AN1016

AN1016
Detecting Small Capacitive Sensors Using the MCP6291 and
PIC16F690 Devices
Author:
Kumen Blake and Steven Bible
Microchip Technology Inc.
INTRODUCTION
Target Audience
This application note is intended for hardware and
firmware design engineers that need to accurately
detect small capacitance values.
Goals
•
•
•
•
Detect small capacitances (e.g., 0.5 pF to 6.5 nF)
Use minimal number of external components
Give simple firmware solution
Highlight design tradeoffs and alternatives
Description
This application note shows how to use a PICmicro®
microcontroller and minimal external circuitry to detect
small capacitances. The design is based on an operational amplifier (op amp) integrator. A capacitive
humidity sensor is used to illustrate this type of
application.
The design is measured to verify the theory and design
choices. Alternatives and modifications to this design
are briefly discussed.
References to documents that treat these subjects in
more depth and breadth have been included in the
“References” section.
The appendices give detailed information that supports
the text of this application note.
INTEGRATOR SOLUTION
This section describes a design that accurately
measures small capacitances. It uses dual slope
integration to measure the sensor’s capacitance. Using
an integrator for measuring small capacitive sensors
has three main advantages:
• Any sensor parasitic capacitance (i.e., case-toground stray) is forced to the correct voltage by
the op amp.
• The parasitic capacitance in parallel is much
smaller than other methods.
• The measured waveform has a constant slope,
which improves the timing accuracy.
Block Diagram
Figure 1 shows the block diagram of the integrator
solution. The “Square Wave Source” voltage (VINT) is
converted to a square wave current (IINT). IINT is then
passed to an integrator comprised of an op amp and
the sensor capacitor (CSEN). The “Integrator” outputs a
voltage triangle wave (VSEN) whose slope depends on
CSEN. The “Threshold Crossing Detector” tells when
VSEN is above or below two reference voltages: a lower
voltage (VRL) and a higher voltage (VRH).
The “Magnitude Control” firmware routine changes the
polarity of VINT so that VSEN goes past both VRL and
VRH by the desired amount. The “Timing Count”
firmware routine counts the time elapsed for VSEN to go
from VRL to VRH (t1), and to go from VRH to VRL (t2).
The “Calculations” firmware routine calculates CSEN
then the relative humidity (RH) from that capacitance.
Related Demo Board
The measurements for this application note were made
on the Humidity Sensor PICtail™ Demo Board, which
is discussed in the user’s guide (DS51594) [15]. This
board is further described by:
• Order Number: PIC16F690DM-PCTLHS
• Assembly Number: 102-00084R1
© 2005 Microchip Technology Inc.
DS01016A-page 1
AN1016
is a triangle wave whose slope depends on CSEN. The
firmware, comparator and reference (VREF) in U1
control the circuit as described before.
Square Wave
Source
VINT
The power supply voltages (VDD_DIG and VDD) were
assumed to vary between 3.0V and 5.5V. This design
uses 1% resistors for low cost. The SR latch and
Timer1 in U1 time the rise and fall times of the VSEN
triangle wave.
(square wave)
Voltage-to-Current
Conversion
IINT
(square wave)
Integrator
VDD_DIG
C1
VSEN (triangle wave)
100 nF
Threshold Crossing
Detector
Magnitude Control
U1
PIC16F690
P1
RINT
VINT 6.65 MΩ
IINT
P4
Timing Counter
Timer1
FIGURE 1:
Integrator Block Diagram.
Figure 2 shows the timing of the main waveforms. The
supply voltages are VDD and ground (0V). The current
IINT has a positive value of IINTP, and a negative value
of –IINTM (IINTP and IINTM are nearly equal magnitudes).
IINT
VDD
–IINTM
VRH
t1
t2
100 nF
Comparator
VRH
U2
MCP6291
VREF
CCM
100 nF
VDD
RCM1
20 kΩ
RCM2
20 kΩ
Op Amp Integrator Circuit.
EQUATION 1:
VRL
0V
Timing Diagram.
Circuit
Figure 3 shows the circuit. The PICmicro® microcontroller (U1) outputs a logic level at pin P1, making the
voltage, VINT, either 0V or VDD. The components
external to U1 form an inverting (Miller) integrator. VSEN
DS01016A-page 2
SR
Latch
The voltage VSEN will have a constant positive
(negative) slope when VINT is 0V (VDD):
VDD
VREF
C2
2: RINT is chosen to minimize the effort to
calculate CSEN.
