AD8605/AD8606/AD8608
5000
4500
4000
3500
3000
2500
2000
1500
1000
500
Figure 49 shows a scope of the output at the snubber circuit.
The overshoot is reduced from over 70% to less than 5%, and
the ringing is eliminated by the snubber. Optimum values for RS
and CS are determined experimentally.
2
3mW/cm
V
= ±2.5V
= 1
S
A
R
R
C
C
V
L
S
L
S
= 10kΩ
= 90Ω
2
2mW/cm
= 1000pF
= 700pF
2
1mW/cm
0
350
450
550
650
750
850
WAVELENGTH (nm)
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
in the increased IB. No performance degradation occurs due to
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.
TIME (10µs/DIV)
Figure 49. Capacitive Load Drive with Snubber
Table 5 summarizes a few optimum values for capacitive loads.
Table 5.
CL (pF)
RS (Ω)
100
70
CS (pF)
1000
1000
800
500
1000
2000
60
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 ꢀ. 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
current (IB) parameter is impacted when the package is
illuminated directly by high intensity light. No degradation in
electrical performance is observed due to illumination by low
intensity (0.1 mW/cm2) ambient light. Figure 50 shows that IB
increases with increasing wavelength and intensity of incident
light; IB can reach levels as high as 4500 pA at a light intensity of
3 mW/cm2 and a wavelength of 850 nm. The light intensities
shown in Figure 50 are not normal for most applications, that is,
even though direct sunlight can have intensities of 50 mW/cm2,
office ambient light can be as low as 0.1 mW/cm2.
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.
Rev. H | Page 17 of 24
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