AN1494

AN1494
Using MCP6491 Op Amps for Photodetection Applications
Author:
Yang Zhen
Microchip Technology Inc.
INTRODUCTION
Low input bias operational amplifiers (op amps) are
often required for a wide range of photodetection
applications, in order to reduce current error and
improve the accuracy of the output signal.
The typical photodetection applications are listed
below:
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
Smoke Detectors
Flame Monitors
Airport Security X-Ray Scanners
Light Meters
Brightness Controls
Bar Code Scanners
Pulse Oximeters
Blood Particle Analyzers
CT Scanners
Automotive Headlight Dimmers
Twilight Detectors
Photographic Flash Controls
Automatic Shutter Controls
Optical Remote Controls
Optical Communications, etc.
This application note discusses the features of Microchip’s MCP6491 low input bias current op amps [1], the
characteristics of photodiodes, and the strengths of the
active photodiode current-to-voltage converter (i.e.
photodiode amplifier), compared to the passive version. Next, the focus shifts to the design techniques of
photodiode amplifier circuitry. Several key design
points are discussed in order to improve the circuit’s
performance. Then, a practical application example
with PSpice simulation results is provided, to help illustrate the design techniques in depth. In addition, the
noise analysis of the photodiode amplifier and the
design of a companion low pass filter are discussed.
Finally, the PCB techniques that help reduce the current leakage are briefly introduced.
 2013 Microchip Technology Inc.
MCP6491 LOW INPUT BIAS
CURRENT OP AMPS
Microchip’s MCP6491 family of op amps has low input
bias current (150 pA, typical at 125°C) and rail-to-rail
input and output operation. The MCP6491 family is
unity gain stable and has a gain bandwidth product of
7.5 MHz (typical). These devices operate with a singlesupply voltage as low as 2.4V, while only drawing
530 μA/amplifier (typical) of quiescent current. These
features make the MCP6491 family of op amps well
suited for photodiode amplifier, pH electrode amplifier,
low leakage amplifier, and battery-powered signal conditioning applications, etc.
Features:
• Low Input Bias Current
- 1 pA (typical at 25°C)
- 8 pA (typical at 85°C)
- 150 pA (typical at 125°C)
• Low Quiescent Current:
- 530 µA/amplifier (typical)
• Low Input Offset Voltage:
- 1.5 mV (maximum)
• Rail-to-Rail Input and Output
• Supply Voltage Range: 2.4V to 5.5V
• Gain Bandwidth Product: 7.5 MHz (typical)
• Slew Rate: 6 V/µs (typical)
• Unity Gain Stable
• No Phase Reversal
• Small Packages
- Singles in SC70-5, SOT-23-5
• Extended Temperature Range
- -40°C to +125°C
Related Parts
• MCP6481: 4 MHz, Low Input Bias Current Op
Amps [2]
• MCP6471: 2 MHz, Low Input Bias Current Op
Amps [3]
DS01494A-page 1
AN1494
PHOTODETECTION APPLICATIONS
There are many detectors which can be used for photodetection applications, such as photodiodes, phototransistors, photoresistors, phototubes, photomultiplier
tubes, charge-coupled devices, etc.
In this application note, we will focus on the photodiode, as it is the most common photodetector and
widely used for the detection of intensity, position, color
and presence of light.
Photodiode
The photodiode is a type of photodetector capable of
converting light to a small current which is proportional
to the level of illumination.
FEATURES
The photodiode’s features can be summarized as
below:
•
•
•
•
•
•
•
Wide spectral response
Excellent linearity
Low noise
Excellent ruggedness and stability
Small physical size
Long lifetime
Low cost
• Series Resistance (RS)
RS is the resistance of the wire bonds and contacts of
the photodiode. An ideal photodiode should have no
series resistance, but the typical value is on the order
of tens of Ω, which is much smaller than RJ. The RS is
used to determine the linearity of the photodiode under
zero bias conditions. For most of applications, it can be
ignored.
• Junction capacitance (CJ)
CJ is directly proportional to the junction area and
inversely proportional to the diode reverse bias voltage.
For a small area diode at zero bias, the typical value is
on the order of tens of pF.
