AN60590 PSoC 3, PSoC 4, and PSoC 5LP Temperature Measurement with a Diode.pdf

AN60590
PSoC® 3, PSoC 4, and PSoC 5LP – Temperature Measurement with a Diode
Authors: Vivek Shankar Kannan, Julie Chen
Associated Project: Yes
Associated Part Family: All PSoC 3, PSoC 4, and PSoC 5LP parts
Software Version: PSoC Creator™ 3.1 CP1 or later
Related Application Notes: For a complete list of the application notes, click here.
®
AN60590 explains diode-based temperature measurement using PSoC 3, PSoC 4, and PSoC 5LP. The
temperature is measured based on the diode forward bias current dependence on temperature. This application note
details how the flexible analog architecture of PSoC 3, PSoC 4, and PSoC 5LP enables you to measure diode
temperatures using a single PSoC device.
Contents
Introduction .......................................................................2
The Diode Equation...........................................................3
Measuring Diode Temperature .....................................3
Transistor as Diode ......................................................3
Measuring Diode Temperature Using
PSoC 3 and PSoC 5LP .....................................................4
Functionality .................................................................5
Component Configuration.............................................5
Firmware Details ..........................................................6
Measuring Diode Temperature Using PSoC 4 ..................7
Functionality .................................................................8
Component Configuration.............................................9
Firmware Details ........................................................ 10
Antiparallel Diode Topology Using
PSoC 3 and PSoC 5LP ................................................... 11
Functionality ............................................................... 11
CY8CKIT-025 Temperature Sensor EBK ........................ 12
Multiple Antiparallel Diode Topology
Using PSoC 3 and PSoC 5LP ......................................... 13
Multiple-Diode Sensing Using PSoC 4 ............................ 13
Design Considerations for PSoC 3 and PSoC 5LP ......... 15
Transistor Selection .................................................... 15
Selection of the IDAC Calibration Resistor ................. 15
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Design Considerations for PSoC 4.................................. 16
Selection of the IDAC Calibration Resistor ................. 16
Selection of Opamp External Resistors ...................... 16
Performance Measures for PSoC 3 and PSoC 5LP ........ 17
Ideality Factor of the Transistor Diode........................ 17
IDAC Current Ratio .................................................... 17
ADC Error ................................................................... 17
Summary of Error Sources ......................................... 18
Temperature Test ....................................................... 18
Performance Measures for PSoC 4 ................................ 19
Ideality Factor of the Transistor Diode........................ 19
IDAC Current Ratio .................................................... 19
Opamp Error .............................................................. 19
ADC Error ................................................................... 20
Summary of Error Sources ......................................... 20
Temperature Test ....................................................... 20
Project Summary............................................................. 21
Summary ......................................................................... 21
Related Application Notes ............................................... 21
About the Authors ........................................................... 21
Worldwide Sales and Design Support ............................. 23
Document No. 001-60590 Rev. *H
1
PSoC® 3, PSoC 4, and PSoC 5LP – Temperature Measurement with a Diode
Introduction
Temperature is the most common and frequently
Table 1 offers a comparison of four different types of
measured
environmental
variable.
Temperature
temperature sensors, enabling you to choose the
measurements typically use one of the four sensors:
appropriate one for your application. This application note
thermocouple, thermistor, diode, and resistance
focuses on diodes. Refer to the Related Application Notes
temperature detector (RTD). The primary criteria for
section for references on using RTD, thermocouple, and
choosing a sensor are cost, accuracy, and temperature
thermistor sensors with PSoC.
range.
Table 1. Comparison of Temperature Sensors
Parameter
RTD
Thermocouple
Thermistor
Diode
Temperature range
(0 °C)
–200 to +850
–250 to +2350
–100 to +300
–50 to +150
Sensitivity at 25 °C
0.387 Ω/°C
40 µV/°C (K-type)
416 Ω/°C
250 µV/°C
Accuracy
High
Medium to high
Medium
Low
Linearity
Good
Fair
Poor
Good
Typical cost (US $)
$3–$80
$3–$15
$0.2–$10
<$0.2
Typical usage
Surface mount for
onboard temperature
<100 meters
Surface mount for
onboard temperature
Onboard temperature
3- and 4-wire up to a few
hundred meters
Leaded for <1 meter
Resource requirement
Excitation current,
amplifier, ADC, reference
resistor
Amplifier, ADC, voltage
reference, and another
temperature sensor for
cold junction
Excitation current, ADC,
reference resistor
Excitation current,
amplifier, ADC
Response time
Slow
Fast
Fast
Slow
Computational
complexity (best possible
accuracy)
High
Very high
Very high
Medium
Diode-based temperature measurement is suited for low-cost temperature sensing applications with temperature ranges from
–50 °C to +150 °C and an accuracy of 2 °C to 5 °C. With on-chip current DACs and a 20-bit Delta Sigma ADC, PSoC 3 and
PSoC 5LP enable high-resolution, accurate temperature measurement using only an external diode and a calibration resistor.
PSoC 4 has on-chip current DACs and a 12-bit SAR ADC, so it can do medium accuracy and resolution temperature
measurement. Table 2 gives a detailed comparison between PSoC 3/PSoC 5LP and PSoC 4 of resolution, accuracy at 125 °C
worst case, and numbers of I/O pins for single-diode and multiple-diode sensing.
Table 2. Comparison of PSoC 3/PSoC 5LP and PSoC 4 for Diode Temperature Sensing
Resolution
PSoC 3 and
PSoC 5LP (20-bit
ADC
configuration)
PSoC 4
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0.1 °C
1 °C
Accuracy
2.37 °C
5.7 °C
Number of I/O Pins for Single
Diode
3
6
Document No. 001-60590 Rev. *H
Number of I/O Pins for Multiple Diode
(N ≥ 2)
Depends on the available I/O pins. N
diodes needs (N + 2) I/O pins.
Note that the diodes here are configured as
antiparallel diode pairs.
Depends on the available I/O pins. N
diodes needs (2N + 6) I/O pins.
Note that the diodes here are not
configured as antiparallel diode pairs.
2
PSoC® 3, PSoC 4, and PSoC 5LP – Temperature Measurement with a Diode
Diode-based temperature measurement is typically used
in one of the following two ways:


Most CPU processors and some application-specific
integrated circuits (ASICs) provide access to thermal
diodes in their architecture to measure the
temperature of the processor core. This temperature
measurement is used for thermal management
functions such as fan speed control to cool the
processor core. PSoC 3, PSoC 4, and PSoC 5LP can
be used to interface with those thermal diodes to
measure the core temperature and perform fan speed
control.
It is also possible to use general-purpose transistors
(such as 2N3904) in the diode-connected
configuration for temperature measurement in any
application. The reason to opt for a general-purpose
transistor would be the extremely low transistor cost
along with less stringent accuracy requirements.
Measuring Diode Temperature
The technique for measuring the diode temperature is
based on applying two different known currents to flow
through the diode and measuring the diode voltage in
each case.
For two different currents
and , such that
,
1
Equation 3
Taking natural logarithm on both sides,
Equation 4
Using Equation 2 in Equation 4, the temperature (T) in
Kelvin is given by:
The Diode Equation
The following equation gives the current
forward-biased diode:
through a
Equation 5
Equation 1
Where,
Where,
– The diode forward voltage drop
– The difference in diode forward voltage
drops for the two currents I2 and I1.
– The reverse saturation current
c – A constant given by:
– A constant (called “ideality factor”) that has a value
between 1 and 2, depending on the material and the
physical structure of the diode
Transistor as Diode
– The thermal voltage given by:
Equation 2
Where,
– Boltzmann’s constant
– The absolute temperature in Kelvin
–
The
magnitude
of
electronic
charge
The temperature-dependent factors in Equation 1 are
and
. The reverse saturation current typically doubles
for every 5 °C rise in temperature.
depends on the
physical properties of the diode.
is directly proportional
to the temperature.
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Equation 6
Equations 5 and 6 are derived for a diode. For the
temperature measurement application, this document
uses a transistor in a diode-connected configuration
instead of a standalone diode because the variations in
the ideality factor ‘ŋ’ due to manufacturing processes are
less in transistors compared to diodes. For example,
2N3904 transistors have a constant ideality factor of
1.004, which is used in this application note.
In the diode-connected configuration, the collector and
base terminals of the transistor are shorted together
externally. Figure 1 shows that this configuration is for
both NPN and PNP transistors. The transistor operates in
the active region with the collector-base junction reverse
biased and the base-emitter junction forward biased. The
voltage (V) across the base and emitter terminals of the
transistor is the equivalent of the diode voltage. The
collector current (Ic) is the one responsible for the
generation of the voltage ‘V’ because Ic is the current
entering (NPN) or leaving (PNP) the collector-base
junction.
Document No. 001-60590 Rev. *H
3
PSoC® 3, PSoC 4, and PSoC 5LP – Temperature Measurement with a Diode
Figure 1. Transistor as Diode
NPN
Voltage sensitivity is the change in voltage for a 1 °C or
1 K difference in temperature, and it is equal to (1/c)
based on Equation 5.
PNP
I
For
, you get
≈ 3860 Kelvin/volt.
For this value of ‘c’, the voltage sensitivity is 259 µV/°C.
Ie = I
Equation 7
Ic
C
B
Ib
+
V = Vbe
E
Ie = I
E
+
B
V = Veb
-
Ib
C
Ic
I
In the temperature measurement application, the ratio of
the bias currents I1 and I2 is considered as the ratio of the
collector currents as well. This assumption is valid for
transistors like 2N3904 that have a constant current gain
hFE over the bias current operating range (10 µA –
200 µA).
Measuring Diode Temperature Using
PSoC 3 and PSoC 5LP
PSoC 3 and PSoC 5LP have a powerful analog
architecture that enables the accurate measurement of
diode temperature. The implementation in PSoC 3 and
PSoC 5LP is based on Equation 5. Figure 2 is the top
design
schematic
(TopDesign.cysch)
for
this
implementation showing the external diode-connected
transistor and the external calibration resistor. This
calibration resistor is used to accurately calculate the
IDAC current ratio ‘N’ given in Equation 6.
You can calculate the voltage sensitivity of the transistordiode to temperature based on Equation 5 and Equation 6.
Figure 2. Single-Diode Topology for PSoC 3 and PSoC 5LP
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Document No. 001-60590 Rev. *H
4
PSoC® 3, PSoC 4, and PSoC 5LP – Temperature Measurement with a Diode
Figure 3. IDAC Configuration for PSoC 3 and PSoC 5LP
Functionality

The current DAC (IDAC) forces the two known
currents I1, I2 through the transistor diode.

The corresponding base-emitter voltages V1 and V2
are measured using the Delta Sigma ADC.

Equation 5 is then used to calculate the temperature
in firmware.

To ensure that the IDAC current ratio ‘N’ (N = I2/I1) in
Equation 5 is computed accurately, a calibration
resistor is connected at the output of IDAC in series
with the transistor diode. The ratio of voltages across
the calibration resistor gives the IDAC current ratio.
This calibration removes the error due to IDAC offset,
nonlinearity in temperature measurement. By using
the high-resolution Delta Sigma ADC, the current ratio
can be calculated very accurately.

The Analog Multiplexer Component in PSoC
Creator™ is used to multiplex the ADC inputs to
perform IDAC calibration, temperature calculation,
and ADC offset calculation.

The ADC offset calculation is performed by shorting
the internal analog ground (VSSA) to the two ADC
inputs and measuring the ADC output. This ADC
offset correction is used while doing the IDAC
calibration to measure the current ratio accurately.

