AD8628/AD8629
FUNCTIONAL DESCRIPTION
The AD8628/AD8629 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 of offset voltage error over their
entire temperature range of −40°C to +125°C, making these
amplifiers ideal for a variety of sensitive measurement
applications in harsh operating environments.
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 1,000, its output has 5 mV
of error due to the 1/f noise. But the AD8628/AD8629 eliminate
1/f noise internally, and thereby greatly reduce output errors.
The AD8628/AD8629 achieve a high degree of precision
through a patented combination of auto-zeroing and chopping.
This unique topology allows the AD8628/AD8629 to maintain
their low offset voltage over a wide temperature range and over
their operating lifetime. The AD8628/AD8629 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%.
The internal elimination of 1/f noise is accomplished as follows.
1/f noise appears as a slowly varying offset to AD8628/AD8629
inputs. Auto-zeroing corrects any dc or low frequency offset.
Therefore, the 1/f noise component is essentially removed,
leaving the AD8628/AD8629 free of 1/f noise.
One of the biggest advantages that the AD8628/AD8629 bring
to systems 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 LTC2050
and LMC2001. 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.
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
family use 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 (SNR) 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.
120
LTC2050
105
(89.7nV/√Hz)
90
75
60
45
30
LMC2001
(31.1nV/√Hz)
The AD8628 is among the few auto-zero amplifiers offered in
the 5-lead TSOT-23 package. This provides a significant
improvement over the ac parameters of the previous auto-zero
amplifiers. The AD8628/AD8629 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 provide true 16-bit accuracy, making them the best
choice for very high resolution systems.
15
0
AD8628
(19.4nV/√Hz)
MK AT 1kHz FOR ALL 3 GRAPHS
10 12
0
2
4
6
8
FREQUENCY (kHz)
Figure 49. Noise Spectral Density of AD8628 vs. Competition
Rev. C | Page 14 of 20
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