AN692

M
AN692
Using a Digital Potentiometer to Optimize a Precision
Single-Supply Photo Detection Circuit
Author:
Photodiode Characteristics
Bonnie C. Baker
Microchip Technology Inc.
A photodiode can be operated in the Photovoltaic or
Photoconductive mode, as shown in Figure 2.
INTRODUCTION
Photodiodes bridge the gap between light and
electronics. Many times, precision applications (such
as CT scanners, blood analyzers, smoke detectors,
position sensors, IR pyrometers and chromatographs)
utilize the basic transimpedance amplifier circuit that
transforms light energy into a usable electrical voltage.
In these circuits, photodiodes are used to capture the
light energy and transform it into a small current. This
current is proportional to the level of illumination from
the light source. A preamplifier then converts the
current (in amperes) from the photodiode sensor into a
usable voltage level.
RF
+5V
Light
Source
ISC
MCP601
VOUT
VOUT = +ISCRF
FIGURE 1:
In this precision
photosensing configuration, a photodiode (in the
Photovoltaic mode) is used to capture the
luminance energy from a light source. The effects
of the variability, due to alignment problems, is
reduced by using a potentiometer for the resistive
element in the feedback loop of the amplifier.
Light
Source
Light
Source
-VBIAS
b) Photodiode
a) Photodiode
configured in
configured in
Photovoltaic mode
Photoconductive mode
FIGURE 2:
The two modes that
photodiodes can be used in are: (a) Photovoltaic
and (b) Photoconductive. In the Photovoltaic
mode, the photodiode is biased with zero volts
which optimizes the sensor’s accuracy. In the
Photoconductive mode, the diode is reverse
biased in order to optimize the responses to step
functions.
A photodiode configured in the Photovoltaic mode is
zero biased. In this mode, the light-to-current response
of the diode is maximized for light sensitivity and
linearity, making it well suited for precision applications.
A photodiode configured in the Photoconductive mode
has a reverse voltage bias applied. In this mode, the
photodiode is optimized for fast response to light
sources. An ideal application for a diode configured in
the Photoconductive mode is digital communication.
The key elements that influence the circuit
performance of each mode are the photodiode
capacitance (CPD) and the photodiode leakage current
(IL), as shown in Figure 3. These parasitic elements
can effect the precision and speed of photo detection
circuits.
This application note will show how the adjustability of
the digital potentiometer can be used as an advantage
in photosensing circuits. Initially, photodiode
characteristics will be looked at, followed by various
digital potentiometer circuits that use photodiodes in
the Photoconductive and Photovoltaic modes.
 2004 Microchip Technology Inc.
DS00692B-page 1
AN692
RS
+
ISC IL
DPD
CPD RPD
VOUT
-
When light illuminates on the photodiode, current (ISC)
flows from the anode to the cathode of the device. The
transfer function of light-to-photodiode current is equal
to the following:
EQUATION
I SC = Radiant Flux Energy/Flux Responsivity
where:
FIGURE 3:
The photodiode can be
described with an ideal current source (ISC) that is
a result of radiant flux energy from light, an ideal
diode (DPD ), a junction capacitance (CPD ),
leakage current (IL ), a parasitic series resistor
(RS ) and a shunt resistor (RPD ).
The junction capacitance (CPD) is determined by the
width of the depletion region between the p-type and
n-type material of the photodiode. The depletion
region width of the photodiode is inversely proportional
to the diode’s reverse bias voltage. Wider depletion
regions will increase the magnitude of the junction
capacitance. Conversely, wider depletion regions
(found with PIN photodiodes) have broader spectral
responses.
Values of the junction capacitance of silicon
photodiodes in the Photovoltaic mode (zero bias) range
from approximately 20 pF to 25 pF, up to several
thousand pico farads. Values of the junction
capacitance of silicon photodiodes in the Photoconductive mode (with a reverse bias of -10V) are generally
ten times lower. This reduced parasitic capacitance
facilitates high-speed operation. However, the linearity
and offset errors are not optimized.
A reverse bias voltage across the photodiode will
cause an increase in leakage current, IL. When the
reverse bias voltage exceeds several millivolts,
linearity starts to be compromised in precision circuits.
With large voltages, this leakage current can be high
enough to make the diode only useful in digital
applications.
The shunt resistance (RPD), also called “dark”
resistance, is measured with zero volts across the
element. At room temperature, the magnitude of this
resistance typically exceeds 100 MΩ. In most circuits,
this resistance is generally ignored.