ID is the small leakage current that flows through
photodiode under reverse bias conditions. It exists
even when there is no illumination and approximately
doubles for every 10°C rise in temperature. There is no
dark current under zero bias conditions.
The equivalent circuit for a photodiode is shown below,
in Figure 1.
RS << RJ
ID
Ideal
Diode
RJ represents the resistance of the zero-biased photodiode junction. An ideal photodiode will have an infinite
RJ, but the actual value of RJ is typically on the order of
thousands of MΩ, which depends on the photodiode
material, and decreases by a factor of 2 for every 10°C
rise in temperature. The high value of RJ yields the low
noise current of the photodiode.
• Dark Current (ID)
EQUIVALENT CIRCUIT
Light
• Junction Shunt Resistance (RJ)
OPERATION MODES
There are two operation modes for the photodiode, the
photovoltaic mode and the photoconductive mode, as
shown in Figure 2 and Figure 3. The two modes have
their own strengths and drawbacks, and mode selection is dependent on the target application.
• Photovoltaic Mode
I
RJ
CJ
This mode has zero voltage potential across the photodiode. No dark current flows through the photodiode,
the linearity and sensitivity are maximized, and the
noise level is relatively low (RJ’s thermal noise only),
which make it well suited for precision applications.
I = current source
(photocurrent generated by the incident light)
RJ = junction shunt resistance
CJ = junction capacitance
RS = series resistance
ID = dark current
FIGURE 1:
Circuit.
A Photodiode Equivalent
A photodiode can be represented by a current source
(I), a junction shunt resistance (RJ), and a junction
capacitance (CJ) in parallel with an ideal diode. The
series resistance (RS) is connected with all other components in series. Dark current (ID) only exists under
reverse bias conditions.
DS01494A-page 2
Light
FIGURE 2:
Photovoltaic Mode.
 2013 Microchip Technology Inc.
AN1494
• Photoconductive Mode
This mode has a reverse bias voltage placed across
the photodiode. The reverse bias voltage reduces the
diode junction capacitance and shortens the response
time. Therefore, the photoconductive mode is suitable
for high speed applications (e.g., high speed digital
communications). The main drawbacks of this mode
include dark current appearance, non-linearity, and
high noise level (RJ’s thermal noise and ID’s shot
noise).
Light
VBIAS  0V
FIGURE 3:
Photoconductive Mode.
Photodiode Current-to-Voltage Converter
ACTIVE PHOTODIODE CURRENT-TOVOLTAGE CONVERTER
The active photodiode current-to-voltage converter is
also called a photodiode amplifier. Based on the photodiode operating modes, two circuit implementations of
photodiode amplifiers are shown in Figure 5 and
Figure 6. For the strengths and drawbacks of each
implementation, please refer to the section “Operation
Modes”.
Both implementations have a large resistor (RF) in the
feedback loop. The output resistance of the photodiode
amplifier is roughly equal to RF/AOL, where AOL is the
open loop gain of the op amp. Therefore, the output
resistance becomes very small and the loading effects
can be ignored.
For the photoconductive mode amplifier, the biasing
voltage is equal to VBIAS. For the photovoltaic mode
amplifier, the biasing voltage is just zero. Both biasing
voltages do not change when the photocurrent varies,
so the photodiode's frequency response will not be
affected.
These strengths of the photodiode amplifier make it
widely used in photodetection applications.
This circuit is used to convert the photodiode’s small
output current to a measurable voltage. Typically, there
are two types of circuit implementations, which are
passive and active versions.
RF
PASSIVE PHOTODIODE CURRENT-TOVOLTAGE CONVERTER
The Passive Photodiode Current-to-Voltage Converter
is implemented by only passive components, as shown
in Figure 4. Its output resistance is roughly equal to the
value of large resistor (RF) and the output voltage is
equal to I*RF.
The large RF can cause loading effects for subsequent
load resistance and capacitance, such as an inaccurate VOUT and a relatively long response time.
Moreover, the variation of photocurrent can cause the
photodiode’s biasing voltage to be unstable, which will
change the junction capacitance (CJ) and affect the
frequency response of photodiode.
–
I
VDD
VOUT
MCP6491
Light
+
V OUT = I  R F
FIGURE 5:
diode Amplifier.
