1 http://www.national.com
June 2009
Rev – 1.1
Photodetector Sensor Board
SP1202S03RB
User’s Guide
© 2009 National Semiconductor Corporation.
2 http://www.national.com
Table of Contents
1.0 Introduction..........................................................................................................................................3
2.0 Board Assembly...................................................................................................................................3
3.0 Quick Start ...........................................................................................................................................3
4.0 Functional Description.........................................................................................................................5
4.1 Operational Modes.................................................................................................................5
4.1.1 The Computer Mode.........................................................................................5
4.1.2 The Stand-Alone Mode....................................................................................5
4.2 Signal Conditioning Circuitry................................................................................................5
4.2.1 The Transimpedance Amplifier.......................................................................5
4.2.2 Light Source.......................................................................................................5
4.3 Power Supply.........................................................................................................................5
4.4 ADC Reference Circuitry.......................................................................................................5
4.5 ADC clock .............................................................................................................................6
4.6 Digital Data Output................................................................................................................6
4.7 Power Requirements and Settings..........................................................................................6
5.0 Installing and Using the Photodetector Sensor Board..........................................................................6
5.1 Board Set-up..........................................................................................................................6
5.2 Quick Check of Analog Functions.........................................................................................6
5.3 Quick Check of Software and Computer Interface Operation...............................................6
5.4 Troubleshooting.....................................................................................................................7
6.0 Board Specifications ............................................................................................................................7
7.0 Example Hardware Schematic .............................................................................................................8
8.0 Example Bill of Materials ....................................................................................................................9
Appendix: Summary Tables of Test Points and Connectors.....................................................................10
Summary Tables of Test Points and Connectors (cont'd) ..........................................................................11
3 http://www.national.com
1.0 Introduction
The Photodiode Sensor Board (SP1202S03RB),
along with the Sensor Signal Path Control Panel
(Sensor Panel) software and SPUSI2 USB
Interface Dongle are designed to ease the design of
circuits using various photodiodes with National’s
amplifiers and Analog-to-Digital converters (ADCs).
Use the WEBENCH® Photodiode Sensor Designer
to determine appropriate ICs and passives to
achieve your signal path requirements:
http://www.national.com/analog/webench/sensors/p
hotodiode
See Figure 1 for component placement and Figure
2 for the example board schematic. The circuit for
the photodiode sensor consists of a
transimpedance amplifier (current to voltage
converter) for operation in the photoconductive
mode. Also, the board has circuitry to drive a light
source (LED) which can be mounted in close
proximity to the photodiode. The LED current can
be either continuous or switched with the current
level adjustable using a multi turn potentiometer.
The values for the LED driver are not calculated by
Sensor Designer. The idea here is to provide a
generic driver stage, allow the user to select a light
source and adjust the current accordingly. The light
source is not included in the kit. It should be chosen
to be commensurate with the photodiode
wavelength and optical sensitivity.
The outputs are a voltage output for the
photocurrent (received optical power and a voltage
output for the LED current (transmitted optical
power).
2.0 Board Assembly
This Photodetector Sensor Board comes as a bare
board that must be assembled. Refer to the
example Bill of Materials for a description of
component values. The values for the photodiode
circuit are calculated using WEBENCH Sensor
Designer. The values for the LED driver are not
calculated by Sensor Designer.
3.0 Quick Start
Refer to Figure 1 for locations of test points and
major components. This Quick Start procedure
provides 5V excitation for the sensor.
1. Place the J1 jumper across pins 2 & 3. This
applies a negative 5VDC bias to the
photodiode.
2. Place the J3 jumper across pins 2 & 3. This
sets the ADC reference voltage to 4.096VDC.
3. Place the J4 jumper across pins 2 & 3. This
sets the –VCC to -5VDC. Do not use this
setting for an Op Amp that has a VCC of 5VDC
or less.
4. Connect the Differential Pressure Sensor Board
to the SPUSI2 board via 14-pin header J2.
