AD8551/AD8552/AD8554

–13–

REV. 0

FREQUENCY – kHz

0

140

0

10

1

2345

6

789

20

40

60

80

100

120

Figure 50. Spectral Analysis of AD855x Output in Unity

Gain Configuration

FREQUENCY – kHz

0

140

0

10

1

2345

6

789

20

40

60

80

100

120

Figure 51. Spectral Analysis of AD855x Output with

+60 dB Gain

When an input signal is applied, the output will contain some

degree of Intermodulation Distortion (IMD). This is another

characteristic feature of all autocorrection amplifiers. IMD will

show up as sum and difference frequencies between the input sig-

nal 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 ampli-

fier. Figure 52 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 would add up to only –120 dB relative

to the input signal.

FREQUENCY – kHz

0

0

10

1

2345

6

789

20

40

60

80

100

120

OUTPUT SIGNAL

1Vrms @ 200Hz

IMD < 100 Vrms

Figure 52. Spectral Analysis of AD855x in High Gain with

a 1 mV Input Signal

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

zero clock frequency feedthrough will not affect the precision of the

measurement system. Should it be desired, the clock frequency

feedthrough can be reduced through the use of a feedback capaci-

tor around the amplifier. However, this will reduce the bandwidth

of the amplifier. Figures 53a and 53b 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.

@ 200Hz

100

100k

3.3nF

Figure 53a. Reducing Autocorrection Clock Noise with a

Feedback Capacitor

FREQUENCY – kHz

0

0

10

1

2345

6

789

20

40

60

80

100

120

Figure 53b. Spectral Analysis Using a Feedback Capacitor