User's Guide TIDU443 – August 2014 MSP430F2274 Transimpedance Amplifier (TIDM-TIA) User's Guide 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. Figure 1. TIA Block Diagram 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. TIDU443 – August 2014 Submit Documentation Feedback MSP430F2274 Transimpedance Amplifier (TIDM-TIA) User's Guide Copyright © 2014, Texas Instruments Incorporated 1 Acronyms, Terms, and Definitions 2 www.ti.com 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 Figure 2. TIA Hardware Description 2 MSP430F2274 Transimpedance Amplifier (TIDM-TIA) User's Guide Copyright © 2014, Texas Instruments Incorporated TIDU443 – August 2014 Submit Documentation Feedback Hardware Description www.ti.com 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. 3.5 3.5.1 Feedback Resistors 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: ( Vout = – Iphoto ´ RF ) where • • Iphoto is the current from the photodiode RF is the resistance of the feedback resistor. TIDU443 – August 2014 Submit Documentation Feedback MSP430F2274 Transimpedance Amplifier (TIDM-TIA) User's Guide Copyright © 2014, Texas Instruments Incorporated (1) 3 Hardware Description 3.5.2 www.ti.com 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. Figure 3. Feedback Resistor Jumpers 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. 4 MSP430F2274 Transimpedance Amplifier (TIDM-TIA) User's Guide Copyright © 2014, Texas Instruments Incorporated TIDU443 – August 2014 Submit Documentation Feedback Hardware Description www.ti.com 3.5.3 Feedback Capacitor Using an external resistor may result in a noisier output signal because there is a noise reducing capacitor calibrated for the onboard feedback resistor’s resistance. Selecting a feedback capacitor value is determined by Equation 2 (see the MSP430F2274 datasheet for details). CF = 1 + 8pRFCi fGBW 4pRF fGBW where • • • RF is the feedback resistance Ci is 10 pF fGBW is the gain-bandwidth product of the op-amp (determined by its slew rate mode) (2) In general, a larger feedback capacitance will result in a slightly smaller bandwidth. For more information on selecting a feedback capacitor, see the Transimpedance Considerations for HighSpeed Amplifiers and Compensate Transimpedance Amplifiers Intuitively resources in Section 6. 3.6 Operational Amplifier The TIDM-TIA uses the MSP430F2274’s integrated op-amp with a feedback resistor to provide the current-to-voltage conversion. Using the op-amp is preferred to simply feeding the photodiode’s current across a resistor. A lone feedback resistor has a tradeoff between a large gain and a small response time. Using the onboard op-amp with a feedback resistor allows for a faster response time, large gain, and better signal-to-noise ratio of output signals. There is also a feedback capacitor added across the feedback resistor. Its job is to act as a low-pass filter, reducing noise and oscillation created by the opamp (see Section 3.5.3 for details on changing the capacitor). The complete circuit involving the op-amp can be seen in Figure 4 where IS1 simulates the photodiode as a current source. Figure 4. Operational Amplifier Circuit TIDU443 – August 2014 Submit Documentation Feedback MSP430F2274 Transimpedance Amplifier (TIDM-TIA) User's Guide Copyright © 2014, Texas Instruments Incorporated 5 Hardware Description www.ti.com The transfer function of the output is then seen in Equation 3: RF VOUT = ´ IPK 1 + SCFRF (3) Voltage (V) As mentioned in the previous section, the TIDM-TIA has a feedback resistor with a resistance of 2.37 MΩ. This combination of photodiode, feedback resistor, and op-amp should provide the ideal output voltage seen in SPICE simulations in Figure 5. Currents created by the photodiode range from 0 to −1.5 µA, which are used as test values. 