VRL
0V
VCM
Note 1: CCG is the sensor’s case-to-ground parasitic capacitance. CCG should be placed
at the op amp’s inverting input pin to
improve the op amp’s stability and
eliminate any dynamic current through
CCG.
IINTP
VSEN
CSEN
CCG
FIGURE 3:
0V
VDD
FIGURE 2:
VSEN
P3
Calculations
VINT
P2
V INT – V CM
I INT = ----------------------------R INT
Δ V SEN
I INT
---------------- = ------------- , I INT is constant
Δt
C SEN
V INT – V CM
I INT = ----------------------------- ,
RINT C SEN
V INT is constant
The voltage reference, VREF, is set to one of two levels:
a lower reference voltage, VRL (0.125VDD), and an
upper reference voltage, VRH (0.500VDD). VRL was
selected to be within the op amp’s output voltage
© 2005 Microchip Technology Inc.
AN1016
range. VRH was selected to be within the comparator’s
common mode input voltage range when VDD goes as
low as 3.0V. The comparator detects where VSEN is
located relative to VRH and VRL.
The voltage, VSEN, is a triangle waveform that goes
outside the levels, VRL and VRH; this allows the circuit
to settle after changing directions, and gives time for
code overhead. The firmware sets the logic level at pin
P1 low (VINT = 0V) when VSEN needs to increase, and
sets it high (VINT = VDD) when VSEN needs to
decrease. During measurements, the microcontroller
pins are put into the states shown in Table 1.
TABLE 1:
PIN STATES
EQUATION 4:
1
C SEN ≥ -------------------------------------------2 π R INT f GBWP ε LG
Where:
fGBWP
=
op amp’s Gain Bandwidth Product
εLG
=
allowable error due to loop-gain
For instance, given a 0.5% accuracy requirement, and
using the MCP6291 for U2, gives:
•
•
•
εLG = 0.005
fGBWP = 10 MHz
CSEN > 0.6 pF
Pin States
CSEN Extraction Equations
P1
P2
Positive Slope (measure t1)
0
input
Negative Slope (measure t2)
1
input
The measurements return timer counts, k1 and k2,
which are related to the measurement times (t1 and t2)
as follows:
Measurement Steps
Note 1:
P2 is high impedance, and is always
connected to the comparator.
The IINT values shown in Figure 2 (IINTP and –IINTM)
have opposite signs and approximately equal magnitudes. The magnitudes are not always equal because
VCM is not always equal to VDD_DIG/2. This produces
elapsed times (t1 and t2) that are only approximately
equal as show in Equation 2.
EQUATION 2:
V RH – V RL
V RH – V RL
t2 = ----------------------------- = ---------------------------- • C SEN R INT
ΔVSEN ⁄ Δt
V DD – V CM
Ensuring Op Amp Stability and Accuracy
The op amp (U2) can behave poorly, or even oscillate,
if CSEN is not properly constrained. The parasitic
capacitance from the op amp’s inverting input to ground
(the sensor’s CCG and the op amp’s CCM) also affects
its stability. It is recommended that a unity gain stable
op amp (such as the MCP6291) be used and that CSEN
be set as follows:
EQUATION 3:
2
C SEN ≥ ----------------------------------2 π RINT f GBWP
Where:
=
Timer Counts:
t1
V RH – VRL C SEN R INT
k 1 = ------------- = -------------------------- • ------------------------V CM
T CLK
T CLK
t2
VRH – V RL C SEN RINT
k 2 = ------------- = ---------------------------- • ------------------------VDD – VCM
T CLK
T CLK
Where:
TCLK
V RH – V RL
V RH – VRL
t1 = ----------------------------- = -------------------------- • C SEN R INT
ΔVSEN ⁄ Δt
V CM
fGBWP
EQUATION 5:
op amp’s Gain Bandwidth Product
To maintain accuracy, the op amp feedback loop needs
sufficient loop-gain. This translates to the following
requirement on CSEN:
© 2005 Microchip Technology Inc.