The second parasitic diode resistance (RS) is known as
the series resistance of the diode. This parasitic
resistance typically ranges from 10 to 1,000Ω. Due to
the small size of this resistor, it only has an affect on the
frequency response of the circuit well past the
bandwidth of operation.
DS00692B-page 2
ISC = the current produced by the photodiode
with units in amperes/cm2.
Radiant Flux Energy = the light energy with
units in watts/cm2.
Flux Responsivity = the measure of the photodiode’s sensitivity with units in watts/
amperes.
Photovoltaic Mode Circuits
A practical way to design a precision photosensing
circuit is to place a photodiode in a Photovoltaic mode.
This can be done by placing the device across the
inputs of a CMOS input amplifier and a resistor in the
feedback
loop.
The
single-supply
circuit
implementation of this circuit is shown in Figure 4.
CF
PB
PA RF
CRF
+5V
MCP41100
Digital
Potentiometer
Light
Source
ISC
MCP601
VOUT
VOUT = +ISCR F
FIGURE 4:
This is a standard, singlesupply, transimpedance amplifier circuit with the
photodiode in the Photovoltaic (zero bias) mode.
In this circuit, the light source illuminates the
photodiode, causing diode current to flow from cathode
to anode. Since the input impedance of the inverting
input of the MCP601 CMOS amplifier is extremely high,
the current generated by the photodiode flows through
the feedback resistor (RF). In this configuration, the
feedback resistor is implemented with a digital
potentiometer (MCP41100).
 2004 Microchip Technology Inc.
AN692
The current-to-voltage transfer function of this circuit is:
EQUATION
Another circuit configuration that can be used for
Photovoltaic mode circuits is shown in Figure 5.
V O UT = I SC × R F
RF – B RF – A
with a single pole at 1/(2pRF(CRF + CF))
RF – W
where:
VOUT = the voltage at the output of the
operational amplifier in volts.
ISC = the current produced by
photodiode with units in amperes.
LIGHT
SOURCE
MCP41010
Digital
Potentiometer
+5V
ISC
the
VOUT
MCP601
RF = a digital potentiometer that is serving
as the feedback resistor with units in ohms.
CRF = the parasitic capacitance of the digital
potentiometer with units in farads. This
parasitic capacitor can cause oscillation
with some digital potentiometer settings. If
this occurs, place a 100 pF to 500 pF in
parallel (CF) with the digital potentiometer,
as shown in Figure 4.
The programmed value of the digital potentiometer
(RF) is equal to:
EQUATION
D CO DE × R NOMINAL
R F = -------------------------------------------------n
2
where:
DCODE = the programmed code to the digital
potentiometer.
VOUT = + ISC (1+RF – A/RW)RF – B
RF – B>>R F – A
FIGURE 5:
In this precision lightsensing circuit, the potentiometer is to implement
a T-network style feedback loop. This configuration provides higher gains while using a lower
value potentiometer.
In this circuit, the digital potentiometer is configured to
form a T-network. The digital potentiometer is a good fit
in this circuit because of its low wiper resistance and
resistor adjustability. The potentiometer’s A and B
resistive elements are used in this circuit so that the
gain versus the potentiometer digital code is linear.
The transfer function of this circuit is:
EQUATION
RNOMINAL = the nominal resistance of the
digital pot from PA to PB.
n = the number of bits that the digital
potentiometer has. In the case of Microchip
digital potentiometers, the ‘n’ is equal to
eight.
If the digital potentiometer is programmed to equal
50 kΩ (DCODE = 128), the maximum current from the
photodiode is 75 µA and the maximum output voltage
(VOUT) is 3.75V. With this configuration, the digital
potentiometer capacitance (CRF) is 75 pF. As a result,
the frequency response of the circuit is equal to
1/2πRFCRF or 42.4 kHz.
V OU T
1 + R F – A
------------- = R F – A +  ---------------------- RF – B
 R

I SC
W
where:
RF – A = the A portion of the digital
potentiometer resistor.
RF – B = the A portion of the digital
potentiometer resistor.
RW = the parasitic resistance through the
wiper.
Circuit flexibility is added with the inclusion of a digital
potentiometer, as opposed to a standard resistor. By
changing the value of the digital potentiometer, the
maximum output voltage (VOUT) can be adjusted. This
kind of flexibility accommodates alignment problems
between the light source and the photodiode.
