(
)
out
photo
F
V
– I
R
=
´
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Hardware Description
3.1
JTAG Connector
The part number of the 14-pin JTAG connector is SBH11-PBPC-D07-ST-BK from Sullins Connector
Solutions.
The JTAG header provides a 4-wire method of programming and powering the TIDM-TIA. A MSP-
FET430UIF can be used to program and debug the TIDM-TIA.
3.2
Powering the TIDM-TIA
Use the power select jumper (JP1) to switch between JTAG and external power sources for the board.
Placing a jumper between pins
V
and
Int
allows power to come from the JTAG connection. Placing a
jumper between pins
V
and
Ext
allows the board to be powered by applying voltage to pin
V
on the J1
jumper just to the left of JP1. A voltage of 3.3 V is recommended, but voltages can range from 1.8 to 3.6 V
for the MSP430F2274.
By default, power by the JTAG connection should be selected. Do not apply voltage to J1 when powering
the TIDM-TIA by JTAG.
3.3
Measuring Current Consumption
The current consumed by the TIDM-TIA can easily be measured by connecting an ammeter between the
two pins of jumper JP2. When not measuring the current, leave a connection between these two pins.
3.4
Photodiode
The part number of the photodiode is SFH 2701 from OSRAM Opto Semiconductors Inc. Find more
information on the
SFH 2701 datasheet
.
The photodiode is the key component to the TIDM-TIA as it creates a current to be manipulated. The
magnitude of the current depends on both the intensity of light hitting its active sensor and the wavelength
of that light. Wavelengths between 400 and 1050 nm affect the photodiode. Currents from
−
0.1 to
−
100
µA can be obtained depending on the light hitting the photodiode. With indoor lighting, current output can
range from around
−
0.1 to around
−
8 µA depending on the light source's proximity. Sunlight causes the
photodiode to output currents of
−
30 µA and lower depending on the brightness of the day.
To measure the current produced by the photodiode, connect an ammeter between both pins of jumper
JP5. This measurement helps when choosing an external feedback resistor (explained in
Section 3.5.2
).
This photodiode can also be replaced for similar designs. For example, a photodiode optimized to be
sensitive to infrared light may be used with similar board functionality. To use a different photodiode,
remove the SFH 2701 and blue-wire a new photodiode to its solder pads.
3.5
Feedback Resistors
3.5.1
Onboard Feedback Resistor
The TIDM-TIA has an onboard 2.37-M
Ω
feedback resistor connected between the inverting input of the
integrated op-amp and its output. This feedback resistor provides the gain across the op-amp. Default
gain with the onboard feedback resistor is very high and optimized for low-light, indoor situations where
the photodiode only produces
−
0.1 to
−
1.5 µA of current. The feedback resistor helps convert the current
from the photodiode into a voltage readable by the ADC. Output voltage is calculated using
Equation 1
:
where
•
I
photo
is the current from the photodiode
•
R
F
is the resistance of the feedback resistor.
(1)
3
TIDU443 – August 2014
MSP430F2274 Transimpedance Amplifier (TIDM-TIA) User's Guide
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