The DieTemp Component is used to measure the
PSoC die temperature. Whenever the internal die
temperature changes by 10 °C, the current ratio ‘N’ is
recalculated to ensure that any IDAC current drift over
temperature does not affect the temperature
measurement accuracy. The die temperature is used
instead of the external diode temperature since it is
possible for the diode and the PSoC to be in different
thermal zones.
Component Configuration
Current DAC (IDAC) Configuration
The IDAC configuration settings are shown in Figure 3.
The IDAC is configured for current sourcing mode, and the
IDAC data register is updated by the CPU in firmware. The
two currents that are sourced by the IDAC to flow through
the diode are 10 µA and 200 µA, respectively. This makes
the ratio factor ‘N’ in Equation 6 equal to ‘20’. For currents
in this range, the self-heating of the diode is also at a
minimum, and its effect on temperature measurement can
be ignored without compromising the accuracy of the
measured temperature.
ADC Configuration
The Delta Sigma ADC configuration settings are shown in
Figure 4.
The ADC is set to differential input mode since the voltage
is measured across the two terminals—the base and
emitter terminals (Vbe) of the transistor. The differential
mode is used to perform a 4-wire measurement technique,
which eliminates the effect of PCB trace resistance and
internal analog routing resistance on the measured diode
voltage. A detailed discussion on the effects of PCB trace
resistances and best practices for mixed-signal board
layout is given in the application note AN57821 – PSoC 3,
PSoC 4, and PSoC 5LP Mixed Signal Circuit Board Layout
Considerations.
The voltage Vbe is usually about 0.5 V–0.7 V at room
temperature for the bias currents that are used in this
application. The voltage drop (Vbe) changes by about
0.25 mV for every 1 °C increase in temperature. So in the
region of temperature measurement (which is the diode
operating temperature of –50 °C to +150 °C), the Vbe
voltage is a positive value less than 1 V. Thus the ±1.024V range is chosen for the ADC input range. The ADC bit
resolution is set to the maximum available 20 bits. In the
±1.024-V range, 20-bit ADC configuration, the voltage
resolution is 2 µV. A 0.1 °C temperature resolution
corresponds to a 25-µV voltage resolution. So with this
ADC configuration, it is possible to measure the
temperature to a resolution of less than 0.1 °C.
For a 1 °C resolution, 16-bit ADC configuration, which has
a resolution of 32 µV, should be more than enough. With
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Document No. 001-60590 Rev. *H
5
PSoC® 3, PSoC 4, and PSoC 5LP – Temperature Measurement with a Diode
the lower end PSoC 3 and PSoC 5LP families, which only
have a 12-bit Delta Sigma ADC, a resolution of 2 °C can
be obtained.
Figure 4. ADC Configuration for PSoC 3 and PSoC 5LP
Figure 5. Temperature Measurement Algorithm for
PSoC 3 and PSoC 5LP
Configure Amux_Adc multiplexer to select
the base-emitter terminals as the ADC
differential input
Set the IDAC output to the low current I1
and measure the corresponding diode
voltage Vbe1
Set the IDAC output to the high current I2
and measure the corresponding diode
voltage Vbe2
Calculate diode temperature based on the
measured voltages using the diode
equation
Return the measured diode temperature
The reference for the ADC is the on-chip 1.024-V
reference voltage. The Buffer Mode is configured for
Level Shift mode, as the negative input of the ADC would
be very close to VSSA (analog ground). See the ADC
component datasheet in PSoC Creator to learn about
these settings in detail.
Firmware Details
The project A_SingleDiode provided with the application
note has the implementation done as shown in Figure 2.
The files SingleDiode.h and SingleDiode.c in the project
provide the C functions related to the diode temperature
measurement.
The
function
InitializeDiodeMeasurement() is called once at the
start of the main code. This function configures and starts
the Components (ADC, IDAC, AMUX, DieTemp) required
for temperature measurement. It also does a IDAC
calibration by calling the function CalibrateIdac(),
which is used to calculate the IDAC current ratio ‘N’ used
in the temperature measurement.
In the diode temperature measurement project, the IDAC
current ratio calculation is done every time the PSoC die
temperature changes by more than 10 °C. This is because
the PSoC 3 and PSoC 5LP IDAC INL parameter varies
with temperature (refer to the device datasheet for typical
graphs). This results in a change in the IDAC current ratio,
which in turn causes a small error in the temperature
calculation. So the ratio calculation is done for every 10 °C
change by calling the function CalibrateIdac() in the main
code.
An averaging routine is performed on the voltages
measured using the ADC to reduce any noise in
temperature measurement. The number of samples used
for averaging is decided based on the ADC configuration.
A 12-bit ADC configuration requires a higher number of
averaging samples than a 20-bit ADC configuration. The
definitions in the SingleDiode.h file, in which the number of
samples to be averaged is chosen based on the ADC bit
resolution, support this determination.
The function GetDiodeTemperature() is the function
that you should call when a temperature measurement
needs to be done. Figure 5 shows the algorithm
implemented by this function.
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Document No. 001-60590 Rev. *H
6
PSoC® 3, PSoC 4, and PSoC 5LP – Temperature Measurement with a Diode
Measuring Diode Temperature Using
PSoC 4
PSoC 4 also has a powerful analog architecture that
enables the measurement of diode temperature. The
implementation is based on Equation 5.
With the 12-bit SAR ADC, the quantization error is 0.5 mV
with 1.024 internal references. The ∆Vbe change in voltage
per °C is about 0.25 mV. Thus, signal amplification is
necessary. Two on-chip opamps in PSoC 4 can be
configured as the differential amplifier, as Figure 6 shows.
Figure 6. Two Opamps Configured as Differential Amplifier
V+
ADC+
R2
R1
R3
ADCV-
The differential gain is equal to (
).
This circuit can even be simplified by removing R3, which
reduces the external resistors from three to two. The other
benefit of using the asymmetric resistors rather than the
common three resistors is that it can result in a bigger gain
and keep the common voltage in the range. If R3 is
removed, the gain is (
).
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What you want to amplify is the ∆V in Equation 5, not the
Vbe itself. Otherwise, the gain needs to be limited to a
small value since Vbe is usually about 0.5 V–0.7 V at
room temperature for the bias currents that are used in
this application. To get a high gain and keep the common
mode voltage within the opamp’s operational spec, one
capacitor is introduced to store the Vbe at low current
(V1). Then, Vbe at higher current (V2) and V1 are used as
the two inputs of the differential amplifier, and the output is
sampled by ADC.
IDAC output may differ from the set value due to the
offset, nonlinearity in temperature measurement. One
external resistor can be used to calibrate these errors. The
actual IDAC current I1, I2 can be measured by the voltage
across the external calibration resistor (Rcal) under I1 and
I2. To reduce the INL effect of ADC, the voltage across
Rcal at I1 is amplified by high gain (
), and the
voltage across Rcal is not amplified by setting the gain
equal to 1. The gain change can be implemented by an
analog multiplexer.
Figure 7 is the top design schematic (TopDesign.cysch)
for this implementation, showing the external diodeconnected transistor, external calibration resistor, external
resistors for internal opamps (R1, R2), and one capacitor
(C1). This calibration resistor is used to accurately
calculate the IDAC current ratio ‘N’ given in Equation 6.
R1, R2 is the external resistors for the two internal
opamps, which is configured as the differential amplifier.
Capacitor C1 is used to store the Vbe voltage at I1 by
shorting AMux_SENS Chan 0 and Chan 1, which will be
compared with the Vbe at I2 current by the differential
amplifier and then sent to the SAR ADC.
Document No. 001-60590 Rev. *H
7
PSoC® 3, PSoC 4, and PSoC 5LP – Temperature Measurement with a Diode
Figure 7. Single-Diode Topology for PSoC 4
Functionality
The current DAC (IDAC) forces the two known currents I1,
I2 through the transistor diode and the external calibration
resistor.
Offset measurement is the first step of the system
initialization. The ADC offset calculation is performed by
shorting opamp1+ and opamp2+. To implement it, Chan 1
of AMux_EXT and Chan 0 of AMux_SENS are selected.
The AMux_GAIN is used to change the differential
amplifier gain since different opamp gain introduces
different offset. You need to measure two offsets, low gain
offset and high gain offset, which will be used under
different conditions. The offsets include the opamp offset
and ADC offset. Meanwhile, note that the IDAC needs to
be enabled to set a suitable bias for the opamp inputs.
Otherwise, Q1 is open and acts as a high-impedance
point, which is very sensitive to system noise. The current
flows through Pin_EXT to Rcal, Q1 to GND. To make
offset measurement more accurate, suitable bias voltage
needs to be provided. Thus, for the high-gain offset, the
IDAC is set to low current; for the low-gain offset, the
IDAC is set to high current.
connected at the output of the IDAC in series with the
transistor (diode). The ratio of voltages across the
calibration resistor gives the IDAC current ratio. To
measure this voltage, Chan 0 of AMux_EXT is selected
and Chan 0 of AMux_SENS is selected. Set the
differential amplifier gain to high (1 + R1/R2) at the lower
current (I1), and set the gain to 1 at high current I2 to
reduce the INL effect by setting AMux_GAIN.
In the ∆Vbe measurement, what really matters is the ∆Vbe
at I1, I2. To get the ∆Vbe, one capacitor, C1, is used to
store the Vbe at I1 by setting AMux_SENS Chan 0 and
Chan 1 both connected, and then the cap C1 disconnects
from transistor Q1 by setting AMux_SENS Chan 0
disconnected and Chan 1 connected. Next, change the
current to I2. The difference between the Vbe at I2 and VC1
