AD8605/AD8606/AD8608
Rev. D | Page 15 of 20
R
S
C
S
R
L
C
L
V+
V–
4
2
3
8
1
AD8605
V
IN
200mV
02731-D-049
Figure 48. Snubber Network Configuration
TIME (10
µ
s/DIV)
VOLTAGE (100mV/DIV)
V
S
= ±2.5V
A
V
= 1
R
L
= 10k
Ω
R
S
= 90
Ω
C
L
= 1,000pF
C
S
= 700pF
02731-D-048
Figure 49. Capacitive Load Drive with Snubber
Table 5 summarizes a few optimum values for capacitive loads.
Table 5.
C
L
(pF) R
S
(Ω) C
S
(pF)
500 100 1,000
1,000 70 1,000
2,000 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 (MicroCSP package option) is essentially
a silicon die with additional post fabrication 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
(I
B
) 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/cm
2
) ambient light. Figure 50 shows that I
B
increases
with increasing wavelength and intensity of incident light;
I
B
can reach levels as high as 4500 pA at a light intensity of
3 mW/cm
2
and a wavelength of 850 nm. The light intensities
shown in Figure 50 are not normal for most applications, i.e.,
even though direct sunlight can have intensities of 50 mW/cm
2
,
office ambient light can be as low as 0.1 mW/cm
2
.
When the MicroCSP 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 I
B
. 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 bands (NUV, 200 nm to 400 nm); therefore,
they can penetrate more deeply into the active silicon. Incident
light with wavelengths greater than 1100 nm has no photo-
electric effect on the AD8605ACB because silicon is trans-
parent to wave lengths 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 in the NUV or NIR bands.
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 MicroCSP 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.
WAVELENGTH (nm)
3500
0
350
INPUT BIAS CURRENT (pA)
2500
3000
2000
500
1000
1500
450 550 650 750 850
1mW/cm
2
4000
4500
5000
3mW/cm
2
2mW/cm
2
02731-D-050
Figure 50. AD8605ACB Input Bias Current Response to Direct Illumination of
Varying Intensity and Wavelength
MICROCSP ASSEMBLY CONSIDERATIONS
For detailed information on MicroCSP PCB assembly and
reliability, refer to ADI Application Note AN-617 on the ADI
website
www.analog.com.
