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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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

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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.

4

MSP430F2274 Transimpedance Amplifier (TIDM-TIA) User's Guide TIDU443–August 2014

Figure 3. Feedback Resistor Jumpers

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F i GBW

F

F GBW

1 8 R C f

C

4 R f

+ p

=

p

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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).

where

• RFis the feedback resistance

• Ciis 10 pF

• f

is the gain-bandwidth product of the op-amp (determined by its slew rate mode) (2)

GBW

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 High-

Speed 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.

Hardware Description

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Figure 4. Operational Amplifier Circuit

5

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Photodiode Current (uA)

OA Voltage Output (V)

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8

0

0.5

1

1.5

2

2.5

3

3.5

D002

Input Current (A)

Voltage (V)

0 5E-7 1E-6 1.5E-6 2E-6

0

0.3

0.6

0.9

1.2

1.5

1.8

2.1

2.4

2.7

3

3.3

3.6

3.9

D001

F

OUT PK

F F

R

V    I

1 SC R

= - ´

+

Hardware Description

The transfer function of the output is then seen in Equation 3:

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.

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(3)

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.

Figure 6. Op-Amp Voltage Output Characterization

For more information on the characteristics of the operational amplifier, see the MSP430F2274 datasheet.

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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.

Hardware Description

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12

34

56

78

910

1112

1314

JTAG

SBH11-PBPC-D07-ST-BK

GND

123

J1

TSW-103-07-G-S

1

2

3

JP1

TSW-103-07-G-S

TEST/SBWTCK

RST/NMI

TCK
TMS
TDI
TDO

Green

12

D1

GND
GND

VCC

0.01µF

C1
DNP

GND

GND

0.1µF

C5

10µF

C4

GND

560

R1

P1.0 P1.0

TDO

TDI

TMS

TCK

RST/NMI

47k

R2

OA0I1 OA0O

OA0I0
OA0O
OA0I1

GND

VCC430D

VCC430A

VCC430D

VCC430D

TEST/SBWTCK

GND

1

2

D2
SFH 2701

0.1µF

C3

10µF

C2

VCC430A

GND

1

2

JP2

TSW-102-07-G-S

VCC
VCC

VCC

VCC

1

2

3

J2

TSW-103-07-G-S

1pF

C6

2.37Meg

R3

1

2

3

JP3

TSW-103-07-G-S

1
2
3

JP4

TSW-103-07-G-S

R4

DNP

DVSS

1

P2.7/XOUT

2

P2.6/XIN

3

DVSS

4

RST/NMI/SBWTDIO

5

P2.0/ACLK/A0/OA0I0

6

P2.1/TAINCLK/SMCLK/A1/OA0O

7

P2.2/TA0/A2/OA0I1

8

P3.0/UCB0STE/UCA0CLK/A5

9

P3.1/UCB0SIMO/UCB0SDA

10

P3.2/UCB0SOMI/UCB0SCL

11

P3.3/UCB0CLK/UCA0STE

12

AVSS

13

AVCC

14

P4.0/TB0

15

P4.1/TB1

16

P4.2/TB2

17

P4.3/TB0/A12/OA0O

18

P4.4/TB1/A13/OA1O

19

P4.5/TB2/A14/OA0I3

20