=
microcontroller’s instruction period
The measurement timer counts will be averaged
together before calculating CSEN; the reason why is
illustrated in Equation 6. Since the error in VCM is in the
denominator of the k1 and k2 equations, and the
nominal VCM is VDD/2, we have:
EQUATION 6:
VCM Error Analysis Equations:
k
k 1 = -----------1+ε
k
k 2 = ----------1–ε
k
k 1 + k 2 = -------------21–ε
Where:
k
=
ideal count (when VCM = VDD/2 exactly)
ε = relative error (caused by VCM error)
=
=
VCM_Error
VCM
=
VCM_Error
VDD – VCM
VCM_Error
VDD / 2
DS01016A-page 3
AN1016
For instance, a +5% error in k1 (a -5% error in k2)
becomes a -0.25% error in the average ((k1 + k2)/2);
this is a very significant improvement in accuracy.
Greater accuracy can be achieved by reducing the
original error in k1. For example, reducing the k1 error
to +2% gives a -0.04% error in (k1 + k2)/2.
The extraction equations are below in Equation 7.
These equations assume CSEN is constrained as
described in Equation 3 and Equation 4. The constant,
B1, is the circuit’s resolution in units of pF / count. RINT
(see Figure 3) was chosen to make it easy to convert
k1 and k2 into CSEN (making B1 = 0.100 pF / count).
Pre-calculated Constant:
VDD ⁄ 2
T CLK
B1 = -------------------------- • ------------V RH – V RL RINT
( coded form )
The following assembly code modules (for the
PIC16F690) make up the Humidity Sensor project:
The HS1101LF humidity sensor from Humirel is
described in detail in its data sheet [1]. It has a relative
humidity (RH) accuracy of about ±2%, and its nominal
capacitance ranges from 162 pF to 193 pF.
195
190
CSEN (pF)
185
180
175
170
165
160
20
30
40
The circuit and microcontroller need to be initialized. It
is necessary to drive VSEN to a known point before
starting the capacitance measurements; it could be
either above or below VRL when starting.
Modular Code
HUMIDITY SENSOR
10
Additional Requirements
Extra delay before starting each measurement
improves the accuracy. It gives the op amp time to
settle, and allows the firmware time to prepare for the
next measurement.
The firmware actually multiplies the sum (k1 + k2) by
the pre-calculated constant B1/2.
0
This algorithm is implemented in the firmware for the
Humidity Sensor PICtail™ Demo Board [15]. The
firmware can be downloaded from Microchip’s website
(www.microchip.com).
Averaging (8 times) is included in this algorithm. The
timer counts need to be converted to CSEN, then to RH.
EQUATION 7:
Extraction Equation:
k1 + k 2
C SEN ≈ ----------------- • B 1
2
B1
≈ ( k 1 + k 2 ) ⎛ ------⎞ ,
⎝ 2⎠
FIRMWARE
50 60
RH (%)
70
80
90 100
FIGURE 4:
HS1101LF Humidity
Sensor’s Nominal Capacitance with TA = +25°C.
• main.inc - contains I/O port and global defines
used throughout the project
• main.asm - contains the main executive routine
including configuration bit assignments
• initialize_f690.asm - initializes the
PIC16F690 to known initial values
• capacitance.asm - reads capacitance using a
dual slope integration technique; Table 2 shows
the algorithm for this module
• humidity.inc - contains PwLI table segment
values
• humidity.asm - contains PwLI routine to
convert capacitance to %RH humidity
• ssc.asm - contains Synchronous Serial Communications (SSC), a synchronous serial communications protocol between a target PICmicro
microcontroller unit and the PICkit™ 1 Flash
Starter Kit or PICkit™ 2 Starter Kit.
• 16f690.lkr - linker script for Humidity Sensor
project
These files can be downloaded from the Microchip web
site (www.microchip.com); and are contained in the
00084R1.zip file.
Since CSEN changes by about 31 pF across the full RH
range, and has a nominal value of 180 pF, it follows
that:
• A 1 pF change in CSEN is a 0.56% change in its
nominal value
• RH changes ≈ 3.2% for each 1 pF change in
CSEN
• RH changes ≈ 6% for each 1% change in CSEN
• RH changes ≈ 0.32% for each increase of 1 in the
count (k) for Figure 3
DS01016A-page 4
© 2005 Microchip Technology Inc.
AN1016
Top Level Algorithm
Capacitance Module
Figure 5 shows the flow chart for the top level program.
This implementation includes averaging eight CSEN
readings together.
Table 2 shows the algorithm for the capacitance
module, and includes the PICmicro microcontroller’s
pin states.
