AD8628/AD8629/AD8630

Rev. E | Page 15 of 24

FUNCTIONAL DESCRIPTION

1/f NOISE

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 amplifi-

ers to be easily configured for high gains without risk of

excessive output voltage errors. The extremely small tempera-

ture 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, 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/AD8630

eliminate 1/f noise internally, and thereby greatly reduce output

errors.

The AD8628/AD8629/AD8630 achieve a high degree of preci-

sion 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 band-

width 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/

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

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 wide-

band 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 (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.

Figure 51 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)

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/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.

Figure 51. Noise Spectral Density of AD8628 vs. Competition