AD8605/AD8606/AD8608

Rev. H | Page 17 of 24

WAVELENGTH (nm)

3500

0

350

2500

3000

2000

500

1000

1500

450

550

650

750

850

4000

4500

5000

Figure 49 shows a scope of the output at the snubber circuit.

The overshoot is reduced from over 70% to less than 5%, and

TIME (10µs/DIV)

Figure 50. AD8605ACB Input Bias Current Response to Direct Illumination of

Varying Intensity and Wavelength

When the WLCSP package is assembled on the board with the

bump side of the die facing the PCB, reflected light from the

PCB surface is incident on active silicon circuit areas and results

illumination of the backside (substrate) of the AD8605ACB.

The AD8605ACB is particularly sensitive to incident light with

wavelengths in the near infrared range (NIR, 700 nm to 1000 nm).

Photons in this waveband have a longer wavelength and lower

energy than photons in the visible (400 nm to 700 nm) and near

ultraviolet (NUV, 200 nm to 400 nm) bands; therefore, they can

penetrate more deeply into the active silicon. Incident light with

wavelengths greater than 1100 nm has no photoelectric effect

on the AD8605ACB because silicon is transparent to wavelengths

in this range. The spectral content of conventional light sources

varies. Sunlight has a broad spectral range, with peak intensity

in the visible band that falls off in the NUV and NIR bands;

fluorescent lamps have significant peaks in the visible but not

the NUV or NIR bands.

Figure 49. Capacitive Load Drive with Snubber

Table 5 summarizes a few optimum values for capacitive loads.

Table 5.

500

100

1000

1000

70

1000

2000

60

800

An alternate technique is to insert a series resistor inside the

feedback loop at the output of the amplifier. Typically, the value

of this resistor is approximately 100 Ω. This method also reduces

overshoot and ringing but causes a reduction in the maximum

output swing.

LIGHT SENSITIVITY

The AD8605ACB (WLCSP package option) is essentially a

silicon die with additional postfabrication dielectric and

intermetallic processing designed to contact solder bumps

on the active side of the chip. With this package type, the die

is exposed to ambient light and is subject to photoelectric

effects. Light sensitivity analysis of the AD8605ACB mounted

on standard PCB material reveals that only the input bias

illuminated directly by high intensity light. No degradation in

electrical performance is observed due to illumination by low

increases with increasing wavelength and intensity of incident

shown in Figure 50 are not normal for most applications, that is,

Efforts have been made at a product level to reduce the effect of

ambient light; the under bump metal (UBM) has been designed

to shield the sensitive circuit areas on the active side (bump

side) of the die. However, if an application encounters any light

sensitivity with the AD8605ACB, shielding the bump side of the

WLCSP package with opaque material should eliminate this

effect. Shielding can be accomplished using materials such as

silica-filled liquid epoxies that are used in flip-chip underfill

techniques.

WLCSP ASSEMBLY CONSIDERATIONS

For detailed information on the WLCSP PCB assembly and

reliability, see Application Note AN-617,

MicroCSP™ Wafer

Level Chip Scale Package.