The pin assignments on the Humidity Sensor PICtail™
Demo Board [15] used for the measurements in this
application note are:
Main
•
•
•
•
Initialize
Call
“capacitance.asm”
Accumulate
CSEN
P1 = RC2 (VINT)
P2 = RC1 (VSEN)
P3 = RA4/T1G
P4 = RC4/C2OUT
Pin P2 is configured as the comparator’s input during
the measurements. This gives the comparator time to
settle before the measurements are made.
TABLE 2:
CAPACITANCE ALGORITHM
Algorithm Steps
Is
count = 8
?
No
Yes
Divide Accumulated
CSEN by 8
Call
“humidity.asm”
Pin States
P1
P2
1
input
0
input
1
input
Initialize VSEN (Note 1)
(Move VSEN to < VRL)
Set VREF to VRL
Detect when VSEN < VRL
Delay
Positive VSEN Slope (Note 2)
(Move VSEN from < VRL to > VRH)
Start count k1 when VSEN = VRL
Set VREF to VRH
FIGURE 5:
Top Level Flow Chart.
The algorithm shown does not include any accuracy
improvements. The user can add code to either correct
the reference levels (VRL and VRH), using the internal
ADC, or calibrate the entire circuit’s errors (including
temperature drift).
Stop count k1 when VSEN = VRH
Delay
Negative VSEN Slope
(Move VSEN from > VRH to < VRL)
Start count k2 when VSEN = VRH
Set VREF to VRL
Stop count k2 when VSEN = VRL
Delay
Note 1:
2:
VRL = 0.125VDD and VRH = 0.500VDD.
These are lower range levels in the
PIC16F690’s VREF (CVREF).
The counts k1 and k2 increment once for
each PICmicro instruction cycle
(TCLK = 0.5 μs)
Relative Humidity Module
Once CSEN has been calculated and averaged, the
relative humidity (RH) for the HS1101LF sensor can be
estimated. The conversion is accomplished using a
piecewise linear interpolation table [11]. Appendix A:
“Piecewise Linear Interpolation Table” contains
details on the design of this table.
© 2005 Microchip Technology Inc.
DS01016A-page 5
AN1016
DESIGN MODIFICATIONS AND
ALTERNATIVES
Possible Modifications
100 nF
SIMPLE MODIFICATIONS
To produce different resolutions (pF / count), change
the RINT value. It makes the code simpler when the
resolution is a simple multiple of 1 pF. One possible set
of values is:
• RINT = 6.65 MΩ for 0.1 pF / count
• RINT = 665 kΩ for 1 pF / count
• RINT = 66.5 kΩ for 10 pF / count
More than one resolution in the same circuit can be
obtained by switching between several RINT resistors
on the Printed Circuit Board (PCB). It is also possible
to use multiple microcontroller pins, one for each RINT
on the PCB. The RINT values not being used would
have their pins set to hi-Z.
The parasitic capacitance (CPAR) in parallel with CSEN
is caused by board and op amp package stray
capacitances. It is typically about 0.5 pF; the calculated
CSEN should be corrected (have CPAR subtracted) by
this amount. CPAR can be measured by leaving having
CSEN open (0 pF).
Many of the errors over relative humidity, supply
voltage and temperature will be consistent over time.
This makes it possble to calibrate out these errors; see
“Error Analysis”.
OP AMP INTEGRATOR WITH REDUCED
CURRENT
The circuit in Figure 6 achieves greater resolution by
attenuating the square wave (VA). The components
RA1, RA2 and RA3 form an attenuator with a DC bias
point at VDD/2 and a gain of 0.0100 V/V. Thus, the current magnitudes IINTP and IINTM will be 100 times
smaller than those produced by the circuit in Figure 3.
This, in turn, produces longer integration times.
This circuit has the following key performance
numbers:
• CSEN needs to be larger than 0.6 pF for a 0.5%
accuracy and for stability
• Resolution ≈ 0.001 pF / count
The attenuator increases the equivalent error at VCM.
This can be handled by using resistors with tighter
tolerances for RA1, RA2 and RA3; Figure 6 uses 1%
resistors for low cost.
VDD_DIG
C1
P1
U1
PIC16F690
RA1
VA 1MΩ
RA3
20 kΩ
P4
Timer1
P2
P3
SR
Latch
Comparator
VREF
RA2
20 kΩ
VSEN
CSEN
VINT
RINT
6.65 MΩ
IINT
CCG
C2
100 nF
U2
MCP6291
CCM
100 nF
VCM
VDD
RCM1
20 kΩ
RCM2
20 kΩ
FIGURE 6:
Op Amp Integrator Circuit
with Reduced Current.
