AD8628/AD8629/AD8630

Rev. F | Page 14 of 20

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 major-

ity 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 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 of the biggest advantages that the AD8628/AD8629/AD8630

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.

MK AT 1kHz FOR ALL 3 GRAPHS

FREQUENCY (kHz)

120

105

90

75

60

45

30

15

0

04

28

610

12

LTC2050

(89.7nV/√Hz)

LMC2001

(31.1nV/√Hz)

AD8628

(19.4nV/√Hz)

Figure 49. Noise Spectral Density of AD8628 vs. Competition