. The signal-

to-noise (SNR) ratio of the output signal is better than 60 dB,

or 0.1%.

FREQUENCY – kHz

0

0

10

1

2345

6

789

20

40

60

80

100

120

Figure 51. Spectral Analysis of AD857x in High Gain with

an Input Signal

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

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

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:

e

e

kTr

i r

n TOTAL

n

s

n s

,

=+

+

()









2

2

1

2

4

(15)

terminal,

k = Boltzmann’s constant (1.38

10

T = Ambient temperature in Kelvin (K = 273.15 +

°C)

√Hz,

√Hz. The e

nated by input voltage noise provided the source resistance is less

than 172 k

Ω. With source resistance greater than 172 kΩ, the

overall noise of the system will be dominated by the Johnson

noise of the resistor itself.

Ω,

which is an impractical value of source resistance.

Hertz, and the equivalent rms noise over a certain bandwidth

can be found as:

ee

BW

n

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 AD857x 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 ampli-

fier requires a substantial amount of time to error correct the

main amplifier back to a valid output. Figure 23 and Figure 24

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 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.