is amplified by selecting AMux_EXT to Chan 1 and
AMux_SENS to Chan 1. AMux_GAIN is set to Chan 0 to
get the high gain.
During the AMux_SENS switching, the C1 charge has
some leakage due to the distributed capacitance of the
AMux_SENS. This acts as a small offset and needs to be
calibrated during the ∆Vbe measurement.
The offset value will be used in the current ratio calibration
and ∆Vbe measurement.
Current ratio calibration is the second step of the system
initialization. To ensure that the IDAC current ratio ‘N’ (N =
I2/I1) is accurate enough, a calibration resistor (R cal) is
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Document No. 001-60590 Rev. *H
8
PSoC® 3, PSoC 4, and PSoC 5LP – Temperature Measurement with a Diode
Figure 9. Opamp Configuration for PSoC 4
Component Configuration
Current DAC (IDAC) Configuration
Figure 8 shows the IDAC configuration settings. The IDAC
is configured for current sourcing mode, and the CPU
updates the IDAC data register in firmware. Different from
PSoC 3 and PSoC 5LP, the current resolution is 1.2 µA/bit
for PSoC 4. The two currents that are sourced by the
IDAC to flow through the diode are 9 * 1.2 µA and 180 *
1.2 µA, respectively. This makes the ratio factor ‘N’ in
Equation 6 equal to ‘20’. For currents in this range, the
self-heating of the diode is also at a minimum, and its
effect on temperature measurement can be ignored
without compromising the accuracy of the measured
temperature.
Figure 8. IDAC Configuration for PSoC 4
ADC Configuratio n
Figure 10 and Figure 11 illustrate the SAR ADC
configuration settings.
The ADC Channel 0 is set as differential and connected to
the outputs of two opamps. Channel 1 is set as single
ended and connected to the die temperature sensor.
Opamp Configuration
The two opamps are configured as the differential
amplifier, which amplifies the difference in voltage
between the positive inputs. Figure 9 shows the
configuration settings.
If the diode is on the far side, it is important to use a 4-wire
measurement technique, which eliminates the effect of
PCB trace resistance and internal analog routing
resistance on the measured diode voltage. The application
note AN57821 – PSoC 3, PSoC 4, and PSoC 5 Mixed
Signal Circuit Board Layout Considerations offers a
detailed discussion on the effects of PCB trace
resistances and best practices for mixed-signal board
layout.
www.cypress.com
With a 1 percent 1.024-V internal reference, the ADC
differential input range is ±1.024 V. The voltage Vbe is
usually about 0.5 V–0.7 V at room temperature for the bias
currents that are used in this application. Using Equation
6, it can be shown that the change in voltage per °C is
about 0.25 mV. There is zero difference between the two
voltages at absolute zero (–273.15 °C), while at room
temperature there should be (273.15 + 25) * 0.25 mV =
74.5 mV difference between the two voltage readings.
From –50 °C to +125 °C, the voltage difference changes
from 55.8 mV to 99.5 mV. Suitable opamp gain should be
set to make the output within 1.024 V. The bit resolution is
set to the maximum available 12 bits. In the ±1.024-V
range, 12-bit ADC configuration, the voltage resolution is
500 µV. A 1 °C temperature change generates about
2.5-mV voltages at the ADC input after amplification, so it
is possible to measure the temperature to a resolution of
less than 1 °C.
Hardware averaging is a unique feature of PSoC 4 that is
useful to eliminate the noise and is time-efficient
compared with the firmware averaging. Here samples
averaged are set to 16.
Document No. 001-60590 Rev. *H
9
PSoC® 3, PSoC 4, and PSoC 5LP – Temperature Measurement with a Diode
Figure 10. SAR ADC General Configuration for PSoC 4
The function GetDiodeTemperature() is the function
that you should call when a temperature measurement
needs to be done. Figure 12 shows the algorithm
implemented by this function. Offset calibration includes
the system offset and the capacitor leakage offset.
Figure 12. PSoC 4 Temperature Measurement Algorithm
Set the IDAC output to the low current I1
Short C1 and Q1
Disconnect C1 from Q1, store the Vbe at
I1on C1
Set the IDAC output to high current I2
Connect opamp1 input + to Q1
Connect opamp2 input+ to C1
Figure 11. SAR ADC Channel Configuration for PSoC 4
Measure the delta Vbe
Offset calibration
Calculate the diode temperature based on
the diode equation 5
Return the measured diode temperature
See the SAR ADC component datasheet in PSoC Creator
to learn about these settings in detail.
Firmware Details
The project P4_SingleDiode provided with the application
note has the implementation done as shown in Figure 7.
The files SingleDiode.h and SingleDiode.c in the project
provide the C functions related to the diode temperature
measurement.
The
function
InitializeDiodeMeasurement() is called once at the
start of the main code. This function configures and starts
the Components (Opamps, ADC, IDAC, AMUX, DieTemp)
required for temperature measurement. It also does a
IDAC
calibration
by
calling
the
function
CalibrateIdac(), which is used to calculate the IDAC
current ratio ‘N’ used in the temperature measurement. In
this function, system offset at different gains is also
measured, which is used in both current ratio calibration
and ∆Vbe measurement.
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In the diode temperature measurement project, the IDAC
current ratio calculation is done every time the PSoC die
temperature changes by more than 10 °C. This is because
the PSoC 4 IDAC INL parameter varies with temperature
(refer to the device datasheet for typical graphs). This
results in a change in the IDAC current ratio, which in turn
causes a small error in the temperature calculation. So the
ratio calculation is done for every 10 °C change by calling
the function CalibrateIdac() in the main code.
A firmware averaging routine is performed to reduce any
noise in temperature measurement. The number of
samples used for averaging is decided based on the ADC
configuration and system noise level, which is defined in
the SingleDiode.h file.
Document No. 001-60590 Rev. *H
10
PSoC® 3, PSoC 4, and PSoC 5LP – Temperature Measurement with a Diode
Antiparallel Diode Topology Using
PSoC 3 and PSoC 5LP
The flexible GPIO pin architecture and the presence of
analog multiplexers in PSoC 3 and PSoC 5LP enable the
temperature measurement of two independent transistor
diodes using only four pins. This is commonly called an
“antiparallel diode connection” in which the base junction
of one transistor is shorted with the emitter junction of the
other transistor and vice versa. This ensures that only one
of the transistors is ON at any instant of time for which the
temperature can be measured. CY8CKIT-025, the
temperature sensor EBK explained in the CY8CKIT-025
Temperature Sensor section, supports the antiparallel
diode topology. Figure 13 shows the top design schematic
of the project illustrating this implementation. The project
B_AntiParallelDiodes_Kit025 provided with the application
note implements the topology given in Figure 13.
Figure 13. Antiparallel Diode Topology for PSoC 3 and PSoC 5LP
Functionality
To measure the temperature of transistor diode Q1, the
IDAC output is connected to Pin_Ic1. Pin_Ic2, which is
configured in the open-drain drive mode, is driven low by
writing a ‘0’ to its data register. This establishes a current
flow path through the transistor Q1. The ADC is then used
to measure the Vbe voltage across Q1 to calculate its
temperature. During the time Q1 is on, transistor Q2 is off,
as its base-emitter junction is reverse biased. After
calculating the temperature of Q1, Pin_Ic2 is driven high
by writing a ‘1’ to its data register. This cuts off the current
flow path through Q1 when not doing the measurement.
To measure the temperature of transistor diode Q2, the
IDAC output is connected to Pin_Ic2. Pin_Ic1, which is
configured in the open-drain drive mode, is driven low by
writing a ‘0’ to its data register. This establishes a current
flow path through the transistor Q2. The ADC is then used
to measure the Vbe voltage across Q2 to calculate its
temperature. During the time Q2 is on, transistor Q1 is off,
as its base-emitter junction is reverse biased. After
calculating the temperature of Q2, Pin_Ic1 is driven high
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by writing a ‘1’ to its data register. This cuts off the current
flow path through Q2 when not doing the measurement.
In Figure 13, two extra pins (Pin_Ircal, Pin_Vrcal2) are
used for IDAC calibration. In the end application, these
two pins can be saved by moving the calibration resistor in
series between the pins Pin_Ic1 and Pin_Vb1e2. This is
not done in the antiparallel diode project because the
CY8CKIT-025 used for demonstrating this project also
uses these calibration resistors for other temperature
sensors like RTD. So the resistor is not connected in
series with the diodes.
In Figure 13, different sets of pins are used for voltage
sensing (Pin_Vb1e2, Pin_Ve1b2) and current carrying
paths (Pin_Ic1, Pin_Ic2). This is essentially a four-wire
sensing topology, which ensures that any voltage drop
across long PCB traces in a remote diode-sensing
application does not affect the temperature measurement.
See the application note “AN57821 – PSoC 3, PSoC 4,
and PSoC 5LP Mixed Signal Circuit Board Layout
Considerations” for details on PCB layout considerations
to be followed in precision analog sensing applications.
Document No. 001-60590 Rev. *H
11
PSoC® 3, PSoC 4, and PSoC 5LP – Temperature Measurement with a Diode
CY8CKIT-025 Temperature Sensor
EBK
The CY8CKIT-025 PSoC Precision Analog Temperature
Sensor Expansion Board Kit (EBK) is used for one of the
three code examples provided in the Project Summary
section.
CY8CKIT-025 comes with two 2N3904 NPN transistor
diodes connected in antiparallel fashion, as shown in
Figure 15. For more details about the transistor hardware
connections, refer to the kit user guide.
Figure 15. 2N3904 Transistors in CY8CKIT-025
This kit, shown in Figure 14, provides four temperature
sensors: thermocouple, thermistor, RTD, and diode. In
addition, interface slots let you plug in your own
thermocouple, thermistor, RTD, and diode. You can
connect the EBK to the development kits available for
PSoC 3 and PSoC 5LP.