P4.6/TBOUTH/A15/OA1I3

21

P4.7/TBCLK

22

P3.4/UCA0TXD/UCA0SIMO

23

P3.5/UCA0RXD/UCA0SOMI

24

P3.6/A6/OA0I2

25

P3.7/A7/OA1I2

26

P2.3/TA1/A3/VREF-/VEREF-/OA1I1/OA1O

27

P2.4/TA2/A4/VREF+/VEREF+/OA1I0

28

P1.0/TACLK/ADC10CLK

29

P1.1/TA0

30

P1.2/TA1

31

P1.3/TA2

32

P1.4/SMCLK/TCK

33

P1.5/TA0/TMS

34

P1.6/TA1/TDI/TCLK

35

P1.7/TA2/TDO/TDI

36

TEST/SBWTCK

37

DVCC

38

DVCC

39

P2.5/ROSC

40

QFN PAD

U1

MSP430F2274IRHA

GND

R4L R4R

R3L R3R

OA0OOA0I1

Ext

Int

GND

1

2

J3

TSW-102-07-G-S

P2.3

P3.6

P1.1

P1.1
P2.3
P3.6

Schematics

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4 Schematics

Figure 7. TIDM-TIA Schematic

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Bills of Materials

5 Bills of Materials

Table 1. BOM

MANUFACTURER SUPPLIER PART

DESIGNATOR DESCRIPTION COMMENT QUANTITY MANUFACTURER SUPPLIER FOOTPRINT

PART NUMBER NUMBER

CAP, CERM, 0.01

C1 µF, 250 V, ±10%, DNP 1 Taiyo Yuden QMK212B7103KG-T Digi-Key 587-1271-1-ND 0805_HV

X7R, 0805
CAP, TA, 10 µF,

C2, C4 10 V, ±20%, 2.5 293D106X0010B2TE3 2 Vishay Sprague 293D106X0010B2TE3 Digi-Key 718-1119-1-ND 3528-21

Ω, SMD
CAP, CERM, 0.1

C3, C5 µF, 25 V, ±10%, 08053C104KAT2A 2 AVX Corporation 08053C104KAT2A Digi-Key 478-3755-1-ND 0805_HV

X7R, 0805
CAP, CERM, 1

C6 pF, 50 V, ±25%, 08055A1R0CAT2A 1 AVX Corporation 08055A1R0CAT2A Digi-Key 478-1292-1-ND 0805_HV

C0G/NP0, 0805

OSRAM Opto

D1 LED, Green, SMD LG L29K-G2J1-24-Z 1 LG L29K-G2J1-24-Z Digi-Key 475-2709-1-ND LG L29K_GREEN

Semiconductors Inc

OSRAM OSRAM Opto

D2 SFH 2701 1 SFH 2701 Digi-Key 475-2967-X-ND SFH 2701 - 1206

Photodiode Semiconductors Inc

J1, J2, JP1, JP3, Header, 100 mil,

TSW-103-07-G-S 5 Samtec Inc TSW-103-07-G-S Digi-Key SAM1029-03-ND TSW-103-07-G-S

JP4 3×1, Gold, TH

Header, 100 mil,

J3, JP2 TSW-102-07-G-S 2 Samtec Inc TSW-102-07-G-S Digi-Key SAM1029-02-ND TSW-102-07-G-S

2×1, Gold, TH
Header

SBH11-PBPC-D07-ST- Sullins Connector SBH11-PBPC-D07-ST- CONN_SBH11-

JTAG (shrouded), 100 1 Digi-Key S9170-ND

BK Solutions BK PBPC-D07-ST-BK

mil, 7×2, Gold, TH
RES, 560 Ω, 5%,

R1 CRCW0603560RJNEA 1 Vishay Dale CRCW0603560RJNEA Digi-Key 541-560GCT-ND 0603

0.1 W, 0603
RES, 47 kΩ, 5%,

R2 CRCW060347K0JNEA 1 Vishay Dale CRCW060347K0JNEA Digi-Key 541-47KGCT-ND 0603

0.1 W, 0603
RES, 2.37 MΩ,

R3 CRCW06032M37FKEA 1 Vishay Dale CRCW06032M37FKEA Digi-Key 541-2.37MHCT-ND 0603

1%, 0.1 W, 0603

R4 RES DNP 1 External Resistor

16-bit Ultra-Low­Power

U1 Microcontroller, MSP430F2274IRHA 1 Texas Instruments MSP430F2274IRHAT Digi-Key 296-21459-1-ND RHA0040B

32-KB Flash, 1-K
RAM, RHA0040B

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References

6 References

1. MSP430F2274: Product Folder

2. MSP-FET430UIF: Tool Folder

3. OSRAM SFH 2701 (PDF)

4. Compensate Transimpedance Amplifiers Intuitively (SBOA055A)

5. Transimpedance Considerations for High-Speed Amplifiers (SBOA122)

6. TIDM-TIA: Design Folder

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