“Photovoltaic Mode” Photo-
RF
–
I
+
VDD
VOUT
MCP6491
RF
Light
I
V OUT = I  R F
–
Light
+
VBIAS
V BIAS  0V V OUT = I  R F
FIGURE 4:
Passive Photodiode
Current-to-Voltage Converter.
 2013 Microchip Technology Inc.
FIGURE 6:
“Photoconductive Mode”
Photodiode Amplifier.
DS01494A-page 3
AN1494
Photodiode Amplifier Key Design Points
Rail-to-Rail Output
Several key design points for the photodiode amplifier
will be analyzed next, in order to improve the circuit’s
performance.
The rail-to-rail output is helpful to maximize the
dynamic output voltage range and improve the signalto-noise ratio (SNR).
OP AMP SELECTION
Wide Gain Bandwidth Product (GBWP) and High
Slew Rate (SR)
Selecting a suitable op amp for the photodiode
amplifier is critical. There are many DC and AC specs
in an op amp data sheet, and the key op amp specs for
the photodiode amplifier are shown and discussed
below.
Low Input Bias Current (IB)
The DC output voltage error due to IB is equal to IB*RF.
IB increases with temperature rise, so the error will be
larger at higher temperature. Usually, the voltage error
can be reduced to IOS*RF by adding a compensation
resistor RC with a value of RFǁRJ in series with the op
amp non-inverting input.
However, at high temperatures, the value of RC is
difficult to determine because the value of RJ
significantly drops with temperature rise. In this
condition, the value of RJ could be less than the value
of RF.
Moreover, RC will develop a noise voltage as the op
amp input noise current flows through it. The RC also
generates a thermal noise voltage. Both noise voltages
will be amplified by the circuit's noise gain. Thus, the
output noise level will increase.
IB also generates a voltage across RC at the op amp's
non-inverting input. This causes the same voltage at
the inverting input. Now, the biasing voltage is no
longer stable, which causes the photodiode's response
to become nonlinear.
Therefore, adding the compensation resistor RC to
reduce the voltage error IB*RF is not an effective
method in general. The op amp should have IB low
enough to keep the voltage error within an acceptable
range of target applications.
Low Input Offset Voltage (VOS)
The DC output voltage error due to VOS is equal to
VOS*(1 + RF/RJ) at room temperature (25°C), which is
about VOS because RF is much less than RJ and the
gain is approximately 1 V/V. At high temperatures, the
error could be much larger because the value of RJ
significantly decreases and the gain can be higher than
1 V/V. Moreover, the VOS drift could make the error
even worse. Therefore, low VOS and low VOS drift will
be very helpful to reduce the output error at high
temperatures.
Common Mode Input Voltage Range
The GBWP and SR should be large enough to meet the
requirement of output step response time, which will be
discussed in more detail later.
Low Input Noise Current Density and Low Input
Noise Voltage Density
When noise current flows through the photodiode
amplifier, resistor noise voltages will result. The op amp
input current noise density (2qI, where q is electron
charge, I is current) is determined by IB, so that lower
IB gives lower op amp input noise current density. The
low input noise voltage density also plays a very important role for the output noise of the photodiode amplifier. It will be amplified by the noise gain so that the
output noise level will be significantly affected. This will
be explained later in this section.
In conclusion, the Microchip’s MCP6491 op amp’s key
features include low IB, low VOS, low VOS drift with temperature, rail-to-rail input/output, wide GBWP, high SR,
low input noise current density and low input noise voltage density, etc. These features make it well suited for
the photodiode amplifier.
FEEDBACK RESISTOR
The value of the feedback resistor (RF) should be set
as large as possible to give a high transimpedance gain
to the photocurrent. Usually, this gain should be high
enough to use most of the op amp’s output voltage
swing when the photocurrent is at its maximum value.
For precision applications, a large resistor with tight tolerance and a low temperature coefficient should be
selected.
It is possible to add more gain with subsequent stages,
however, the noise performance will not be as good as
using a large RF in one stage, which can easily improve
the SNR.
For a given bandwidth f, the thermal noise voltage of
RF is given by 4kTRFf, where k is Boltzmann’s
constant (1.38 x 10-13J/K), T is absolute temperature
(K), RF is feedback resistance (Ω).