Figure 14. Precision Analog Temperature Sensor EBK
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Document No. 001-60590 Rev. *H
12
PSoC® 3, PSoC 4, and PSoC 5LP – Temperature Measurement with a Diode
Multiple Antiparallel Diode Topology Using PSoC 3 and PSoC 5LP
The antiparallel diode topology for two diodes can be extended further to measure the temperature of the multiple antiparallel
diodes configuration. You can use this to do a low-cost temperature sensing solution of multiple thermal zones in an
application using a single PSoC 3 and PSoC 5LP. Figure 16 gives the schematic for measuring the temperature of four diodes
(two pairs).
Figure 16. Multiple Antiparallel Diode Topology Using PSoC 3 and PSoC 5LP
In Figure 16, Q1, Q2 transistors form one antiparallel
diode pair, and Q3, Q4 form the other pair. The number of
pins required in a multiple antiparallel diode topology can
be further reduced by sharing one set of current sourcing
(Pin_Icommon) and voltage sensing (Pin_Vcommon) pins
for all of the diodes, as shown in Figure 16. Note that the
other set of current sourcing and voltage sensing pins
cannot be shared across multiple diode pairs, since it will
lead to the temperature measurement of a diode getting
influenced by the other diodes. The project
C_MultipleParallelDiodes provided with the application
note implements the topology given in Figure 16. This
project can be easily modified to support more than two
diode pairs. This implementation requires (2N + 2) pins for
measuring the temperature of N diode pairs or 2N diodes.
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Multiple-Diode Sensing Using PSoC 4
The single-diode sensing method can be extended to
measure the multiple diodes. You can use this to do a lowcost temperature sensing solution of multiple thermal
zones in an application using a single PSoC 4. Antiparallel
topology is not used here because the temperature
sensing method for PSoC 4 is different from that of
PSoC 3 and PSoC 5LP, making antiparallel topology not
suitable for PSoC 4. Figure 17 gives the schematic for
measuring the temperature of two diodes. More than two
diodes can use the same method.
Four-wire sensing is an electrical measuring technique
that uses separate pairs of current-carrying and voltagesensing electrodes to make more accurate measurements
than traditional 2-wire sensing. For multiple-diode sensing,
the IDAC current flows through AMux_EXT, which is the
main difference from single-diode sensing. You cannot
add another analog multiplexer before opamp, for there
are only two analog buses in PSoC 4; the analog
multiplexer resource is limited.
Document No. 001-60590 Rev. *H
13
PSoC® 3, PSoC 4, and PSoC 5LP – Temperature Measurement with a Diode
As a result, PSoC 4 cannot do true 4-wire sensing.
Fortunately, the internal parasitic resistance of the analog
multiplexer can be calibrated in the firmware. Although the
wire resistance cannot be calibrated, it is usually very
small and ignorable if the diode is mounted on the PCB.
The trace resistance can be calculated with the following
formula:
For example, one trace with 35-µm thickness (1 oz/ft^2),
10-mm trace width, and 1-cm trace length has about
0.0191- resistance.
The ∆Vbe at a different current changes about 0.25 mV for
every 1 °C increase in temperature. A 1- wire resistance
introduces about a 0.216-mV error when measuring the
Vbe voltage, which will introduce about a 0.86 °C error. It is
recommended that the PCB trace resistance be less than
0.1 . Based on the previous calculation, you can see that
the PCB trace resistance is usually small enough. If the
trace is long (>5 cm), use of a thicker trace is
recommended to minimize the temperature error. If the
diode is used over the connector, the wire resistance
should be well considered to guarantee the system
performance.
Figure 17. Multiple-Diode Sensing Using PSoC 4
The temperature measurement method is similar to that of
the single diodes. The only difference is that it requires
parasitic resistance calibration for AMux_EXT, according
to the following method:
AMux_EXT Ch0 resistance introduces an offset error
when measuring the ∆Vbe of Q2, which can be measured
by selecting AMux_EXT to Ch0, AMux_SENS to Ch2. This
offset should be measured twice under low current and
high current, respectively. The AMux_GAIN should be set
to high gain, which is the same as measuring the ∆Vbe of
Q2.
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AMux_EXT Ch2 resistance introduces an offset error
when measuring the ∆Vbe of Q1, which can be measured
by selecting AMux_EXT to Ch1, AMux_SENS to Ch1. This
offset should be measured twice under low current and
high current, respectively. The AMux_GAIN should be set
to high gain, which is the same as measuring the ∆Vbe of
Q1.
AMux_EXT Ch2 resistance introduces an offset error
during the current ratio calibration process, which can be
measured by selecting AMux_EXT to Ch3, AMux_SENS
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PSoC® 3, PSoC 4, and PSoC 5LP – Temperature Measurement with a Diode
to Ch0. Pin_SENSC_CAL is an extra pin added for this
calibration. This offset should be measured twice: at low
current and AMux_GAIN set to high gain; at high current
and AMux_GAIN set to low gain, which is the same as for
current ratio calibration.
Another difference from single-diode sensing is the gain
reduction; the reason is to avoid ADC saturation. Using
170  as the typical parasitic resistance for analog
multiplex, the offset can be as high as 36.7 mV at high
current (216 µA). The maximum ∆Vbe at 125 °C is
99.5 mV. As in the Selection of Opamp External Resistors
section, the maximum value for R1/R2 is 9. So the
maximum R1/R2 for multiple-diode sensing can be derived
as:
In the schematic, 60K and 10K are used because the
matched pair resistor is used. See the Selection of Opamp
External Resistors section to learn the reason for using the
matched pair resistor.
The project P4_MultiDiodes provided with the application
note implements the method given in Figure 17. You can
easily modify this project to support more than two diodes.
This implementation requires (2N + 6) pins for measuring
the temperature of N diodes.
Design Considerations for PSoC 3
and PSoC 5LP
Using the assumed ideality factor
and the two
temperature parameters (Tmeasured, Tideal), the correct
ideality factor can be calculated as given by the following
equation.
Equation 8
It is recommended to perform this test on multiple samples
from the same transistor vendor and to compute the
average of ideality factors to ensure accuracy of this
parameter.
Transistor DC Forward Current Gain
The transistor diode equations used for temperature
measurement are based on the assumption that the ratio
of the bias currents sourced by the IDAC (emitter current
for the transistor) is equal to the ratio of the corresponding
collector currents. This is a valid assumption for transistors
like 2N3904 that have a constant DC forward current gain
(hFE) over the entire bias current operating range (10 µA –
200 µA). If any other transistor is used, you must ensure
that the parameter hFE is constant over the bias current
range. This can be found by looking at the graph showing
the “hFE Vs IC” variation in the transistor datasheet.
Selection of the IDAC Calibration Resistor
An external resistor (Rcal) in series with the diode is used
for calibrating the IDAC so as to calculate the IDAC
current ratio accurately. This resistor need not be
accurate, as the absolute value of the resistance does not
matter when taking the ratio of the voltages.
This section explains the various design considerations for
accurate temperature measurement using a transistor
diode.
From Equation 5,
Transistor Selection
There is a maximum limit on the value of the calibration
resistor that can be used. Two factors determine the
maximum value of the resistance: IDAC compliance
voltage and ADC positive voltage range.
The selection of the right transistor is very important for
accurate temperature measurement. The CY8CKIT-025
uses the 2N3904 NPN transistor, which is the most
commonly used transistor for temperature measurement.
The following factors should be considered when choosing
a transistor for temperature measurement.
Ideality Factor
The ideality factor ( ) of the transistor is involved in
temperature measurement as shown in Equations 5 and 6
on page 3. The datasheets of the general-purpose
transistors do not provide an ideality factor value. But a
correct measurement of the ideality factor is required for
accurate temperature measurement. For the 2N3904
transistors, an ideality factor of 1.004 gave accurate
results during testing. For other general-purpose
transistors, the procedure to calculate the ideality factor is
as follows:
Current ratio, N = I2/I1 = (V2 * Rcal)/(V1 * Rcal) = V2/V1
PSoC 3 and PSoC 5LP IDAC compliance voltage is
(VDDA – 1). VDDA is the analog supply voltage to PSoC 3
and PSoC 5LP. Add an extra 1 V on top of the IDAC
compliance voltage for the worst-case diode voltage drop
while calculating the maximum resistance Ra as given in
Equation 9.
Ra = (Vdda - 2)/Imax
Equation 9
Imax is the maximum current output of the IDAC used for
transistor biasing, which in this application is 200 µA. For
example, VDDA = 3.3 V, Imax = 200 µA results in an Ra of
6.5 kΩ.
The maximum positive voltage that can be measured by
the ADC also determines the maximum resistance R2 as
given in Equation 10.
1.
Assuming an ideality factor
(for example,
1.004), measure the diode temperature (Tmeasured in
Kelvin) using PSoC.
R2 = (ADC_POSITIVE_VOLTAGE_RANGE) / Imax
Equation 10
2.
Measure the ambient temperature ‘Tideal’ (in Kelvin)
using an accurate temperature measurement source.
For the ADC in the ±1.024-V range, positive voltage range
is 1.024 V. With Imax set at 200 µA, R2 is 5.1 kΩ.
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Document No. 001-60590 Rev. *H
15
PSoC® 3, PSoC 4, and PSoC 5LP – Temperature Measurement with a Diode
The maximum value of the calibration resistor that can be
used is governed by the following relation:
Rcal < (Minimum {R1, R2}) - Rrouting
Equation 11
In Equation 11, R1 and R2 are the resistances calculated
as given in Equations 9 and 10. Rrouting is the internal
analog routing resistance of PSoC 3 and PSoC 5LP. This
