Data Sheet

AD8628/AD8629/AD8630

FUNCTIONAL DESCRIPTION

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Rev. M | 15 of 21

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.

Figure 52. First-Order Low-Pass Filter Test Circuit, ×101 Gain and 3 kHz

Corner Frequency

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

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.

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

INPUT OVERVOLTAGE PROTECTION

Although the AD8628/AD8629/AD8630 are rail-to-rail input amplifi-

ers, care should be taken to ensure that the potential difference

between the inputs does not exceed the supply voltage. Under nor-

mal negative feedback operating conditions, the amplifier corrects

its output to ensure that the two inputs are at the same voltage.

However, if either input exceeds either supply rail by more than 0.3

V, large currents begin to flow through the ESD protection diodes in

the amplifier.

These diodes are connected between the inputs and each supply

rail to protect the input transistors against an electrostatic discharge

event, and they are normally reverse-biased. However, if the input