The output signal is given by VSIGNAL = I*RF and the
SNR = 20*log(VSIGNAL/VNOISE). When RF is doubled,
the resistor thermal noise voltage is increased by 2
and the output signal voltage is increased by 2. Thus,
the SNR is increased by 3 dB.
The common mode input voltage range needs to at
least include ground because the non-inverting input of
the op amp is grounded.
DS01494A-page 4
 2013 Microchip Technology Inc.
AN1494
FEEDBACK CAPACITOR
Moreover, the noise gain peaking results in very high
output noise levels (Figure 9), which may severely
degrade the integrity of the output signal.
100
80
Noise Gain (dB)
N
The photodiode amplifier does not always behave as
desired. The gain peaking and step output ringing are
typical phenomena, which could happen in frequency
and time domains (refer to Figure 7 and Figure 8).
60
40
20
Transimp
pedance Gain (V/A)
100M
0
10M
-20
1
10
100
1M
FIGURE 9:
100k
10M
Noise Gain Peaking.
1k
10.00
10
100.00
1,000.00
10,000.00
100,000.00
1,000,000.00 10,000,000.0
100
1k 10k
100k
1M 10M
Frequency (Hz)
CF
FIGURE 7:
Signal Gain (i.e. Transimpedance Gain) Peaking.
RF
6
Step Ou
utput Voltage (V)
1M
These phenomena make the photodiode amplifier
unstable. A small capacitor (CF) can be added in the
feedback loop to eliminate the gain peaking, step output ringing and noise gain peaking issues (refer to
Figure 10).
10k
100
1.00
1
1k
10k 100k
Frequency (Hz)
I
–
5
Ideal
Diode
4
VDD
MCP6491
RJ
CJ
+
VOUT
3
2
V OUT = I  R F
1
0
0
FIGURE 8:
0.5
1.0
1.5
2.0
Time (ms)
Output Ringing.
 2013 Microchip Technology Inc.
2.5
3.0
FIGURE 10:
Photodiode Amplifier Using
Feedback Capacitor.
In the next section, we will discuss the stability of the
photodiode amplifier, explain why the amplifier will be
stable after adding a feedback capacitor, and learn how
to determine the value of the feedback capacitor to get
the optimum output response.
DS01494A-page 5
AN1494
AMPLIFIER STABILITY ANALYSIS
Figure 11 shows the noise gain bode plot of the photodiode amplifier in log-log scale. It is important to clarify
the difference between the noise gain and the signal
gain because the system stability is dependent on the
characteristics of noise gain, not signal gain.
The noise gain is the gain seen by a testing voltage
source in series with the op amp non-inverting input,
which is equal to the signal gain when the signal is
applied to the op amp non-inverting input.
(dB)
AOL
-20 dB/dec
with adding CF (Stable)
GN
f1
f2
f3
• For an unstable photodiode amplifier, the net
slope between GN and AOL is equal to
+40 dB/decade as shown in Figure 11, where the
dotted line of GN intercepts the curve of AOL. The
dashed line shows the extended GN curve without
adding CF.
• For a stable photodiode amplifier, the net slope
between GN and AOL is equal to +20 dB/decade
as shown in Figure 11, where the solid line of GN
intercepts the curve of AOL. The solid line shows
the GN curve with adding CF.
The explanation on the noise gain Bode plot is shown
below:
without adding CF (Unstable)
+20 dB/dec
The stability of the system is determined by the net
slope between the noise gain (GN) and the open loop
gain (AOL) at the frequency where they cross over.
f (Hz)
1
f 1 = -------------------------------------------------------------------------------2   R J  R F    C J + C OP + C F 
1
f 2 = ----------------------------2  R F  C F
GBWP
GBWP
f 3 = ----------------- = --------------------------------C J + COP
GN
1 + ----------------------CF
Where
RJ = junction shunt resistance
RF = resistance of feedback resistor
CJ = junction capacitance
CF = feedback capacitance
COP = op amp input capacitance
= CCM + CDM
CCM = op amp common mode input capacitance
CDM = op amp differential mode input capacitance
GBWP = op amp gain bandwidth product
GN = noise gain
AOL = op amp open loop gain
f1 = the location of GN’s first zero
f2 = signal gain bandwidth
(i.e. transimpedance gain bandwidth)
• When f is less than f1:
- GN is equal to 1 + RF/RJ, which is roughly
equal to 1 V/V or 0 dB when RF << RJ.