routing resistance is dependent on the analog routing
connections in the project. You can find more details on
the analog routing resistances in the application note
AN58827 – PSoC 3 and PSoC 5 Internal Analog Routing
Considerations. A worst-case value of 2.2 kΩ can be
assumed for Rrouting based on the value given in the
datasheet. For a Vdda of 3.3 V or more, where the ADC
range is the limiting factor on Rcal, it is recommended to
use a 3-kΩ calibration resistor. This ensures that the
current ratio can be accurately calculated even when
using the lower priced families of PSoC 3 and PSoC 5LP
devices, which have only a 12-bit ADC.
Design Considerations for PSoC 4
This section explains the various design considerations for
accurate temperature measurement using a transistor
diode.
The transistor selection part is the same as for PSoC 3
and PSoC 5LP.
Selection of the IDAC Calibration Resistor
An external resistor (Rcal) in series with the diode is used
for calibrating the IDAC so as to calculate the IDAC
current ratio accurately. This resistor need not be
accurate, as the absolute value of the resistance does not
matter when taking the ratio of the voltages.
From Equation 5,
Current ratio, N = I2/I1 = (V2 * Rcal)/(V1 * Rcal) = V2/V1
There is a maximum limit on the value of the calibration
resistor that can be used. The maximum value of the
resistance is determined by three factors: IDAC
compliance voltage, opamp output voltage range, and
ADC positive voltage range.
PSoC
4
IDAC
compliance
voltage
is
(VDDA + 0.5). VDDA is the analog supply voltage to PSoC 4.
Add an extra 1 V on top of the IDAC compliance voltage
for the worst-case diode voltage drop while calculating the
maximum resistance Ra as given in Equation 12.
Ra = (VDDA – 0.5)/Imax
Equation 12
Imax is the maximum current output of IDAC used for
transistor biasing. For example, VDD = 3.3 V, Imax = 216 µA
results in an Ra of 12.9 kΩ.
The maximum output voltage for opamp is (VDDA – 0.2).
Assuming the diode voltage drop is 1 V, it determines the
maximum resistance Rb as follows.
As Figure 7 shows, R1, R2 is the external resistors of the
two opamps. V1-, V2- refers to the negative input voltage of
opamp1 and opamp 2, respectively.
At low current, the differential opamp gain is set to high:
1 + R1/ R2.
V1- = 1 + Rcal* Imin
V2- = 1
Voutput 1 = V1- + (R1/ R2)*( V1- - V2- )
Voutput 2 = V2- = 1
So, Voutput 1 = (1 + R1/ R2)*Rcal* Imin + 1
At high current, the differential opamp gain is set to low: 1.
V1- = 1 + Rcal* Imax
V2- = 1
Voutput 1 = V1- + ( V1- - V2- )
Voutput 2 = V2- = 1
So, Voutput 1 = 2*Rcal* Imax +1
Equation 14
Voutput 1 at high current is the worst case. For high current,
the gain is set to 1. For example, VDD = 3.3 V, Imax = 216
µA results in an Rb of 4.8 kΩ using Equations 13 and 14.
The maximum positive voltage that can be measured by
the ADC also determines the maximum resistance RC as
given by Equation 15.
RC = (ADC_POSITIVE_VOLTAGE_RANGE) / Imax
Equation 15
For the ADC in the ±1.024-V range, positive voltage range
is 1.024 V. With Imax set at 216 µA, Rc is 4.7 kΩ.
The maximum value of the calibration resistor that can be
used is governed by the following relation:
Rcal < (Minimum {Ra, Rb, Rc}) - Rrouting
Equation 16
In Equation 16, Ra and Rb are the resistances calculated
as given in Equations 12 and 14. Rrouting is the internal
analog routing resistance of PSoC 4. This routing
resistance is dependent on the analog routing connections
in the project. The typical value should be smaller than
200 . In the project P4_SingleDiode, use a 4.3-kΩ
calibration resistor.
Selection of Opamp External Resistors
The different amplifier is made from the two opamps in
PSoC 4, which is used to amplify the ∆Vbe and voltage
across the calibration resistor (VRal). R1, R2 sets up the
gain of the differential amplifier. Also set the common
mode voltage on the output of each opamp. It should meet
two conditions:
1. The differential output should be less than the SAR
ADC reference voltage: 1.024 V.
2.
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Equation 13
The common mode voltage on the opamp’s output
should be within (0, Vdda – 0.2 V).
Document No. 001-60590 Rev. *H
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PSoC® 3, PSoC 4, and PSoC 5LP – Temperature Measurement with a Diode
Gain = 1 + R1/R2, ∆Vbe_max = 100 mV at 125 °C
IDAC Current Ratio
R1/R2_max = 9 according to condition 1.
The current ratio ‘N’ is calculated using an external
resistor and the internal ADC in PSoC. So, if there is any
variation in the output current due to IDAC offset, the INL
gets canceled out to the resolution extent permitted by the
resistor and the ADC. For example, a 3-kΩ calibration
resistor used with a 12-bit ADC can measure current
changes to the resolution of 0.17 µA (500 µV / 3 kΩ =
0.17). Here, 500 µV is the resolution of the ADC in 12-bit
mode.
When measuring the ∆Vbe, if the gain = 10, the voltage
stored in the capacitor is about 0.6 V, and the Vbe is about
0.7 V. The output on opamp1 is about 1.7 V, and the
output on opamp 2 is 0.6 V, both with the common voltage
range.
So R1/R2_max = 9, according to conditions 1 and 2.
When measuring VRal, the IDAC calibration resistor can
guarantee the common voltage within the range based on
the previous analysis.
R1, R2 accuracy, especially the tracking accuracy (R1/R2
accuracy), is critical to ensure the system performance. To
guarantee about 5 °C temperature accuracy, 0.1 percent
tracking accuracy is needed. A discrete 0.1 percent
tolerance resistor can be expensive. Thus, it is
recommended to use the matched pair resistor: R1/2 = 9,
ratio tolerance: 0.1 percent. To reduce the AMux_GAIN
parasitic resistance effect, a 10-k and 90-k pair is
better than a 1-k and 9-k pair. The SOT23 surface
mount voltage divider could be an option.
For multiple-diode sensing, the R1/R2 should be reduced
to 6 to avoid ADC saturation, because the AMux_EXT
introduces offset voltage. For detailed information, please
refer to the Multiple-Diode Sensing Using PSoC 4 section.
Performance Measures for PSoC 3
and PSoC 5LP
This section discusses the different factors that affect the
accuracy of the diode temperature measurement using
PSoC 3 and PSoC 5LP. This includes the error caused by
the PSoC components in the design (ADC, IDAC) and
also the external transistor diode. The analysis in this
section is based on the following equation:
Equation 17
Ideality Factor of the Transistor Diode
The following equation gives the error due to the ideality
factor:
Equation 18
is the expected temperature in Kelvin for correct
ideality factor
, and
is the measured
temperature in Kelvin for assumed ideality factor
.
It can be inferred from Equation 18 that the error due to
wrong ideality factor increases with increasing
temperature. A 0.1 percent error in ideality factor
would
cause
a
measurement error of 0.36 °C at a temperature of 85 °C.
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The following equation gives the temperature
measurement error due to the error in current ratio
measurement:
Equation 19
Nideal is the current ratio for the actual IDAC currents and
Nmeasured is the current ratio measured by the external Rcal
and ADC combination. Note that Tideal should be in Kelvin
in Equation 19. With the use of an external calibration
resistor and ADC for IDAC current ratio calculation, the
error in the calculated current ratio will be due to the ADC
measurement error – specifically, the error caused due to
the ADC INL. ADC measurement error is discussed in the
next section.
ADC Error
The ADC has three sources of error: offset error, gain
error, and ADC nonlinearity. From Equation 5, the offset
error is canceled by taking the difference of two voltages,
V1 and V2.
Gain Error
PSoC 3 and PSoC 5LP Delta Sigma ADC is factory
calibrated for a gain error of ±0.2 percent in a subset of
ADC configurations, which includes the ±1.024-V
configuration used in this implementation. This 0.2 percent
gain error also includes the ADC reference error. The 0.2
percent ADC gain error results in a 0.2 percent error in the
corresponding temperature. The generic equation given in
Equation 20 is for a gain error of k, where the temperature
Tideal should be in Kelvin for the error calculation.
∆T= Tideal - Tmeasured = Tideal
Equation 20
The error due to 0.2 percent gain error at 85 °C
0.72 °C.
is
If the ambient temperature (temperature of the PSoC
device) is different from 25 °C, ADC gain drift causes
additional error. The Delta Sigma ADC has a gain drift of
50 ppm/°C. It would be 3,000 ppm or 0.3 percent for an
ambient temperature of 85 °C. This causes an error of
1.1 °C based on Equation 17. But if the diode is at a
remote location compared to PSoC, then PSoC can be at
a lower temperature compared to the diode, in which case
the error due to gain drift is less.
Document No. 001-60590 Rev. *H
17
PSoC® 3, PSoC 4, and PSoC 5LP – Temperature Measurement with a Diode
ADC INL
The INL of an ADC at any point is the difference between
the ideal ADC count and the actual ADC count at that
point after gain and offset corrections have been done.
The datasheet specifies the maximum INL of all points
across process, voltage, and temperature (PVT). The
PSoC 3 and PSoC 5LP Delta Sigma ADC has an INL of
±32 least significant bits (LSb) in ±1.024-V mode; 32 LSb
corresponds to 62.5 μV for a 20-bit resolution and
±1.024-V range.
Note that the INL error not only affects the measurement
of (V2-V1), but also introduces an error in the current ratio
measurement because the current ratio is calculated
based on the voltage measured by the ADC across Rcal.
The maximum positive worst-case error occurs when ADC
measurements are affected by INL as described below:
Table 3. Temperature Measurement Error Sources
Error
Source
Error in
20-Bit
ADC
Mode
Error in
12-Bit
ADC
Mode
ADC
Gain
Error
±0.72 °C
±0.72 °C
ADC
Gain Drift
±1.1 °C
±1.1 °C
Worst-case error at 85 °C
assuming diode and PSoC
are in the same thermal
zone. This error will be
lower for remote diodes.
ADC INL
±0.75 °C
±6 °C
Assuming the worst-case
INL specifications for the
ADC. Typical value will be
much lower.
Ideality
Factor
±0.36 °C
±0.36 °C
For a 0.1 % error in ideality
factor at 85 °C. This error is
due to the transistor itself
and not the PSoC signal
chain.

