Datasheet

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
101
OUTPUT SIGNAL (dB)
–20
–40
–60
–80
–100
–120
2 3 4
5 6
7
8 9
V
SY
= 5V
A
V
= 0dB
01101-056
Figure 56. Spectral Analysis of AD8552 Output in Unity Gain Configuration
FREQUENCY (kHz)
0
–140
0
101
OUTPUT SIGNAL (dB)
–20
–40
–60
–80
–100
–120
2 3 4 5 6 7 8 9
V
SY
= 5V
A
V
= 60dB
01101-057
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
101
OUTPUT SIGNAL (dB)
–20
–40
–60
–80
–100
–120
2 3 4 5 6 7 8 9
V
SY
= 5V
A
V
= 60dB
01101-058
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
V
IN
= 1mV rms
@ 200Hz
01101-059
Figure 59. Reducing Autocorrection Clock Noise Using a Feedback Capacitor
FREQUENCY (kHz)
0
0 101
OUTPUT SIGNAL
–20
–40
–60
–80
–100
–120
2 3 4 5 6 7 8 9
V
SY
= 5V
A
V
= 60dB
01101-060
Figure 60. Spectral Analysis Using a Feedback Capacitor