- The zero of GN is located in f1.
• When f is between f1 and f2:
- GN increases by +20 dB/dec.
- The pole of GN is located in f2, which is equal
to 1/(2*RF*CF). This is also the signal gain
bandwidth.
• When f is between f2 and f3:
- GN is equal to 1 + (CJ + COP)/CF.
- The crossover frequency of AOL and GN is
located in f3, which is equal to GBWP/GN.
• When f is larger than f3:
- GN is determined and limited by AOL, which
decreases by -20 dB/dec.
The value of CF affects the location of f2, which
determines the signal gain bandwidth and the phase
margin of the photodiode amplifier.
When CF becomes larger, the phase margin will be
increased, which makes the system more stable with
less gain peaking, step overshoot and noise gain
peaking. However, this also will result in smaller signal
gain bandwidth and longer output response time.
Table 1 below shows the percent overshoot as a result
of different phase margins.
TABLE 1:
Phase Margin (°)
Overshoot (%)
45
25
55
13.3
65
4.7
75
0.008
f3 = noise gain bandwidth
FIGURE 11:
DS01494A-page 6
Noise Gain Bode Plot.
 2013 Microchip Technology Inc.
AN1494
For most photodetection applications, the optimum
value of CF is typically considered when the phase
margin is 65°, which gives a negligible gain peaking
and 4.7% overshoot at output, while keeping reasonable signal gain bandwidth and response time.
The value of CF at 65° phase margin is approximately
shown in Equation 1 where RF << RJ is assumed.
EQUATION 1:
MCP6491 op amp’s typical GBWP is 7.5 MHz and its
input capacitance is COP = CCM + CDM = 12 pF.
To make the photodiode amplifier stable, a feedback
capacitor CF is needed. Based on Equation 1, the
value of CF is 1 pF when the amplifier’s phase margin
is 65°.
At room temperature (25°C), the DC voltage error at
output due to IB and VOS of MCP6491 is given by
IB*RF + VOS = 1 pA*10 MΩ + 1.5 mV = 1.51 mV.
The graphs in Figure 13 — Figure 17 show the related
output response plots with and without adding CF.
C J + C OP
CF  2  ---------------------------------------2  R F  GBWP
These plots are PSpice simulation results
by using MCP6491 op amp Spice macro
model, which is free on the Microchip web
site at www.microchip.com. The model is
intended to be an initial design tool. Bench
testing is a very important part of any
design and cannot be replaced with
simulations.
Note:
In Figure 11, the maximum signal gain bandwidth is
achieved at 45° phase margin when f2 is equal to f3,
and the corresponding value of CF will be half of the
one shown in Equation 1.
If we consider the effect of RF’s parasitic capacitance,
CF will be the value of the one shown in Equation 1
minus RF’s parasitic capacitance.
APPLICATION EXAMPLE
Here we provide an example to illustrate the circuit’s
performance improvement in frequency and time
domains after the feedback capacitor is added.
In Figure 12, the photodiode’s RJ = 2000 MΩ at 25°C,
CJ = 100 pF, MCP6491 op amp’s VDD = 5.5V,
RF = 10 MΩ, and assume VOUT switches between 2V
and 4V for the two alternating illumination levels.
Transimp
pedance Gain (V/A)
Normally, the parasitic capacitance is less than 0.1 pF
for a surface mount resistor due to its small size. Thus,
the effect of the parasitic capacitance can be ignored.
1G
1.00E+09
without CF
1.00E+08
100M
1.00E+07
10M
with CF
1.00E+06
1M
1.00E+05
100k
1.00E+04
10k
1.00E+03
1k
1.00E+02
100
1
10
FIGURE 13:
CF
100
1k 10k 100k
Frequency (Hz)
1M
10M
Signal Gain vs. Frequency.
6
10 MΩ
–
2000
MΩ
100
pF
VDD
MCP6491
+
VOUT
V OUT = I  RF
Step O
Output Voltage (V)
I
without CF
5
with CF
4
3
2
1
0
0
0.5
1.0
1.5
2.0
2.5
3.0
Time (ms)
FIGURE 12:
Example.
