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.

MK AT 1kHz FOR ALL 3 GRAPHS

FREQUENCY (kHz)

120

105

90

75

60

45

30

15

0

04

28

610

12

COMPETITOR A

(89.7nV/√Hz)

COMPETITOR B

(31.1nV/√Hz)

AD8628

(19.4nV/√Hz)

Figure 49. Noise Spectral Density of AD8628 vs. Competition