 2004 Microchip Technology Inc.
DS00692B-page 3
AN692
This formula can be further worked by using the
following substitutions:
EQUATION
R F – A = R F – NOM – R F – B
R F – NOM D n
R F – B = ----------------------------n
2
where:
RF – NOM = the nominal resistance across the
digital potentiometer. In Figure 5, this value is
equal to 10 kΩ.
Dn = the programmed digital code of the
potentiometer.
n = the number of bits of the digital
potentiometer. In Figure 5, this value is equal
to eight.
Given all of the above calculations, the graph in
Figure 6 shows the gain of this T-network circuit for the
entire digital code range of the MCP41010. The
resistive values used in this graph are:
The primary sources of error that effect the
performance of this circuit are amplifier offset voltage,
amplifier noise and digital potentiometer noise.
The actual offset voltage of the amplifier will produce a
gain error in the lower codes. For instance, an offset
voltage of 0.35 mV will produce a 4.2% error when the
digital potentiometer is set to code 50. When the offset
of the amplifier is 0.1 mV, the gain error of the circuit is
1% with the same digital potentiometer code.
In cases where this circuit is used for precision sensing,
the noise response of the circuit should be kept as low
as possible. The two factors that effect the overall noise
originate from the amplifier and the resistive network. In
order to achieve the lowest possible noise in this circuit
R F – B >> RF – A. The range of digital potentiometer
codes that meet this criteria is from codes 233 to 255.
Photoconductive Mode Circuits
The response of a photodiode can be configured in the
Photoconductive mode, as shown in Figure 7.
RF – NOM = 10 kΩ (data sheet typical)
RW = 25Ω (data sheet typical)
PB PA
+5V
Transimpedance Amplifier Gain (V/A)
1200000
Gain Calculations Use
Typical Values
for R F-NOM and R W
1000000
Light
Source
ISC
MCP601
RF
MCP41050
Digital
Potentiometer
VOUT
800000
600000
-5V
400000
Resistive Range
that will Give the
Lowest Noise
Response
200000
0
0
50
100
150
200
250
Digital Code of Potentiometer
VOUT = ISC * RF
FIGURE 7:
When a photodiode is
configured in the Photoconductive mode, the
diode is reversed biased in order to reduce the
diode parasitic capacitance.
FIGURE 6:
This graph shows the gain
versus digital code of the circuit shown in
Figure 7.
DS00692B-page 4
 2004 Microchip Technology Inc.
AN692
CONCLUSION
REFERENCES
The two modes that a photosensing circuit can be
configured are: Photovoltaic and Photoconductive.
Photovoltaic configurations are best suited for
precision circuits, while Photoconductive configurations are best suited for higher speed, digital circuits. If
real-time adjustability of photodiode current to voltage
gain is an issue in these photo detection circuits, a digital potentiometer can effectively be used to achieve
this goal.
“Keeping the Signal Clean in Photosensing
Instrumentation”, Bonnie C. Baker, SENSORS,
June 1997.
This application note presents three photosensing
circuits configured with a digital potentiometer for realtime adjustments that can be used to calibrate LED/
photodiode alignment variability.
“The Eyes of the Electronics World”,
http://www.chipcenter.com/analog/tn006.htm,
Bonnie C. Baker, Knowledge Center, Analog,
January, 1998.
“Comparison of Noise Performance between a FET
Transimpedance Amplifier and a Switched Integrator”,
Bonnie C. Baker, Burr-Brown Application Note, AB-057,
January 1994.
“Optoelectronics”, Reston Publishing Company, Inc.,
Robert G. Seippel, 1981.
“Photodiode Amplifiers”, Jerald Graeme, McGraw Hill,
1996.
“Design a Precision, Single-Supply Photo Detection
Circuit”, http://www.chipcenter.com, Bonnie C. Baker,
Knowledge Center, Online Tools, June, 1999.
 2004 Microchip Technology Inc.
DS00692B-page 5
AN692
NOTES:
DS00692B-page 6
 2004 Microchip Technology Inc.
Note the following details of the code protection feature on Microchip devices:
•
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•
Microchip believes that its family of products is one of the most secure families of its kind on the market today, when used in the
intended manner and under normal conditions.
•
There are dishonest and possibly illegal methods used to breach the code protection feature. All of these methods, to our
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•
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•
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