Photodiode Amplifier Circuit
 2013 Microchip Technology Inc.
FIGURE 14:
Step Output Response.
DS01494A-page 7
AN1494
Photodiode Amplifier Noise Analysis
100
Figure 18 shows the noise model of the photodiode
amplifier.
without CF
60
CF
40
VN_RF
with CF
20
RF
–
+
Noise Gain (dB)
N
80
0
–
-20
10
100
Total Outp
put RMS Noise Voltage
De
ensity (nV/Hz)
1M
RJ
VN_RJ
100k
+
IN+
VOUT
VN
without CF
10k
Where
VN_RJ = RJ’s noise voltage density
VN_RF = RF’s noise voltage density
1k
with CF
VN = op amp input noise voltage density
IN-, IN+ = op amp input noise current density
100
FIGURE 18:
1
10
100 1k 10k 100k 1M 10M
Frequency (Hz)
FIGURE 16:
Total Output RMS Noise
Voltage Density vs. Frequency.
7
5
3
2
with CF
1
0
100 1k 10k 100k 1M
Frequency (Hz)
10M
FIGURE 17:
Total Output RMS Noise
Voltage vs. Frequency.
Although the added CF eliminates a lot of output noise,
we still need to further reduce the noise in order to
improve the SNR and achieve better signal integrity.
Now we will focus on the noise analysis of the photodiode amplifier.
DS01494A-page 8
• Hand Calculation
• PSpice Simulation
The typical input noise voltage density and input noise
current density of MCP6491 are 19 nV/Hz and
0.6 fA/Hz, respectively. The input noise voltage
density vs. frequency plot can be found in the
MCP6491 data sheet. The 1/f noise is dominant in the
lower frequencies while the thermal noise is dominant
in the higher frequencies.
4
10
Two ways to quickly estimate total output root-meansquare (RMS) noise are provided:
The resistor voltage noise density is given by
VN = 4kTR and is spectrally flat. For a 1 kΩ resistor,
the VN is 4 nV/Hz.
6
1
Noise Model.
NOISE ESTIMATED BY HAND CALCULATION
without CF
0.1
MCP6491
CJ
Noise Gain vs. Frequency.
10
0.1
Total Outpu
ut RMS Noise Voltage
(mV)
IN-
10M
–
+
FIGURE 15:
1k
10k 100k
Frequency (Hz)
–
+
1
VDD
The total output RMS noise is calculated by the square
root of the sum of the squared values of the individual
output noise contributors. Each output noise contributor is calculated by integrating its squared output noise
density over the equivalent noise bandwidth in a
square root. The output noise density is calculated by
multiplying its input noise density by an appropriate
gain. Note that the worst output noise contributor will
dominate the total output RMS noise.
 2013 Microchip Technology Inc.
AN1494
For a single pole system, the equivalent noise bandwidth is equal to the -3 dB bandwidth multiplied by 1.57.
Because there is no resistor in series with the op amp's
non-inverting input, IN+ does not contribute to output
noise.
TABLE 2:
Input Noise
Density
Output Noise
Density
Equivalent Noise
Bandwidth
VN
VN*GN
1.57*Noise Gain
Bandwidth
IN-
IN-*RF
1.57*Signal Gain
Bandwidth
VN_RJ
VN_RJ*(RF/RJ)
1.57*Signal Gain
Bandwidth
VN_RF
VN_RF
1.57*Signal Gain
Bandwidth
Note 1: The noise gain bandwidth is given by
GBWP/GN.
Notice that the op amp’s input noise voltage density
(VN) needs to be multiplied by noise gain GN to get the
corresponding output noise density, and the noise gain
bandwidth is much larger than the signal gain bandwidth. This makes VN dominate the total output RMS
noise voltage.
NOISE ESTIMATED BY PSPICE SIMULATION
Figure 19 shows the MCP6491 op amp input noise
voltage density spectrum simulation plot by using the
MCP6491 op amp Spice macro model in PSpice, which
matches the noise density spectrum plot of MCP6491
data sheet well.
