AD8628/AD8629

Rev. C | Page 14 of 20

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.

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%.

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.

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.

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 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.

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