AD8551/AD8552/AD8554

Rev. C | Page 17 of 24

Because the AD855x amplifiers are self-correcting op amps, they

do not have increasing flicker noise at lower frequencies. In

essence, low frequency noise is treated as a slowly varying offset

error and is greatly reduced as a result of autocorrection. The

correction becomes more effective as the noise frequency

approaches dc, offsetting the tendency of the noise to increase

exponentially as frequency decreases. This allows the AD855x

to have lower noise near dc than standard low noise amplifiers

that are susceptible to 1/f noise.

INTERMODULATION DISTORTION

The AD855x can be used as a conventional op amp for gain/

bandwidth combinations up to 1.5 MHz. The auto-zero correction

frequency of the device is fixed at 4 kHz. Although a trace

amount of this frequency feeds through to the output, the

amplifier can be used at much higher frequencies. Figure 56

shows the spectral output of the AD8552 with the amplifier

configured for unity gain and the input grounded.

The 4 kHz auto-zero clock frequency appears at the output with

less than 2 μV of amplitude. Harmonics are also present, but at

reduced levels from the fundamental auto-zero clock frequency.

The amplitude of the clock frequency feedthrough is proportional

to the closed-loop gain of the amplifier. Like other autocorrection

amplifiers, at higher gains there is more clock frequency

feedthrough. Figure 57 shows the spectral output with the

amplifier configured for a gain of 60 dB.

FREQUENCY (kHz)

0

–140

010

1

–20

–40

–60

–80

–100

–120

2

3

4

5

678

9

Figure 56. Spectral Analysis of AD8552 Output in Unity Gain Configuration

FREQUENCY (kHz)

0

–140

01

1

–20

–40

–60

–80

–100

–120

2

3

4

5

678

9

0

Figure 57. Spectral Analysis of AD855x Output with +60 dB Gain

When an input signal is applied, the output contains some

degree of intermodulation distortion (IMD). This is another

characteristic feature of all autocorrection amplifiers. IMD

appears as sum and difference frequencies between the input

signal and the 4 kHz clock frequency (and its harmonics) and is

at a level similar to, or less than, the clock feedthrough at the

output. The IMD is also proportional to the closed-loop gain of

the amplifier. Figure 58 shows the spectral output of an AD8552

configured as a high gain stage (+60 dB) with a 1 mV input

signal applied. The relative levels of all IMD products and

harmonic distortion add up to produce an output error of

−60 dB relative to the input signal. At unity gain, these add

up to only −120 dB relative to the input signal.

IMD < 100µV rms

OUTPUT SIGNAL

1V rms @ 200Hz

FREQUENCY (kHz)

0

010

1

–20

–40

–60

–80

–100

–120

23

456

7

89

Figure 58. Spectral Analysis of AD8552 in High Gain with a 1 mV Input Signal

For most low frequency applications, the small amount of auto-

zero clock frequency feedthrough does not affect the precision

of the measurement system. If it is desired, the clock frequency

feedthrough can be reduced through the use of a feedback

capacitor around the amplifier. However, this reduces the

bandwidth of the amplifier. Figure 59 and Figure 60 show a

configuration for reducing the clock feedthrough and the

corresponding spectral analysis at the output. The −3 dB

bandwidth of this configuration is 480 Hz.