AD8551/AD8552/AD8554
0
؊20
0
؊20
V
A
= +5V
= +60dB
OUTPUT SIGNAL
1Vrms @ 200Hz
SY
V
A
= +5V
= 0dB
V
SY
V
؊40
؊40
؊60
؊60
؊80
؊80
IMD < 100Vrms
؊100
؊120
؊140
؊100
؊120
0
0
1
2
3
4
5
6
7
8
9
10
1
2
3
4
5
6
7
8
9
10
FREQUENCY – kHz
FREQUENCY – kHz
Figure 50. Spectral Analysis of AD855x Output in Unity
Gain Configuration
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.
0
V
A
= +5V
= +60dB
SY
؊20
؊40
V
؊60
؊80
3.3nF
؊100
؊120
؊140
100k⍀
100⍀
V
= 1mV rms
@ 200Hz
0
IN
1
2
3
4
5
6
7
8
9
10
FREQUENCY – kHz
Figure 51. Spectral Analysis of AD855x Output with
+60 dB Gain
Figure 53a. Reducing Autocorrection Clock Noise with a
Feedback Capacitor
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.
0
V
A
= +5V
= +60dB
SY
V
؊20
؊40
؊60
؊80
؊100
؊120
0
1
2
3
4
5
6
7
8
9
10
FREQUENCY – kHz
Figure 53b. Spectral Analysis Using a Feedback Capacitor
REV. 0
–13–
ADI [ ADI ]