5. Connect a USB cable between the SPUSI2
board and a PC USB port. GRN LED D1 on the
Photodiode Sensor Board and D1 on the
SPUSI2 board should come on if the PC is on.
6. If not already installed, install the Sensor Panel
software on the PC. Run the software.
4 http://www.national.com
Figure 1. Component and Test Point Locations
J3
VREF
Select
J4
-VCC
Select
TP2
GND
J1
Bias
Select
TP8
VREF TP9
CS* TP1
Analog
TIA Ou
t
TP10
GND
TP14
DIN
TP13
VCC3V
J2
SPUSI2
Conn.
TP12
DOUT
TP11
SCLK
TP6
Isense - TP4
GND TP3
Current
Meter
Analog
Ou
t
TP5
Isense +
TP7
Generator
In
S1
LED
On/Off
J5
Generator
In
VR1
Current
Adjust
5 http://www.national.com
4.0 Functional Description
The Photodiode Sensor Board component and test
point locations are shown in Figure 1. The board
schematic is shown in Figure 2.
4.1 Operational Modes
This board may be use in one of two modes: the
Computer Mode using the SPUSI2 USB Interface
Dongle or the Stand-Alone Mode without the use of
the SPUSI2 USB Interface Dongle and a PC.
4.1.1 The Computer Mode
The board is intended for use in the Computer
Mode, where a SPUSI2 board is used with it and
the SPUSI2 board is connected to a PC via a USB
port. Power to both boards is provided via USB.
4.1.2 The Stand-Alone Mode
The Stand-Alone Mode does not use the SPUSI2
board to capture data and upload it to a PC. To use
the board this way, the user must provide +5V at
pin 14 of header J2 as well as provide ADC clock
and Chip Select signals to the ADC at pins 3 and 1,
respectively, of J24. ADC data output is available at
pin 5 of J2. Test Points may also be used to
insert/read these signals. The range of frequencies
for the ADC clock is 500KHZ to 1 MHz. The CS rate
can be as low as desired, as but no faster than 17
times the ADC clock rate.
4.2 Signal Conditioning Circuitry
The output of the TIA (transimpedance amplifier) is
on TP1. This is a voltage that is proportional to
photocurrent. The values for RF and CF are
calculated by the WEBENCH Sensor Designer. RF
is determined by the full scale input voltage of the
ADC and the maximum output photocurrent of the
photodiode. CF is determined based on photodiode
capacitance and an estimate of stray capacitance
on the inverting input of OP Amp U1. The current
flowing in the LED is measured and scaled by Op
Amp U2 and appears on TP3. These voltages
appear on the inputs of ADC U4. The digital output
of the ADC appears on J2 Pin 5.
4.2.1 The Transimpedance Amplifier
In the photoconductive mode, the cathode of the
photodiode is connected to the inverting pin of an
op amp with the non inverting pin grounded. To
maintain the virtual ground on the inverting pin, the
op amp must provide current from its output
through the RF to the photodiode.
So: V out = IPHOTODIODE * RF.
RF is determined by knowing the maximum
photocurrent, sometimes referred to as I SHORT
CIRCUIT of the photodiode, and the full scale input
value of the ADC.
So: RF = VFS/ISHORT CIRCUIT
Because the photodiode has capacitance, RF and
CDIODE form a pole in the noise gain transfer
function. This can create stability issues and is
compensated for by CF. WEBENCH Sensor
Designer calculates the value of CF for a 45 0
phase margin to insure stability.
4.2.2 Light Source
Provisions for a light source for test purposes are
on the board.
The user can select an LED with appropriate
wavelength and output power to compliment the
photodiode selection. It may be necessary to
modify component values in the LED driver to
optimize performance.
S1 is provided as an ON/OFF control for the LED
drive circuit. It controls the gate voltage of an NCH
MOSFET. In the open position, the mosfet is
conducting and the current source is enabled.
