AN4451
Application note
Signal conditioning for a UV sensor
Nicolas Aupetit
Introduction
This application note describes the analog conditioning circuit used for a high impedance sensor that
acts like a current sensor. It explains how to condition a signal coming from the sensor - in this case an
ultraviolet (UV) sensor - and how to improve performance.
While sunlight is important for our health, overexposure to it carries significant health risks. For example,
sunburn is caused by the UV radiation contained in sunlight. Measurement of UV is important from a
medical point of view, but for various other reasons too. The detection of UV rays is important in the
industrial domain, particularly to detect flame in a blue flame oil burner or in some fire detectors.
Knowing the right levels of UV for plant growth is also important. Low levels of UV light have a positive
effect on plant growth and seed germination but, higher levels can be harmful and even toxic. UV is part
of our life and if it is not well controlled it can cause damage. Consequently, UV sensors are very
important.
March 2014
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Contents
AN4451
Contents
1
How does the UV sensor work ....................................................... 3
1.1
Signal conditioning of the UV sensor ................................................ 3
1.2
Application example .......................................................................... 4
1.3
Input offset voltage (Vio) ................................................................... 4
1.4
Feedback resistance ......................................................................... 4
1.5
Input bias current (Iib) ....................................................................... 5
1.6
UV sensor equivalent circuit .............................................................. 5
1.7
Resistors Rs and Rj .......................................................................... 5
1.8
Capacitors Cj, Cin, and Cf ................................................................. 6
2
3
Stability of the UV sensor ............................................................... 8
Noise reduction ............................................................................. 11
4
Output voltage limitation .............................................................. 12
5
Conclusion ..................................................................................... 13
6
Revision history ............................................................................ 14
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1
How does the UV sensor work
How does the UV sensor work
A UV sensor works as a photodiode. When a UV source irradiates the sensor, UV radiation
is converted into a small current proportional to the UV radiation. Obviously, the delivered
current depends on the UV sensor used. Formula (1) shows a rough estimation of the
photocurrent (Ip) given for a particular active chip area (AChip).
where:
Achip is the active chip area in m
2
Schip is the chip’s spectral sensitivity in AW
-1
Eλ is the spectral irradiance of the UV light source which can be measured in mWcm nm
-2
1.1
-1
Signal conditioning of the UV sensor
The UV sensor generates a small current, generally a few nA, which is proportional to the
UV insulation. Figure 1 exhibits a transimpedance amplifier configuration that is used to
convert this current into an adequate voltage that can be read by the ADC of the
microcontroller.
Figure 1: Transimpedance amplifier
Cf
Rf
Uv sensor
STM32 microcontroller
Vcc
I_Uv
Vout
+
Rn
ADC
Cn
The op-amp converts the current generated by the sensor into a voltage thanks to the Rf
resistor. The output voltage sensed by the ADC is theoretically given in formula (2):
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How does the UV sensor work
1.2
AN4451
Application example
The UV sensor under consideration in this application note is the GUVA-C22SD from the
Genuv Company (see Figure 2).
Figure 2: UV sensor application
10pF
8.2MΩ
UV radiation
Index 4
Vcc
104nA
-
Uv sensor
GUVA-C22SD
OA1MPA
Vout
+
The GUVA-C22SD sensor delivers a current of 26 nA/UV index. Rf is set @ 8.2 MΩ.
Consequently, this sensor receives UV radiation with an index of 4 (Vout = 26 nA * 4 *
8.2 MΩ = 852.8 mV).
1.3
Input offset voltage (Vio)
As the ideal op-amp does not exist, it must be accepted that the op-amp itself has an
impact. For example, an op-amp adds a DC offset on the output which is directly linked to
the input voltage offset (see formula (3)).
Consequently, it is better to choose an op-amp with a low input voltage offset (Vio). The
OA1MPA is a good op-amp in this respect as it offers a maximum Vio of 200 µV. By taking
the Vio into account, the Vout becomes: Vout = 26 nA * 4 * 8.2 MΩ ± 200 µV. Thus, Vout is
in the range [852.6 mV:853 mV] which is an error of 234 ppm!
1.4
Feedback resistance
As the current generated by the UV sensor is extremely small, it is advised to use the
largest feedback resistor possible to benefit from the ADC performance. Even though it
may seem paradoxical, a large feedback resistor also helps to improve the SNR.
