Datasheet

AD8628/AD8629/AD8630
Rev. I | Page 14 of 24
FUNCTIONAL DESCRIPTION
The AD8628/AD8629/AD8630 are single-supply, ultrahigh
precision rail-to-rail input and output operational amplifiers.
The typical offset voltage of less than 1 µV allows these amplifiers
to be easily configured for high gains without risk of excessive
output voltage errors. The extremely small temperature drift
of 2 nV/°C ensures a minimum offset voltage error over their
entire temperature range of −40°C to +125°C, making these
amplifiers ideal for a variety of sensitive measurement applica-
tions in harsh operating environments.
The AD8628/AD8629/AD8630 achieve a high degree of precision
through a patented combination of auto-zeroing and chopping.
This unique topology allows the AD8628/AD8629/AD8630 to
maintain their low offset voltage over a wide temperature range
and over their operating lifetime. The AD8628/AD8629/AD8630
also optimize the noise and bandwidth over previous generations
of auto-zero amplifiers, offering the lowest voltage noise of any
auto-zero amplifier by more than 50%.
Previous designs used either auto-zeroing or chopping to add
precision to the specifications of an amplifier. Auto-zeroing
results in low noise energy at the auto-zeroing frequency, at the
expense of higher low frequency noise due to aliasing of wideband
noise into the auto-zeroed frequency band. Chopping results in
lower low frequency noise at the expense of larger noise energy
at the chopping frequency. The AD8628/AD8629/AD8630
family uses both auto-zeroing and chopping in a patented ping-
pong arrangement to obtain lower low frequency noise together
with lower energy at the chopping and auto-zeroing frequencies,
maximizing the signal-to-noise ratio for the majority of
applications without the need for additional filtering. The
relatively high clock frequency of 15 kHz simplifies filter
requirements for a wide, useful noise-free bandwidth.
The AD8628 is among the few auto-zero amplifiers offered in
the 5-lead TSOT package. This provides a significant improvement
over the ac parameters of the previous auto-zero amplifiers. The
AD8628/AD8629/AD8630 have low noise over a relatively wide
bandwidth (0 Hz to 10 kHz) and can be used where the highest
dc precision is required. In systems with signal bandwidths of
from 5 kHz to 10 kHz, the AD8628/AD8629/AD8630 provide
true 16-bit accuracy, making them the best choice for very high
resolution systems.
1/f NOISE
1/f noise, also known as pink noise, is a major contributor to
errors in dc-coupled measurements. This 1/f noise error term
can be in the range of several µV or more, and, when amplified
with the closed-loop gain of the circuit, can show up as a large
output offset. For example, when an amplifier with a 5 µV p-p
1/f noise is configured for a gain of 1000, its output has 5 mV of
error due to the 1/f noise. However, the AD8628/AD8629/AD8630
eliminate 1/f noise internally, thereby greatly reducing output errors.
The internal elimination of 1/f noise is accomplished as follows.
1/f noise appears as a slowly varying offset to the AD8628/AD8629/
AD8630 inputs. Auto-zeroing corrects any dc or low frequency
offset. Therefore, the 1/f noise component is essentially removed,
leaving the AD8628/AD8629/AD8630 free of 1/f noise.
One advantage that the AD8628/AD8629/AD8630 bring to
system applications over competitive auto-zero amplifiers is their
very low noise. The comparison shown in Figure 49 indicates
an input-referred noise density of 19.4 nV/√Hz at 1 kHz for
the AD8628, which is much better than the Competitor A
and Competitor B. The noise is flat from dc to 1.5 kHz, slowly
increasing up to 20 kHz. The lower noise at low frequency is
desirable where auto-zero amplifiers are widely used.
02735-046
MK AT 1kHz FOR ALL 3 GRAPHS
FREQUENCY (kHz)
VOLTAGE NOISE DENSITY (nV/√Hz)
120
105
90
75
60
45
30
15
0
042861012
COMPETITOR A
(89.7nV/√Hz)
COMPETITOR B
(31.1nV/√Hz)
AD8628
(19.4nV/√Hz)
Figure 49. Noise Spectral Density of AD8628 vs. Competition