Also connected to the gate of the mosfet is a
connector for a signal generator. A switching signal
can be connected here to observe the transient
response of the transimpedance amplifier. The rise
times and switching frequencies are somewhat
limited by the current source components and the
mosfet switch so the user may want to install
different components to achieve higher
performance for the current source in this mode.
The current source consists of U3, Q1, VR1 and
R7. The op amp will make sure the voltage at the
CT of VR1 appears on the inverting pin. This
voltage then appears across R7 (minimal VON for
Q2). ILED = VR1CT/R7. Turning VR1 clockwise
increases LED current. The differential amplifier U2
measures the current flowing in the LED by
measuring the Voltage drop across R4. This
voltage drop is scaled by the gain setting resistors.
ILED = Analog_V2/(R3/(R2*R4))
4.3 Power Supply
In the computer mode, power to this board is
supplied through header J2 and ultimately from the
host PC via USB. In most cases, the only voltage
needed for the Photodetector Sensor board is the
+5V from the USB connection.
The supply voltage source for the ADC (VREF on
the schematic) is selected with J3 to be either the
4.1V from U5, or +5V from J2.
4.4 ADC Reference Circuitry
The single-ended ADC122S101 uses its supply
voltage as its reference, so it is important that its
supply voltage be stable and quiet. A 4.1V
reference voltage is provided by U5, an accurate
LM4120-4.1.
6 http://www.national.com
4.5 ADC clock
The ADC clock signal is provided external to the
board at header J2. The frequency of this clock
should be in the range of 500KHZ MHz to 1 MHz. A
CS (Chip Select) signal is also required at J2. See
the ADC data sheet for timing requirements.
4.6 Digital Data Output.
The digital output data from the ADC is available at
14-pin header J2. All digital signals to and from the
ADC are present at this connector socket.
4.7 Power Requirements and Settings
Voltage and current requirements for the
Photodetector Sensor Board are:
Pin 14 of J2: +5.0V at 30 mA
Pins 2 and 4 of J2: Ground
With J4 connected from pin 2 to pin 3, the op amps
in the circuit have a -5VDC on the -VCC terminals.
This will yield a more accurate result for zero
current flow in the LED and zero light level received
in the photodiode. J4 pin 1 to pin 2 places GND on
the –VCC pin.
The Photodiode can be reversed biased by
connecting J1 from pin 2 to pin 3. This will reverse
bias the photodiode by -5VDC and reduce the
diode capacitance. This will reduce the effects of
noise gain peaking due to capacitance on the
inverting input of the transimpedance amplifier. J1
pin 1 to pin 2 connects the photodiode anode to
GND.
5.0 Installing and Using the Photodetector
Sensor Board
This Photodetector Sensor board requires power as
described above.
5.1 Board Set-up
Refer to Figure 1 for locations of connectors, test
points and jumpers on the board.
1. Connect the Photodetector Sensor board to a
SPUSI2 USB Interface Dongle.
2.Be sure all jumpers are in place per Table 1,
below.
Table 1 - Jumper Default Positions
Jumper Pins
Shorted FUNCTION
J1 2-3 Neg. Photodiode Bias
J3 2 - 3 4.1V ADC Reference
J4 2 - 3 -5VDC for -VCC
3. Connect a USB cable to the SPUSI2 board and
a PC.
4. Confirm that GRN LED D1 on the
Photodetector Sensor board is on, indicating
the presence of power to the board.
5. Be sure that the correct light source is installed
in close proximity to the photodiode.
6. Be sure the current source values are chosen to
drive the Light Source with the correct current
levels.
7. For more accurate light power measurements it
may be necessary to fashion a light shield to
cover both the light source and the photodiode.
Sufficient room around these two components
has been provided for a light shield.
5.2 Quick Check of Analog Functions
Refer to Figure 1 for locations of connectors and
test points and jumpers on the board. If at any time
the expected response is not obtained, see Section
5.4 on Troubleshooting.
