Data Sheet

AD8628/AD8629/AD8630

FUNCTIONAL DESCRIPTION

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Rev. M | 14 of 21

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 sen-

sitive measurement applications 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 ampli-

fier 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 arrange-

ment 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 (0.1 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-refer-

red 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.

Figure 49. Noise Spectral Density of AD8628 vs. Competition

PEAK-TO-PEAK NOISE

Because of the ping-pong action between auto-zeroing and chop-

ping, the peak-to-peak noise of the AD8628/AD8629/AD8630 is

much lower than the competition. Figure 50 and Figure 51 show

this comparison.

Figure 50. AD8628 Peak-to-Peak Noise