Data Sheet

AD8551/AD8552/AD8554

Rev. F | Page 17 of 24

INTERMODULATION DISTORTION

The AD8551/AD8552/AD8554 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

0

10

1

–20

–40

–60

–80

–100

–120

2

3

4

5

6

7

8

9

VSY = 5V

AV = 0dB

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

FREQUENCY (kHz)

0

–140

0

10

1

–20

–40

–60

–80

–100

–120

2

3

4

5

6

7

8

9

VSY = 5V

AV = 60dB

Figure 57. Spectral Analysis of AD8551/AD8552/AD8554 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

0

10

1

–20

–40

–60

–80

–100

–120

2

3

4

5

6

7

8

9

VSY = 5V

AV = 60dB

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.

100Ω

100kΩ

3.3nF

VIN = 1mV rms

@ 200Hz

Figure 59. Reducing Autocorrection Clock Noise Using a Feedback Capacitor

FREQUENCY (kHz)

0

0

10

1

–20

–40

–60

–80

–100

–120

2

3

4

5

6

7

8

9

VSY = 5V

AV = 60dB

Figure 60. Spectral Analysis Using a Feedback Capacitor