AD8628/AD8629

Rev. C | Page 15 of 20

PEAK-TO-PEAK NOISE

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

chopping, the peak-to-peak noise of the AD8628/AD8629 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. LTC2050 Peak-to-Peak Noise

NOISE BEHAVIOR WITH FIRST-ORDER LOW-PASS

FILTER

The AD8628 was simulated as a low-pass filter 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.

470pF

OUT

100k

Ω

IN

1k

Ω

Figure 52. Test Circuit: First-Order Low-Pass Filter—×101 Gain

and 3 kHz Corner Frequency

FREQUENCY (Hz)

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

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 Test Circuit

The measured noise spectrum of the test circuit 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 LTC2050.

3dB FILTER BANDWIDTH (Hz)

10

1

0.1

10

100

10k

1k

LTC2050

AD8551

AD8628

Figure 55. 3 dB Filter Bandwidth in Hz