Effectively, resistance noise is thermal noise which is defined as: en_Rf = √4*K*T*Rf,
-23
where K is the Boltzmann’s constant (1.38x10 J/K) and T is Temperature ºK (T ºC +
273.15). The SNR is expressed in formula (4).
By using a large feedback resistor, the SNR is improved by √Rf.
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1.5
How does the UV sensor work
Input bias current (Iib)
By choosing a large feedback resistor while simultaneously trying to achieve a high gain,
the input bias current of the op-amp causes another DC offset: Iib * Rf. Consequently, a
low bias current op-amp is needed to obtain the highest sensitivity. To achieve this, it is
extremely important to choose a CMOS op-amp such as the OA1MPA which offers a very
low Iib (10 pA @25 °C). The total output voltage must also be considered (see formula (5)).
If we consider the UV sensor application described in this document (see Figure 2) and if
we use the OA1MPA as the transimpedance amplifier, the total Vout is:
Vout = 26 nA * 4 * 8.2 M Ω ±200 µV ±10 pA * 8.2 MΩ. Consequently, Vout is in the range
[852.5 mV:853.1 mV]. This represents an error of 330 ppm compared to the theoretical
value.
1.6
UV sensor equivalent circuit
Figure 3 exhibits the equivalent circuit of the photodiode, where Cj and Rj represent
respectively the junction capacitor and the shunt resistor of the diode junction.
Figure 3: Equivalent circuit of UV sensor
Rs
Serial res
Ip
photocurrent
1.7
Cj
Rj
Resistors Rs and Rj
The resistance of the output source, Rs, is generally negligible. On the contrary, the diode
shunt resistance, Rj, should be as high as possible. For example, regarding the UV sensor
GUVA-C22SD, Rj is in the range 100 GΩ. Effectively, without any current in the
photodiode, an output of 0 V is theoretically expected. However, in reality Vout is as shown
in formula (6).
Clearly, it is extremely important to have a low Vio and an UV sensor with a high Rj to limit
the offset error on the output.
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How does the UV sensor work
1.8
AN4451
Capacitors Cj, Cin, and Cf
In general, transimpedance amplifiers are prone to oscillate. To understand the stability of
this kind of architecture, it is important to take into consideration all components of the
amplifier, even parasitic components, as shown in Figure 4.
Figure 4: Components that need to be considered in a transimpedance amplifier
Cf
Rf
Vcc
Rs
Ip
photocurrent
Cj
-OA1MPA
+
Cin
Rj
Vout
The noise gain of the above configuration determines the stability of the circuit. Generally,
Rf is high to provide enough gain to convert the current of the UV sensor into a
measureable voltage. However, Rf combined with Cin and Cj creates a pole which ensures
instability of the circuit. In Figure 5, we can see that for a high value of Rf (8.2 MΩ) and no
feedback capacitor (Cf), the output of the OA1MPA is unstable.
Figure 5: OA1MPA output response to a small current signal without feedback capacitance,
Rf = 8.2 MΩ
3
1.E-07
2.5
1.E-07
8.E-08
2
6.E-08
1.5
4.E-08
1
Vout (V)
I_UV (A)
Current from the UV sensor
Vout of the OA1MPA
2.E-08
0.5
0.E+00
0
0.01
0.02
0.03
Time (s)
0.04
0.05
0
0.06
By adding a small capacitor (Cf) across Rf, oscillations or gain peakings are suppressed
and the output is stabilized (see Figure 6).
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How does the UV sensor work
Figure 6: OA1MPA output response to a small current signal with feedback capacitance,
Cf = 10 pF and Rf = 8.2 MΩ
3
1.E-07
2.5
Current form the UV sensor
8.E-08
2
6.E-08
1.5
4.E-08
1
Vout (V)
I_UV (A)
1.E-07
Vout of the OA1MPA
0.5
2.E-08
0.E+00
0
0.01
0.02
0.03
Time (s)
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0.05
0
0.06
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Stability of the UV sensor
2
AN4451
Stability of the UV sensor
UV sensors are generally very capacitive. For example, the GUVA-C22SD UV sensor has
an internal capacitance of 100 pF and the OA1MPA adds an additional input capacitance of
3 pF. Such capacitance has a direct impact on the stability of the system. This section
describes how to calculate the minimum value of the Cf capacitor (see Figure 7). Cf is a
feedback capacitor which is added in parallel with the transimpedance resistor. Its function
is to stabilize the system. As Cf limits the bandwidth, it therefore minimizes noise. In the
formulas below, Cin and Cj are considered as a unique capacitor, Cg, (see Figure 7). The
serial resistor, Rs, which has a resistance of about 100 Ω is neglected.