V2 measured by the ADC is equal to the actual V2 +
INL error

V1 measured by the ADC is equal to the actual V2 –
INL error

The voltage across Rcal when I2 is sourced as
measured by the ADC is equal to the actual voltage –
INL error

The voltage across Rcal when I1 is sourced as
measured by the ADC is equal to the actual voltage +
INL error
Again, note that it is very rare for all these conditions to
happen together. Nonetheless, the worst-case error
occurs when these do happen together. The Microsoft
Excel workbook attached with this application note can be
used to estimate this worst-case error.
In the 20-bit mode, considering an ADC INL of ±32 LSB,
the error is ±0.75 °C. In the 12-bit mode, the ADC has an
INL of ±1 LSB; this corresponds to a temperature error of
±6 °C.
Note that these are worst-case errors,
case INL across PVT and using it for
This pessimistic approach indicates the
due to INL. In practice, the error due to
lower.
using the worsterror calculation.
worst-case limits
INL will be much
Temperature Test
The temperature measurement accuracy of the different
ADC configurations was practically tested in the lab using
a Temptronic temperature forcing system and sweeping
the temperature of the enclosed chamber from 0 to
100 °C. A MicroTherma measurement device with a
thermocouple sensor, placed in the same chamber as the
transistor diode, was used as a temperature reference.
The results for the different configurations are given in
Table 4 and Table 5. Since the resolution in the 12-bit
mode was >1 °C, a minimum-maximum routine was
implemented to display the maximum and minimum
temperatures measured by PSoC for a fixed temperature.
This was used in calculating the temperature
measurement accuracy given in the following tables.
Table 4. Temperature Results in 12-Bit ADC Mode
Diode Min
T (°C)
Diode Max
T (°C)
Min Diff
10.8
11
13
0.2
2.2
19.4
19.1
21.3
–0.3
1.9
27.2
27.3
28.8
0.1
1.6
35.6
35.5
37.7
–0.1
2.1
47.1
46.3
48.6
–0.8
1.5
57.4
57.2
59.1
–0.2
1.7
65.4
65.1
67
–0.3
1.6
75.5
74.9
76.8
–0.6
1.3
85.6
85.1
87.2
–0.5
1.6
96.6
96
98
–0.6
1.4
Reference
T (°C)
Summary of Error Sources
Table 3 gives the temperature error due to the various
error sources at a measurement temperature of 85 °C. In
the table, all error sources except the ideality factor are
due to the PSoC signal chain. The errors due to the PSoC
signal chain are for worst-case conditions. The
Temperature Test section gives the practically observed
result of the temperature measurement for the different
ADC configurations.
Note that the final error cannot be considered as a
summation of all errors; this is because all the errors are
correlated. The Microsoft Excel workbook attached with
this application note can be used to estimate the
combined worst-case error.
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(Special)
Comments
Document No. 001-60590 Rev. *H
(°C)
Max
Diff (°C)
18
PSoC® 3, PSoC 4, and PSoC 5LP – Temperature Measurement with a Diode
Table 5. Temperature Results in 20-Bit ADC Mode
Reference
T (°C)
Diode T
(°C)
The 2-LSB INL of ADC, which means a maximum 1-mV
voltage measurement error.
Difference
(°C)
8.8
9.1
0.3
18.2
18.5
0.3
28
27.9
–0.1
Then the total equivalent gain error caused by the ADC
INL in excitation current ratio calibration is +/–0.3 percent.
37.1
37
–0.1
Error due to the opamp gain: 0.1 percent
46.3
46.2
–0.1
So, the I2/I1 total error is about 0.4 percent.
60.8
60.6
–0.2
69.3
69.2
–0.1
The gain error of ADC does not impact the current ratio
calibration because it impacts both I1 and I2 measurement
and will be canceled when you divide I1 by I2.
77.3
77.2
–0.1
87.5
87.6
0.1
94.5
94.7
0.2
In this case, the final temperature error caused by current
ratio calibration is:
Performance Measures for PSoC 4
This section discusses the different factors that affect the
accuracy of the diode temperature measurement using
PSoC 4. This includes the error caused by the PSoC
components in the design (ADC, IDAC), the external
resistors of the opamps, and the external transistor diode.
The analysis in this section is based on the following
equation:
Equation 21
Ideality Factor of the Transistor Diode
The analysis is the same as for PSoC 3 and PSoC 5LP;
see the Ideality Factor section for detailed information.
Generally, a 0.1 percent error in ideality factor would
cause a measurement error of 0.37 °C or 0.37 K at a
temperature of 373 K (100 °C), about 0.4°C at 125 °C.
IDAC Current Ratio
The final temperature measurement accuracy is highly
dependent on the excitation current ratio I2/I1. So you
need to calibrate the IDAC output current before actual
measurement. The basic theory is to measure the voltage
drop on Rcal at I1 and I2 and use the actual N = I2/I1
temperature calculation in Equation 21.
The error is mainly due to the ADC INL and the opamp
gain used at low current.
Error due to INL:
As discussed previously, a 0.1 percent error causes a
0.4 °C error, so the total I2/I1 causes a 0.5 °C temperature
error using the current ratio calibration.
If you do not use the gain 10 amplifier with 1X current
excitation, the 2-LSB INL will cause a 2 percent gain error
in ratio calibration. That means about a 2.8 °C
temperature error finally.
Opamp Error
When the differential amplifier is set to high gain, the gain
is equal to (1 + R1/R2). The R1/R2 ratio tolerance impacts
the gain error of the system; it acts as the ADC gain error.
Since the tolerance is small compared to the resistors’
value, you can simplify the calculation and use the sum of
two tolerances as the worst-case gain error. For example,
if you use two 1 percent resistors, the gain error should be
2 percent.
So, if you use 1 percent accuracy resistors, that leads to
an 8 °C temperature error at 125 °C, which is
unacceptable. It is based on the assumption that the two
resistors are totally independent. However, in practice, you
can use a matched pair resistor, which includes two
resistors in a single package (usually configured as a
divider) and has a very good ratio tolerance. Absolute
tolerance is not critical.
If you use a 0.1 percent ratio tolerance resistor
divider/array, the final temperature error caused by gain
stage is 0.4 °C.
The offset calibration process in firmware takes care of the
opamp offset.
Rcal = 4.3 kΩ
At I1 = 10.8 µA, use the opamps to gain up the VRcal from
46 mV to 460 mV.
At I2 = 216 µA, no gain, VRcal = 928mV.
www.cypress.com
Document No. 001-60590 Rev. *H
19
PSoC® 3, PSoC 4, and PSoC 5LP – Temperature Measurement with a Diode
ADC Error
The ADC has three sources of error: offset error, gain
error, and ADC nonlinearity. The offset calibration process
in firmware takes care of the ADC offset.
Gain Error
The internal voltage reference is 1 percent, and the ADC
gain error is 0.1 percent. Both are considered as the gain
error. So, the temperature error caused by ADC in 125 °C
is about 4.4 °C.
ADC INL
After the opamp amplification, the ADC INL is small
enough to be ignored.
Summary of Error Sources
Table 6. Temperature Measurement Error Sources
IDAC
Current
Ratio
Error at
25 °C
Error at
125 °C
0.4 °C
0.5 °C
Comments
Use 10 x gain at low current.
INL error calculation is the
worst case; in practice, the
error should be much
smaller.
Opamp
Error
0.3 °C
0.4 °C
Use 0.1% ratio tolerance
resistor divider.
ADC
Gain
Error
3 °C
4.4 °C
Use internal ADC reference.
ADC
Gain Drift
–
–
–
Ideality
Factor
0.3 °C
0.4 °C
For a 0.1 % error in ideality
factor at 100 °C. This error is
due to the transistor itself,
not to the PSoC signal chain.
(Special)
www.cypress.com
The temperature measurement accuracy of the different
ADC configurations was practically tested in the lab using
a Temptronic temperature forcing system and sweeping
the temperature of the enclosed chamber from 0 to
100 °C. A MicroTherma measurement device with a
thermocouple sensor placed in the same chamber as the
transistor diode was used as a temperature reference.
Table 7 shows the results for the different configurations.
A minimum-maximum routine was implemented to display
the maximum and minimum temperatures measured by
PSoC for a fixed temperature. This was used in calculating
the temperature measurement accuracy given in the
following table.
Table 7. Temperature Results for PSoC 4
Table 6 gives the temperature error due to the various
error sources. In the table, all error sources except the
ideality factor are due to the PSoC signal chain. The errors
due to the PSoC signal chain are for worst-case
conditions. The Temperature Test section gives the
practically observed result of the temperature
measurement for the different ADC configurations.
Error
Source
Temperature Test
Reference T
(°C)
Diode T
(°C)
Difference
(°C)
1.3
0
–1.3
9.9
8.4
–1.5
19.1
17.9
–1.2
28.5
27.1
–1.4
37.6
36.4
–1.2
47
45.8
–1.2
56.5
55.4
–1.1
67.4
66.2
–1.2
77.1
76
–1.1
86.7
85.6
–1.1
96.8
95.9
–0.9
Document No. 001-60590 Rev. *H
20
PSoC® 3, PSoC 4, and PSoC 5LP – Temperature Measurement with a Diode
Project Summary
Related Application Notes
The AN60590.cywrk workspace provided with the
application note contains six code examples for diode
temperature measurement as follows.

