Datasheet

13
Appendix C: General Application Guide for APDS-9007
The APDS-9007 is an analog current output ambient light
photo sensor whose spectral response is close to the CIE
standard photopic observer. APDS-9007 consists of a pho-
todiode and an IC that performs amplication of the pho-
todiode output signal and conversion to a logarithmic
output current. APDS-9007 is able to produce a high gain
photo current that can be converted to an output voltage
via a standard value external load resistor. APDS-9007 is
then easily integrated into micro-controller that has an
available A/D as shown in Figure C1 below.
The magnitude of the output voltage, Vout is directly pro-
portional to the photo current which is generated by the
brightness of the light source shone on the photo sensor
and the value of the load resistor used, RL. Increasing the
brightness of the light source and/or the value of the load
resistor will increase the magnitude of the output voltage.
The unit used to measure light in terms of what our eyes
perceive is “LUX”. It indicates the intensity to which a
surface is lit or the brightness of the light. Light sources
with the same LUX level appear at the same brightness to
the human eyes. The equipment for “LUX” measurement is
a lux meter.
Selection of the load resistor RL will determine the
amount of current-to-voltage conversion in the circuit.
APDS-9007 allows a maximum saturation output voltage
of (Vcc – 0.5V).
Articial light sources such as uorescent lamps or in-
candescent lamps produced ac noise with a frequency
of 50/60Hz and 100Hz respectively. A capacitor of 10uF,
which acts as a low-pass lter, is recommended to be
added in parallel with the load resistor to lter out these
interferences.
An analog logarithmic current output is able to represent
the wide dynamic range of ambient light brightness
level from near darkness environment to bright outdoor
conditions correctly and accurately. For a logarithmic
output, constant relative changes in the input values are
converted to constant absolute changes in the output
values.
A logarithmic current output is advantageous because
when measuring low brightness levels, small changes in
those levels need to be detected. For example, a change
from 100lux to 200lux needs to be detected. On the other
hand, when measuring high brightness levels, only rela-
tively large changes need to be detected. For example, a
change from 10000lux to 10200lux can be negligible and
a change from 10klux to 20klux needs to be detected
instead.
Hence APDS-9007, with a logarithmic current output, is
able to provide a good relative resolution over the entire
ambient light brightness range. It is able to measure small
current steps at low brightness levels and yet, at high
brightness levels, it is not required to measure the current
at such ne absolute resolutions.
This is illustrated in Figure C2 below. A change from 10lux
to 100lux and a change from 1klux to 10klux will both
give a same relative change which is 10. This is equivalent
to a constant absolute change in output current which is
10uA for both changes. Hence a change from 10lux to
100lux will result in a change of 10uA. Similarly, a change
from 1klux to 10klux will also result in a change of 10uA.
APDS-9007 can be directly connected to the A/D of the
micro-controller via one external load resistor which will
give the same accuracy or resolution over a wide dynamic
range.
I1 = 10uA
I2 = 20uA
�"I = I2 - I1
I1 = 30uA
I2 = 40uA
�"I = I2 - I1
Ev1 = 10lux
Ev2 = 100lux
�"Ev = Ev2/Ev1 = 10
Ev1 = 1klux
Ev2 = 10klux
�"Ev = Ev2/Ev1 = 10
0
5
10
15
20
25
30
35
40
45
1 10 100 1000 10000
Lux
Output Current (uA)
Figure C1. Conguration of APDS-9007
Figure C2. Feature of Logarithmic Output
Micro-
Controller
A/D
C
Light
Source
Pin 6:
ShutDown
(Active High)
Pin 4: VDD
Pin 3 : OUT
Pin 1:�
GND
RL
APDS9007
GND