3.9 3.6 3.3 3 2.7 2.4 2.1 1.8 1.5 1.2 0.9 0.6 0.3 0 0 5E-7 1E-6 Input Current (A) 1.5E-6 2E-6 D001 Figure 5. TIA SPICE Simulation The Tina-TI SPICE model of the op-amp is available on the MSP430F2274 product web page. Characterization data for the photodiode current versus op-amp output voltage can be seen in Figure 6. Results are close to expected output. NOTE: This output will change if using an external feedback resistor or different photodiode. 3.5 OA Voltage Output (V) 3 2.5 2 1.5 1 0.5 0 0 0.2 0.4 0.6 0.8 1 1.2 Photodiode Current (uA) 1.4 1.6 1.8 D002 Figure 6. Op-Amp Voltage Output Characterization For more information on the characteristics of the operational amplifier, see the MSP430F2274 datasheet. 6 MSP430F2274 Transimpedance Amplifier (TIDM-TIA) User's Guide Copyright © 2014, Texas Instruments Incorporated TIDU443 – August 2014 Submit Documentation Feedback Hardware Description www.ti.com 3.7 Analog-to-Digital Converter The 10-bit ADC inside the MSP430F2274 converts voltage at the output of the op-amp into a 10-bit value. Both components are connected to each other inside the MSP430F2274. By default, the ADC uses reference voltages of 0 V (GND) and 3.3 V (VCC) for its samples in single-channel, single-conversion mode, which is configured in software and can be changed for the different operations of the ADC. However, channel A1 of the ADC is the only channel the op-amp output is connected to by default. See the MSP430F2274 user’s guide for more information on the ADC. 3.8 GPIO Pins On the TIDM-TIA, P1.0 from the MSP430F2274 controls the onboard LED. The LED turns on when the photodiode provides a dark reading and turns off for a bright reading. Whether a reading is "dark" or "bright" is determined by the conversion value from the ADC. The TIDM-TIA also offers access to three GPIO pins on the MSP430F2274. Pins P1.1, P2.3, and P3.6 are all available to use through jumper J2. See the MSP430F2274 user’s guide for more information on these pins. TIDU443 – August 2014 Submit Documentation Feedback MSP430F2274 Transimpedance Amplifier (TIDM-TIA) User's Guide Copyright © 2014, Texas Instruments Incorporated 7 Schematics Schematics JTAG 14 12 10 TEST/SBWTCK 8 6 4 2 C1 DNP 13 11 9 7 5 3 1 J1 RST/NMI TSW-103-07-G-S J2 0.01µF TCK TMS TDI TDO GND P1.1 P2.3 P3.6 3 2 1 1 2 3 4 www.ti.com TSW-103-07-G-S SBH11-PBPC-D07-ST-BK R2 47k GND JP1 3 2 1 Ext Int GND VCC TSW-103-07-G-S JP2 2 VCC 1 VCC TSW-102-07-G-S R4 R4L VCC VCC 3 2 1 VCC430A R4R DNP JP3 R3L VCC430D TSW-103-07-G-S OA0I1 R3 R3R 2.37Meg JP4 1 2 3 TSW-103-07-G-S OA0O C6 OA0I1 C2 10µF C4 10µF C3 0.1µF OA0O 1pF C5 0.1µF J3 2 1 U1 TSW-102-07-G-S 2 GND 1 Green R1 P1.0 560 14 38 39 P1.0 P1.1 29 30 31 32 33 34 35 36 TCK TMS TDI TDO GND P3.6 9 10 11 12 23 24 25 26 RST/NMI 5 TEST/SBWTCK 37 AVCC DVCC DVCC P1.0/TACLK/ADC10CLK P1.1/TA0 P1.2/TA1 P1.3/TA2 P1.4/SMCLK/TCK P1.5/TA0/TMS P1.6/TA1/TDI/TCLK P1.7/TA2/TDO/TDI P3.0/UCB0STE/UCA0CLK/A5 P3.1/UCB0SIMO/UCB0SDA P3.2/UCB0SOMI/UCB0SCL P3.3/UCB0CLK/UCA0STE P3.4/UCA0TXD/UCA0SIMO P3.5/UCA0RXD/UCA0SOMI P3.6/A6/OA0I2 P3.7/A7/OA1I2 1 VCC430A VCC430D