AD8551/AD8552/AD8554
0
–20
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.
V
A
= 5V
= 60dB
SY
V
–40
–60
–80
–100
–120
–140
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.
0
1
2
3
4
5
6
7
8
9
10
FREQUENCY (kHz)
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.
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.
0
V
= 5V
SY
A
= 0dB
V
–20
–40
0
V
A
= 5V
= 60dB
OUTPUT SIGNAL
1V rms @ 200Hz
SY
V
–20
–40
–60
–80
–60
–100
–120
–140
–80
IMD < 100µV rms
–100
–120
0
1
2
3
4
5
6
7
8
9
10
FREQUENCY (kHz)
Figure 56. Spectral Analysis of AD8552 Output in Unity Gain Configuration
0
1
2
3
4
5
6
7
8
9
10
FREQUENCY (kHz)
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.
Rev. C | Page 17 of 24
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