AN75511 – PSoC 3/PSoC 5LP – Temperature
Measurement with a Thermocouple

AN66477 – PSoC 3 and PSoC 5LP – Temperature
Measurement with a Thermistor

AN70698 – PSoC 3 and PSoC 5LP – Temperature
Measurement with an RTD

AN58827 – PSoC 3 and PSoC 5LP Internal Analog
Routing Considerations

AN57821 – PSoC 3, PSoC 4, and PSoC 5LP Mixed
Signal Circuit Board Layout Considerations

AN58304 – PSoC 3 and PSoC 5 – Pin Selection for
Analog Designs
These three projects are applicable to PSoC 3 and
PSoC 5LP.

A_SingleDiode: This project implements a single
diode temperature measurement according to the
implementation
shown
in
Figure
2.
This
implementation is recommended for customer end
applications, but it does not work with the CY8CKIT025, which does not have the calibration resistor in
series with the diode.

B_AntiParallelDiodes_Kit025: This project implements
the antiparallel diode topology shown in Figure 13 and
is designed to work with CY8CKIT-025.

C_MultipleParallelDiodes: This project implements the
multiple antiparallel diode topology shown in Figure
16. This project does not work with the CY8CKIT-025
since it supports only two diodes.
The following two projects are applicable to PSoC 4. For
the analog routing resource difference, they do not work
with the CY8CKIT-025.


P4_SingleDiode: This project implements single-diode
temperature
measurement according
to
the
implementation shown in Figure 7.
P4_MultiDiodes: This project implements multiplediode temperature measurement according to the
implementation shown in Figure 17.
About the Authors
Name:
Vivek Shankar Kannan
Title:
Applications Engineer
Background:
Graduated with a B.E. degree in
Electronics and Communication from
Anna University. He is currently
working on PSoC applications.
Name:
Julie Chen
Title:
Applications Engineer
Background:
Graduated with a M.E. degree in
Electronics and Communication from
Zhejiang University. She is currently
working on PSoC applications.
To include the math.h library for the PSoC 4 and PSoC
5LP GCC compiler, go to Project > Build Settings and in
the ARM GCC – Linker – General tab, add the letter “m” in
the Additional Libraries option. This configuration has
already been done in the projects described in this
application note.
Summary
This application note explained how the powerful analog
architecture in PSoC 3, PSoC 4, and PSoC 5LP enables
temperature
measurement
using
general-purpose
transistors. PSoC 3 and PSoC 5LP target the accurate
diode temperature measurement field; PSoC 4 targets the
medium accurate diode temperature measurement field.
PSoC 3, PSoC 4, and PSoC 5LP also have the capability
to do multiple-diode temperature measurements using a
minimum number of pins. PSoC 3, PSoC4, and PSoC
5LP are well suited to do the system management
controller functions by performing control actions based on
the temperature measured, enabling a true system-onchip solution.
www.cypress.com
Document No. 001-60590 Rev. *H
21
PSoC® 3, PSoC 4, and PSoC 5LP – Temperature Measurement with a Diode
Document History
®
Document Title: PSoC 3, PSoC 4, and PSoC 5LP – Temperature Measurement with a Diode – AN60590
Document Number: 001-60590
Revision
ECN
Orig. of
Change
Submission
Date
Description of Change
**
2900858
VVSK
03/29/2010
New Application Note.
*A
3156112
VVSK
01/31/2011
Updated project for FCS.
Changed title to specify AN is for PSoC 3 / PSoC 5.
Changed Associated Part Family to reflect new PSoC 5 part numbering.
Minor text edits.
*B
3287533
VVSK
06/20/2011
Updated the Projects for CY8CKIT-025.
Added new section on using Multiple transistor diodes.
*C
3442928
VVSK
11/19/2011
Project Updates for PSoC Creator 2.0.
Updated template.
*D
3814122
VVSK
11/21/2012
Updated title to read “PSoC® 3 and PSoC 5LP - Temperature Measurement with a
Diode – AN60590”
Updated Associated Part Family as “All PSoC 3 and PSoC 5LP parts”
Updated Software Version as “PSoC® Creator™ 2.1 SP1 or later”
Updated Abstract
Updated Introduction (Updated contents in the section, added Table 1)
Updated The Diode Equation (Updated Measuring Diode Temperature (updated
contents in the section), added Transistor as Diode)
Updated Measuring Diode Temperature Using PSoC 3 and PSoC 5LP (Updated
Functionality
Updated Component Configuration (Updated Current DAC (IDAC) Configuration,
updated ADC Configuration)
Added Firmware Details
Removed 4-Wire Measurement Technique
Removed Choosing the Right Analog Pins
Removed Diode Temperature Measurement Project
Removed Transistor Selection
Removed Selection of the IDAC Calibration Resistor
Removed Error Budget Analysis
Update with PSoC 4 contents
Renamed CY8CKIT-025 for Diode Temperature Measurement‖ as CY8CKIT-025
Temperature Sensor and updated the contents in the same section.
Added Multiple Antiparallel Diode Topology
Added Design Considerations for PSoC 4
Added Performance Measures for PSoC 4
Updated Project Summary
Replaced "PSoC 5" with "PSoC 5LP" in all instances across the document
*E
3991824
VVSK
05/06/2013
No technical updates. Completing Sunset Review
*F
4210595
JCHE
12/05/2013
Updated PSoC 4 contents about diode measuring
*G
4219451
JCHE
12/13/2013
Added the associated project files
*H
4678523
VVSK
03/05/2015
Updated the projects to PSoC Creator 3.1 CP1.
Updated the “Performance Measures for PSoC 3, PSoC 5LP” section
Provided an Excel sheet to calculate the performance
Sunset review
www.cypress.com
Document No. 001-60590 Rev. *H
22
PSoC® 3, PSoC 4, and PSoC 5LP – Temperature Measurement with a Diode
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Document No. 001-60590 Rev. *H
23
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