VCC430D P2.0/ACLK/A0/OA0I0 P2.1/TAINCLK/SMCLK/A1/OA0O P2.2/TA0/A2/OA0I1 P2.3/TA1/A3/VREF-/VEREF-/OA1I1/OA1O P2.4/TA2/A4/VREF+/VEREF+/OA1I0 P2.5/ROSC P2.6/XIN P2.7/XOUT P4.0/TB0 P4.1/TB1 P4.2/TB2 P4.3/TB0/A12/OA0O P4.4/TB1/A13/OA1O P4.5/TB2/A14/OA0I3 P4.6/TBOUTH/A15/OA1I3 P4.7/TBCLK RST/NMI/SBWTDIO TEST/SBWTCK QFN PAD AVSS DVSS DVSS 6 7 8 27 28 40 3 2 GND 15 16 17 18 19 20 21 22 13 1 4 D2 SFH 2701 OA0I0 OA0O OA0I1 P2.3 2 D1 GND GND GND GND GND MSP430F2274IRHA Figure 7. TIDM-TIA Schematic 8 MSP430F2274 Transimpedance Amplifier (TIDM-TIA) User's Guide TIDU443 – August 2014 Submit Documentation Feedback Copyright © 2014, Texas Instruments Incorporated Bills of Materials www.ti.com 5 Bills of Materials Table 1. BOM DESIGNATOR DESCRIPTION COMMENT QUANTITY MANUFACTURER C1 CAP, CERM, 0.01 µF, 250 V, ±10%, X7R, 0805 DNP 1 Taiyo Yuden C2, C4 CAP, TA, 10 µF, 10 V, ±20%, 2.5 Ω, SMD 293D106X0010B2TE3 2 C3, C5 CAP, CERM, 0.1 µF, 25 V, ±10%, X7R, 0805 08053C104KAT2A C6 CAP, CERM, 1 pF, 50 V, ±25%, C0G/NP0, 0805 D1 MANUFACTURER PART NUMBER SUPPLIER SUPPLIER PART NUMBER FOOTPRINT QMK212B7103KG-T Digi-Key 587-1271-1-ND 0805_HV Vishay Sprague 293D106X0010B2TE3 Digi-Key 718-1119-1-ND 3528-21 2 AVX Corporation 08053C104KAT2A Digi-Key 478-3755-1-ND 0805_HV 08055A1R0CAT2A 1 AVX Corporation 08055A1R0CAT2A Digi-Key 478-1292-1-ND 0805_HV LED, Green, SMD LG L29K-G2J1-24-Z 1 OSRAM Opto LG L29K-G2J1-24-Z Semiconductors Inc Digi-Key 475-2709-1-ND LG L29K_GREEN D2 OSRAM Photodiode SFH 2701 1 OSRAM Opto SFH 2701 Semiconductors Inc Digi-Key 475-2967-X-ND SFH 2701 - 1206 J1, J2, JP1, JP3, JP4 Header, 100 mil, 3×1, Gold, TH TSW-103-07-G-S 5 Samtec Inc TSW-103-07-G-S Digi-Key SAM1029-03-ND TSW-103-07-G-S J3, JP2 Header, 100 mil, 2×1, Gold, TH TSW-102-07-G-S 2 Samtec Inc TSW-102-07-G-S Digi-Key SAM1029-02-ND TSW-102-07-G-S JTAG Header SBH11-PBPC-D07-ST(shrouded), 100 BK mil, 7×2, Gold, TH 1 Sullins Connector Solutions SBH11-PBPC-D07-STBK Digi-Key S9170-ND CONN_SBH11PBPC-D07-ST-BK R1 RES, 560 Ω, 5%, 0.1 W, 0603 CRCW0603560RJNEA 1 Vishay Dale CRCW0603560RJNEA Digi-Key 541-560GCT-ND 0603 R2 RES, 47 kΩ, 5%, 0.1 W, 0603 CRCW060347K0JNEA 1 Vishay Dale CRCW060347K0JNEA Digi-Key 541-47KGCT-ND 0603 R3 RES, 2.37 MΩ, 1%, 0.1 W, 0603 CRCW06032M37FKEA 1 Vishay Dale CRCW06032M37FKEA Digi-Key 541-2.37MHCT-ND 0603 R4 RES DNP 1 U1 16-bit Ultra-LowPower Microcontroller, 32-KB Flash, 1-K RAM, RHA0040B MSP430F2274IRHA 1 External Resistor Texas Instruments MSP430F2274IRHAT TIDU443 – August 2014 Submit Documentation Feedback Digi-Key 296-21459-1-ND RHA0040B MSP430F2274 Transimpedance Amplifier (TIDM-TIA) User's Guide Copyright © 2014, Texas Instruments Incorporated 9 References 6 References 1. 2. 3. 4. 5. 6. 10 www.ti.com MSP430F2274: Product Folder MSP-FET430UIF: Tool Folder OSRAM SFH 2701 (PDF) Compensate Transimpedance Amplifiers Intuitively (SBOA055A) Transimpedance Considerations for High-Speed Amplifiers (SBOA122) TIDM-TIA: Design Folder MSP430F2274 Transimpedance Amplifier (TIDM-TIA) User's Guide Copyright © 2014, Texas Instruments Incorporated TIDU443 – August 2014 Submit Documentation Feedback IMPORTANT NOTICE Texas Instruments Incorporated and its subsidiaries (TI) reserve the right to make corrections, enhancements, improvements and other changes to its semiconductor products and services per JESD46, latest issue, and to discontinue any product or service per JESD48, latest issue. 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