Figure 7: Simplified equivalent circuit
Cf
Rf
8.2MΩ
Vcc
GUVA-C22SD
Cg
103pF
OA1MPA
+
Rg
100GΩ
Vout
The open loop transfer function of the system is given in formula (7).
∗
∗
where:
A is the open loop transfer function of the op-amp.
However, by using formula (7), a pole appears and must be considered, as shown in
formula (8).
1
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Stability of the UV sensor
A zero also appears and must be considered, as shown in formula (9).
1
In addition, the low-frequency pole of the op-amp's open loop transfer function must be
considered, as shown in formula (10).
Therefore, the bode diagram of this system can be plotted as shown in Figure 8.
Figure 8: Bode diagram of the open loop transfer function of an application using a UV sensor
We can consider that Rg (100 GΩ) >> Rf (10 MΩ), so fz > fp.
With these considerations,
1
To guaranty stability of the system, the bode diagram must cross the X-axis with a slope of
-20 dB/decade. So, considering Figure 8, and to ensure stability, the gain at frequency fz
must be greater than 1.
Consequently, formula (11) is inferred.
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Stability of the UV sensor
AN4451
A second order equation can be deduced as shown in formula (12).
So, we can deduce the minimum feedback capacitance of Cf to guaranty stability of the
OA1MPA, is shown in formula (13).
In this application, GBP = 120 kHz, Cg = 103 pF, and Rf = 8.2 MΩ. Using formula (13), we
can calculate that the minimum feedback capacitance of Cf is 4.2 pF.
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3
Noise reduction
Noise reduction
The Cf capacitor added in parallel with the Rf resistor helps to stabilize the transimpedance
of the application. It also lowers the bandwidth of the system. The cut off frequency is given
by formula (14).
1
If we consider a feedback capacitance of Cf = 10 pF, the bandwidth is limited to 1.9 kHz
and noise is also reduced on the output. A simple RC filter may also be added on the
output of the OA1MPA to obtain an overall second filter Rn, Cn as shown in Figure1 and as
described in formula (15).
1
If the bandwidth is a critical requirement, choose an op-amp with a higher
bandwidth like the TSV731 (GBP = 900 kHz). In addition, try to reduce the
transimpedance gain (Rf) and add a second stage of voltage gain. The drawback
is higher noise on the output.
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Output voltage limitation
4
AN4451
Output voltage limitation
To improve the sensitivity of the UV sensor, use it in photovoltaic mode i.e. with a zero bias
operation. In this case, the dark current offset is generated by the photodiode leakage. If
the op-amp is used in single supply from GND to Vcc (as shown in Figure1), the Vol output
saturation of the OA1MPA might be a limitation for treating low UV radiation levels.
Despite the fact that the OA1MPA is an output rail-to-rail op-amp, it has a Vol output
saturation voltage of 40 mV @ 25 °C. When it is important to know precisely the current
delivered by a sensor with a low UV intensity, add a reference to avoid Vol limitation as
shown in Figure 9.
Figure 9: How to avoid Vol limitation
10pF
8.2MΩ
Vcc
Uv sensor
OA1MPA
STM32microcontroller
Microcontroller
STM32
Vout
+
Rn
ADC
Cn
Vref=100mV
In the above case, there is an offset of 100 mV at Vout. It is important to connect Vref to
the ADC to obtain a precise calculation.
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5
Conclusion
Conclusion
UV sensors provide an extremely small current depending of the level of UV radiation. To
convert this current to an adequate voltage, a transimpedance amplifier is used. Then, an
ADC can convert the signal into the digital domain. For this kind of application, it is
important to choose a CMOS rail-to-rail amplifier with a low Vio to avoid inducing big errors
on the output. The OA1MPA is a good option for such a UV sensor application. However,
stability must be also taken into account and the right components must be chosen,
particularly the feedback capacitor Cf which helps to stabilize the system, limit bandwidth,
and reduce noise.
Benefit from a wide portfolio of analog switches, voltage references, temperature sensors,
pressure sensors, or microcontrollers to develop your application, by making
STMicroelectronics your one-stop shop as illustrated in Figure 10.
Figure 10: UV sensor evaluation board
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Revision history
6
AN4451
Revision history
Table 1: Document revision history
14/15
Date
Revision
18-Mar-2014
1
DocID025985 Rev 1
Changes
Initial release.
AN4451
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