1. Perform steps 1 through 7 of Section 5.1.
2. Check for 5.0V on VCC and for 4.1V at TP8.
3. Check for -5V at J4 pin 3.
4. Turn S1 ON.
5. Monitor the voltage on TP3.
6. As the potentiometer is adjusted, the DC voltage
on TP3 will vary. Verify the LED current is in the
correct range.
7. As the potentiometer is adjusted, the voltage on
TP1 should also vary based on the amount of
light the source is generating.
This completes the quick check of the analog
portion of the evaluation board.
5.3 Quick Check of Software and Computer
Interface Operation
1. Perform steps 1 through 7 of Section 5.1.
2. Run the Sensor Panel software on the PC.
3. Select the SPI202S03RB Board.
4. Manually enter the following data:
Responsivity of the photodiode under test.
RF
Number of bits
Input Optical Power. This value is
determined by setting the LED current,
going to the LED datasheet and reading the
power out at that LED current and at the
photodiode wavelength.
Reference Voltage
The software will measure the photocurrent of
the photodiode and the LED current.
The software knows the input power, calculates
the received power and estimates the optical
loss between the LED and the photodiode.
This completes the quick check of the software and
computer interface.
7 http://www.national.com
5.4 Troubleshooting
If there is no output from the board, check the
following:
Be sure that the proper voltages and polarities
are present.
Be sure there is a clock signal at TP11 when
trying to capture data.
Be sure there is a voltage at TP3 that varies
with light source current adjust.
Be sure that the voltage on TP1 varies with light
source current adjust.
If the ADC output is zero or a single code, check
the following:
Be sure that the proper voltages and polarities
are present.
Be sure that J2 is properly connected to a
SPIUSI-2 USB Interface Dongle.
6.0 Board Specifications
Board Size: 2.6" x 2.5" (6.6 cm x 6.35 cm)
Power Requirements: +5V (30mA) at J2 pin 14
7.0 Example Hardware Schematic
8 http://www.national.com
9 http://www.national.com
10 http://www.national.com
8.0 Example Bill of Materials
Bill of Materials Photodetector
Board
Project: PD_Ref_Des_Rev_B.PrjPCB
Variant: None
Creation Date:
9/24/2008 10:27:25
Print Date: 24-Sep-08 1:40:52 PM
REV B
Quantity
Designator Description Value Footprint Digikey_PN
7 C1, C2, C4, C5, C6,
C7, C8 Capacitor 0.1uF 603 445-1317-1-ND
1 C3 Capacitor 10uF 1206 445-1391-1-ND
1 C9 Capacitor 10uF 805 587-1295-1-ND
2 C10, C11 Capacit or 470pF 603 445-1307-1-ND
1 C12 Polarized Ca pacitor Value 0805L 478-3265-2-ND
4 C13, C14, C15, C16 Capacit or 1uF 603 445-1604-1-ND
1 CF Capacit or Sensor Designer Sensor Designer Sensor De signer
1 D1 LED, RED User Specified LED
3 J1, J3, J4 Header, 3-Pi n HDR, 1x3 HDR1X3-A S1011E-03-ND
1 J2 Header, 7-Pi n, Dual row HDR2X7H-B
1 J5 Header, 2-Pi n HDR, 1x2 HDR1X2_A WM6502-ND
1 LED1 Typical INFRARED GaAs LED LED-1A
1 PD1 Photo Di ode, 900nm SFH213 Photo-1A 475-1077-ND
2 Q1, Q2 N-Channel Enhancement Mode
Vertical DMOS FET 2N7002 SOT23 2N7002CT-ND
1 RF Resistor Sensor Designer Sensor Des igner Sensor Designer
