REV. A–10–
AD8628
FUNCTIONAL DESCRIPTION
The AD8628 is a single-supply, ultrahigh precision rail-to-rail
input and output operational amplifier. The typical offset voltage
of less than 1 mV allows this amplifier 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 its entire temperature range of –40∞C
to +125∞C, making the AD8628 amplifier ideal for a variety of
sensitive measurement applications in harsh operating environ-
ments. The AD8628 achieves a high degree of precision through
a patented combination of auto-zeroing and chopping. This
unique topology allows the AD8628 to maintain its low offset
voltage over a wide temperature range and over its operating
lifetime. AD8628 also optimizes 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. AD8628 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 (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.
AD8628 is one of the few auto-zero amplifiers offered in the 5-lead
SOT-23 package. It greatly improves the ac parameters of the
previous auto-zero amplifiers. It has 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 up to
5 kHz to 10 kHz, the AD8628 provides true 16-bit accuracy
making it the best choice for very high resolution systems.
1/f Noise
1/f noise, also known as “pink noise,” is a major contributor of
errors in dc-coupled measurements. This 1/f noise error term
can be in the range of several mV 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 mV p-p 1/f
noise is configured for a gain of 1,000, its output will have 5 mV
of error due to the 1/f noise. But AD8628 eliminates 1/f noise
internally and therefore greatly reduces output errors. Here is how
it works: 1/f noise appears as a slowly varying offset to AD8628
inputs. Auto-zeroing corrects any dc or low frequency offset, thus
the 1/f noise component is essentially removed, leaving AD8628
free of 1/f noise.
FREQUENCY (kHz)
120
105
00124
VOLTAG E NOISE DENSITY (nV/ Hz)
60
45
30
15
90
75
26810
AD8628
(19.4nV/ Hz)
LMC2001
(31.1nV/ Hz)
LT C2050
(89.7nV/ Hz)
MK AT 1kHz FOR ALL 3 GRAPHS
Figure 1. Noise Spectral Density of AD8628 vs.
Competition
One of the biggest advantages that AD8628 brings to systems
applications over competitive auto-zero amplifiers is its very low
noise. The comparison shown in Figure 1 indicates an input-
referred noise density of 19.4 nV/÷Hz at 1 kHz for AD8628 that
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.