Texas Instruments MSP430F2274 User Manual
MSP430F2274 Transimpedance Amplifier (TIDM-TIA)
1 Introduction
This manual describes the TIDM-TIA hardware and how to use it. The TIDM-TIA uses a MSP430F2274
microcontroller to convert the current produced by a photodiode into a voltage. The TIDM-TIA converts
current to voltage by using one of the MSP430F2274’s integrated op-amps and an external feedback
resistor. This voltage is then sampled by the ADC on the MSP430F2274 and converted to a 10-bit value.
Resulting conversion values can then be used to turn the LED on or off based on simple logic. Other
GPIO pins are also available to use externally. The entire process of the TIA is indicated in Figure 1.
User's Guide
TIDU443–August 2014
User's Guide
The TIDM-TIA is programmable and powered by a 4-wire JTAG connection or by an external source
through an external voltage pin.
Find more information on the MSP430F2274 datasheet.
For more information on transimpedance amplifiers and their properties, see the Transimpedance
Considerations for High-Speed Amplifiers and Compensate Transimpedance Amplifiers Intuitively
resources in Section 6.
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Figure 1. TIA Block Diagram
1
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Acronyms, Terms, and Definitions
2 Acronyms, Terms, and Definitions
ADC— Analog-to-Digital Converter
CCS— Texas Instruments’ Code Composer Studio
GPIO— General-Purpose Input/Output
JTAG— Joint Test Action Group
LED— Light Emitting Diode
OA— General Purpose Operational Amplifier
Op-Amp— Operational Amplifier
TI— Texas Instruments
TIA— Transimpedance Amplifier
TIDM-TIA— The name of this reference design
Blue-wire— Patch wires added to a circuit board to correct issues or change design.
3 Hardware Description
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MSP430F2274 Transimpedance Amplifier (TIDM-TIA) User's Guide TIDU443–August 2014
Figure 2. TIA Hardware Description
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( )
out photo F
V – I R= ´
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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 MSPFET430UIF 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.
Hardware Description
3.5 Feedback Resistors
3.5.1 Onboard Feedback Resistor
The TIDM-TIA has an onboard 2.37-MΩ 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
• RFis the resistance of the feedback resistor. (1)
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is the current from the photodiode
photo
3
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Hardware Description
3.5.2 External Feedback Resistor
The TIDM-TIA also has an option to use an external feedback resistor for a different voltage gain. Figure 3
shows the functionality of jumpers JP3 and JP4 with regards to selecting a feedback resistor.
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By default, there is a jumper connecting the Int and middle pins of JP3 and JP4 to use the onboard
feedback resistor. Connecting the Ext and middle pins of both JP3 and JP4 allows for the use of an
external feedback resistor. An external resistor can be connected to the TIDM-TIA board by blue-wiring a
resistor to the R4 location on the board at the two holes. Knowing a target output voltage and measuring
the current from the photodiode makes it possible to use Equation 1 to select a proper feedback resistor.
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MSP430F2274 Transimpedance Amplifier (TIDM-TIA) User's Guide TIDU443–August 2014
Figure 3. Feedback Resistor Jumpers
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