AD8628/AD8629/AD8630

Rev. I | Page 15 of 24

PEAK-TO-PEAK NOISE

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

chopping, the peak-to-peak noise of the AD8628/AD8629/

AD8630 is much lower than the competition. Figure 50 and

Figure 51 show this comparison.

BW = 0.1Hz TO 10Hz

TIME (1s/DIV)

Figure 50. AD8628 Peak-to-Peak Noise

BW = 0.1Hz TO 10Hz

TIME (1s/DIV)

Figure 51. Competitor A Peak-to-Peak Noise

NOISE BEHAVIOR WITH FIRST-ORDER, LOW-PASS

FILTER

The AD8628 was simulated as a low-pass filter (see Figure 53)

and then configured as shown in Figure 52. The behavior of the

AD8628 matches the simulated data. It was verified that noise is

rolled off by first-order filtering. Figure 53 and Figure 54 show

the difference between the simulated and actual transfer functions

of the circuit shown in Figure 52.

470pF

OUT

100kΩ

IN

1kΩ

Figure 52. First-Order Low-Pass Filter Test Circuit,

×101 Gain and 3 kHz Corner Frequency

FREQUENCY (kHz)

50

45

40

35

30

25

15

10

5

20

0

0

30

60

100

90

80

70

50

40

20

10

Figure 53. Simulation Transfer Function of the Test Circuit in Figure 52

FREQUENCY (kHz)

50

45

40

35

30

25

15

10

5

20

0

0

30

60

100

90

80

70

50

40

20

10

Figure 54. Actual Transfer Function of the Test Circuit in Figure 52

The measured noise spectrum of the test circuit charted in

Figure 54 shows that noise between 5 kHz and 45 kHz is

successfully rolled off by the first-order filter.

TOTAL INTEGRATED INPUT-REFERRED NOISE FOR

FIRST-ORDER FILTER

For a first-order filter, the total integrated noise from the

AD8628 is lower than the noise of Competitor A.

3dB FILTER BANDWIDTH (Hz)

10

1

0.1

10

100

10k

1k

COMPETITOR A

AD8551

AD8628

Figure 55. RMS Noise vs. 3 dB Filter Bandwidth in Hz