1k
Input No
oise Voltage Density
(nV/Hz)
Table 2 shows the input noise density of each noise
source, the corresponding output noise density and the
equivalent noise bandwidth.
2: The signal gain bandwidth is given by
1/(2*RF*CF).
The GN is dependent on frequency; it is 1 V/V at lower
frequencies and gradually becomes higher with a
maximum of 113 V/V at higher frequencies. Instead of
integrating GN over frequency, we simply use 113 V/V
as the noise gain over the equivalent noise bandwidth
for quick noise estimation.
Thus, the output noise from each contributor can be
estimated, according to Table 2, and the results are
shown in Table 3.
TABLE 3:
Input Noise
Density
Output Noise
Voltage Density
(nV/Hz)
Individual
Output Noise
Voltage
(RMS in µV)
VN
VN*GN = 19*113
IN-
IN-*RF = 6
692
VN_RJ
VN_RJ*(RF/RJ) = 28
4.4
VN_RF
VN_RF = 400
63
1
10
0.1
1
10
100
1k
10k 100k
1M 10M
Frequency (Hz)
FIGURE 19:
MCP6491 Op Amp Input
Noise Voltage Density vs. Frequency.
Figure 20 shows the total output RMS noise voltage
density spectrum.
Total Output RMS Noise Voltage
Density (nV/Hz)
D
For the circuit shown in Figure 12, the noise gain bandwidth is (7.5 MHz)/(113 V/V) = 66 kHz and its equivalent noise bandwidth is 66 kHz*1.57 = 104 kHz. The
signal gain bandwidth is 16 kHz and its equivalent
noise bandwidth is 16 kHz*1.57 = 25 kHz.
100
10k
1k
100
10
0.1
1
10
100
1k
10k 100k 1M 10M
Frequency (Hz)
FIGURE 20:
Total Output RMS Noise
Voltage Density vs. Frequency.
The total output RMS noise is equal to 695 µV, which is
the square root of the sum of the individual squared
output noise values.
 2013 Microchip Technology Inc.
DS01494A-page 9
AN1494
Figure 21 shows the total output RMS noise voltage
spectrum. Within 10 MHz, the total output RMS noise
voltage is 650 µV.
The noise estimated by hand calculation (695 µV) is
similar to the one simulated by PSpice.
For a 4V output voltage signal, the SNR is equal to
20*log(VSIGNAL/VNOISE) = 20*log(4V/650µV) = 76 dB.
In PSpice probe, the trace expression
“SQRT(S(V(ONOISE)*V(ONOISE)))” can
be used to integrate output noise voltage
density over bandwidth.
Tota
al Outp
put RMS Nois
se Volta
age
(μV
V)
Note:
In Equation 2, the low pass filter’s cut-off frequency (fc)
is set to be equal to the maximum allowed signal gain,
which gives the minimum rising time (tR) of the output
step.
For a fixed RF, tR can be further reduced by choosing
an op amp with higher GBWP. The higher GBWP
makes the value of CF smaller based on Equation 1,
and thus makes fc larger.
EQUATION 2:
1
f C = ----------------------------2  R F  C F
700
600
0.35
t R  ---------fC
500
400
Where
300
200
fC = cut-off frequency of low pass filter
100
tR = 10% to 90% rising time (s)
0
1
10
100 1k 10k 100k 1M 10M
Frequency (Hz)
FIGURE 21:
Total Output RMS Noise
Voltage vs. Frequency.
NOISE FILTERING
In Figure 22, a single pole RC low pass filter can follow
the photodiode amplifier to eliminate the noise beyond
the signal gain bandwidth.
CF
RF
I
–
VDD
FIGURE 22:
DS01494A-page 10
In Figure 23, the step output responses are shown for
the low pass filters with different fC. Notice that the filter
with lower fC yields longer tR.
4
3
fC = 16kHz
fC = 1.6kHz
2
fC = 318Hz
1
0
VOUT
R
MCP6491
+
As shown in Figure 12, RF = 10 MΩ, CF = 1 pF, thus fC
is 16 kHz and tR is 22 µs based on Equation 2.
Step O
Output Voltage (V)
0.1
0
2
4
6
8
10
12
Time (ms)
C
Noise Filtering.
FIGURE 23:
Step Output Response vs.