2 R1, R4 Resistor 10.0ohm 603 541-10.0HCT-ND
2 R2, R5 Resistor 1.0Mohm 603 541-1.0MGCT-ND
2 R3, R6 Resistor 47.5kohm 603 541-47.5kHCT-N D
1 R7 Resistor 71.5ohm 603 541-71.5HCT-ND
1 R8 Resistor 10.0kohm 603 541-10.0kHCT-ND
2 R9, R17 Resis tor 0ohm 603 541-0. 0GTR-ND
2 R10, R12 Resistor 180ohm 603 541-180GCT-ND
1 R11 Resistor 330ohm 603 311-330GCT-ND
1 R13 Resistor 47kohm 603 541-47kGCT-ND
1 R14 Resistor 39.2kohm 603 541-39.2kHCT-ND
1 R15 Resistor 12.4kohm 603 541-12.4kHCT-ND
1 S1 Single-Pole, Single-Throw Switch SPST-2 CKN6012-ND
11 TP1, TP3, TP5, TP6,
TP7, TP8, TP9, TP11,
TP12, TP 13, TP 14
Test Po int White Testpoint Keystone
500x 5002K-ND
3 TP2, TP4, TP10 Test Point Black TEST POI N T 5001K-ND
1 U1 Sensor Designer Sensor Designer Sensor Designer Sensor Des i gner
2 U2, U3 Precision, CMOS Input, RRIO, Wide
Supply Range A mplif i ers LMP7701MF MF05A LMP7701MFCT-ND
1 U4 2 Channel, 1 MSPS, 12-Bi t A/D
Converter ADC122S101CIMM MUA08A ADC122S101CIMMCT-ND
1 U5 Precision Micropower Low Dropout
Voltage Reference LM4140ACM-4.1 SOIC8 LM4140BCM-4.1-ND
1 U6 Low Noise Regulated S witched
Capacit o r V ol tage Inverter LM2687MM MUA08A LM2687MMCT-ND
1 VR1 Potentiometer 1K VR5_A 3250W-102-ND
3 Shunt 2 pin shunt S9001-ND
11 http://www.national.com
APPENDIX
Summary Tables of Test Points and Connectors
Test Points on the Photodetector Sensor Board
Identifier Name Function
TP 1 Analog_V1 TIA Analog Output
TP 2 GND Ground
TP 3 Analog_V2 Current Meter Analog Output
TP 4 GND Ground
TP 5 Isense + Current Sense Resistor +
TP 6 Isense - Current Sense Resistor -
TP 7 J5 Pin 1 Waveform Generator Input
TP 8 VREF ADC VDC
TP 9 CS* CS* input for ADC
TP 10 GND Ground
TP 11 SCLK SCLK Input to ADC
TP 12 DOUT SDATA output from ADC
TP 13 VCC_3V3 3.3VDC from SPUSI2 Board
TP 14 DIN ADC DIN from SPUSI2 Board
J1 Jumper – Photodiode Bias Select
Shorted Positions Results
1 - 2 GND on Photodiode Anode
2 - 3 -5VDC on Photodiode Anode
J3 Jumper - VADC_SEL
Shorted Positions Results
1 - 2 +5V for ADC Supply and Reference Voltage
2 - 3 +4.1V for ADC Supply and Reference Voltage
J4 Jumper - -VCC Select
Shorted Positions Results
1 - 2 -VCC = GND
2 - 3 -VCC = -5VDC
12 http://www.national.com
Summary Tables of Test Points and Connectors (cont'd)
J2 Connector - Connection to SPUSI2 Board
J2 Pin Number Voltage or Signal
1 CS* input to ADC
2 Ground
3 SCLK input to ADC
4 Ground
5 SDATA output from ADC
6 no connection
7 DIN to ADC
8 no connection
9 no connection
10 no connection
11 no connection
12 no connection
13 +3.3V from SPUSI2 USB Interface Dongle
14 +5V from SPUSI2 USB Interface Dongle
13 http://www.national.com
The Photodetector Sensor Board is intended for product evaluation purposes only and is not intended for resale to end consumers, is not
authorized for such use and is not designed for compliance with European EMC Directive 89/336/EEC.
National does not assume any responsibility for use of any circuitry or software supplied or described. No circuit patent licenses are implied.
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