Low Pass Filter’s fC.
The low pass filter also serves as an anti-aliasing filter
for the subsequent analog-to-digital converter (ADC).
The ADC’s sampling rate should be at least two times
of the low pass filter’s fc.
 2013 Microchip Technology Inc.
AN1494
We chose R = 100 kΩ and C = 0.1 nF to make the low
pass filter with fc = 16 kHz. The noise generated by the
filter itself is negligible.
Total Outputt Noise Voltage Density
(nV/Hz)
In Figure 24 and Figure 25, the related output RMS
noise spectrum plots with and without filtering are
shown, which are PSpice simulation results.
10k
No low pass filter
In photodetection applications, PCB surface leakage
effects need to be considered. Surface leakage is
caused by humidity, dust or other contamination on the
board. Under low-humidity conditions, a typical
resistance between nearby traces is 1012. A 5V
difference would cause 5 pA of current to flow, which is
greater than the MCP6491 family’s bias current at
+25°C (1 pA, typical).
There are several ways to reduce surface leakage
such as cleaning, coating and guard rings.
1k
Cleaning with isopropyl alcohol helps remove
residues, and coating isolates the surface from
moisture, dust, etc.
100
Adding low pass filter
with fC = 16 kHz
10
0.1
1
10
100 1k 10k 100k 1M 10M
Frequency (Hz)
FIGURE 24:
Total Output RMS Noise
Voltage Density vs. Frequency.
Total Output RMS Noise Voltage
(μV)
PCB Surface Leakage
700
No low pass filter
600
500
400
Adding low pass filter
with fC = 16 kHz
300
200
The more reliable and permanent solution to reduce
surface leakage is using guard rings. As shown in
Figure 26, the guard ring drawn in the dotted line is a
low impedance conductive trace and it surrounds the
sensitive inverting input pin area. The guard ring is
biased at the same voltage as the sensitive inverting
input pin so that there is no leakage current between
itself and the guarded sensitive pin. In a photodiode
amplifier circuit, the guard ring is directly connected to
the op amp’s grounded non-inverting input pin. Thus,
the guard ring blocks the leakage current which would
flow into the sensitive pin, and sinks it to ground.
Moreover, to minimize coupling effects, the circuit
connections within the guard ring should be kept as
short as possible.
For more information on PCB layout techniques,
please refer to Microchip’s AN1258 (“Op Amp
Precision Design: PCB Layout Techniques”).
100
CF
0
0.1
1
10
100 1k 10k 100k 1M 10M
Frequency (Hz)
FIGURE 25:
Total Output RMS Noise
Voltage vs. Frequency.
In Figure 25, the total output RMS noise voltage is
205 µV within 10 MHz.
RF
I
VDD
MCP6491
VOUT
+
For a 4V output voltage signal, the SNR is equal to
20*log(VSIGNAL/VNOISE) = 20*log(4V/205µV) = 86 dB,
which is 10 dB higher than the SNR without filtering.
FIGURE 26:
 2013 Microchip Technology Inc.
–
Guard Ring Technique.
DS01494A-page 11
AN1494
SUMMARY
REFERENCES
This application note reviews the features of Microchip’s MCP6491 low input bias current op amps [1], the
characteristics and operation modes of photodiodes,
then it focuses on designing photodiode amplifier circuitry, and several key design points are discussed in
order to improve the circuit’s performance. The noise
analysis of a photodiode amplifier and the design technique of a low pass filter are also discussed. Finally, the
PCB techniques that help reduce the current leakage
are briefly introduced as well.
[1] MCP6491 Data Sheet, “7.5 MHz Low Input Bias
Current Op Amps”, Microchip Technology Inc.,
DS22321, 2012
DS01494A-page 12
[2] MCP6481 Data Sheet, “4 MHz Low Input Bias Current Op Amps”, Microchip Technology Inc.,
DS22322, 2012
[3] MCP6471 Data Sheet, “2 MHz Low Input Bias Current Op Amps”, Microchip Technology Inc.,
DS22324, 2012
 2013 Microchip Technology Inc.
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•
Microchip products meet the specification contained in their particular Microchip Data Sheet.
•
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
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•
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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ISBN: 978-1-62076-987-4
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DS01494A-page 13
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