Note 1: CCG is the sensor’s case-to-ground parasitic capacitance. CCG should be placed
at the op amp’s inverting input pin to
improve the op amp’s stability and eliminate any dynamic current through CCG.
2: RINT is chosen to minimize the effort to
calculate CSEN.
Other Ciruits
A quick overview of different methods to measure
capacitance is found in AN990, “Analog Sensor Conditioning Circuits - An Overview” (AN990) [5]. Those
designs include an R-C decay and an oscillator.
The R-C decay method [6, 8] is very low cost and easy
to implement. It is difficult to use this method for small
capacitive sensors because of the microcontroller’s
parasitic pin capacitance and leakage currents.
It is quite popular to use a 555 timer and the capacitive
sensor to form an oscillator circuit, which works well for
larger capacitors. Smaller capacitors see greater
inaccuracies due to the 555 timer’s parasitic pin
capacitance and leakage currents. Also, 555 timers
from different vendors behave quite differently.
It is also possible to create other oscillator circuits [7].
They can be quite accurate with proper calibration, and
they can be either simple or complicated.
DS01016A-page 6
© 2005 Microchip Technology Inc.
AN1016
ERROR ANALYSIS
The design in this application note is accurate enough
to make a detailed error analysis worth the effort. The
dominant error sources are covered in this section.
They will be covered in the same sequence they
propagate through the circuit and algorithm. Their
impact on RH accuracy, and possible improvements,
will be sumarized at the end.
Ratiometric Design
The circuit was designed to be ratiometric. This is
accomplished by making IINT, VCM, VRL and VRL
proportional to VDD.
Using a ratiometric design makes the measurement
times independent of power supply voltage (VDD),
eliminating one source of measurement error.
Current (IINT) Imbalance
When IINTP and IINTM are not equal, the timer counts,
k1 and k2, are not equal. This causes an error (ε) in the
calculated counts k1 and k2. “CSEN Extraction Equations” discusses this phenomenon in detail.
Errors in VCM (VCM_Error in Equation 6) contribute to
the current imbalance. The common mode voltage
setting resistors (RCM1 and RCM2) and the op amp (U1)
dominate the VCM errors. If RCM1 is 1% low and RCM2
is 1% high, then the relative error (ε) would would be
+2%. If op amp U1’s input offset voltage (VOS_OA) is
+4.5 mV and VDD is 5.0V, then ε would be +0.2%.
The current IINT is also imbalanced by the op amp input
bias current (IB_OA). This produces a relative error
ε = IB_OA / IINT. This error is largest, for CMOS input op
amps, at high temperatures.
The attenuator in Figure 6 also causes a current (IINT)
imbalance. A mismatch between RA2 and RA3
produces this current mismatch.
Errors in Average Count (k)
The relative error (ε) in k1 and k2 causes a smaller error
of ε2 in the average count, k = (k1 + k2)/2; see “CSEN
Extraction Equations”.
Errors in the average count, k, are produced by the relative error in the following:
•
•
•
•
VREF levels (VRH – VRL)
RINT
Comparator CMRR (change in offset vs. VSEN)
Oscillator frequency
The op amp’s gain-bandwidth product can have a
significant effect on the errors for small CSEN values;
see Equation 4. The smaller CSEN is, the larger this
error is.
Errors in Calculating CSEN
The parasitic capacitance CPAR will cause an error of
about 0.5 pF if no correction is made, and about
±0.1 pF if the correction is made.
The nominal value of B1 is not exactly 0.1 pF / count; it
is approximately 0.10025 pF / count. This error
(+0.25%) has been designed to smaller than most
errors.
The designed circuit’s measurement resolution is
0.1 pF / count. The quantization error cannot be better
than 1/2 this value (0.05 pF / count).
Errors in Calculating RH
“Humidity Sensor” gives basic information on the
Humirel’s HS1101LF capacitive RH sensor. As
explained there, the circuit in Figure 3 has a RH
resolution of about 0.32% / count (3.2% / pF). Also, a
1% error in measuring CSEN produces a 6% RH error.
In addition, there is a ±2% error in the nominal RH
value, and a ±6% error due to temperature variations
(at -40°C and +85°C).
RH is calculated from CSEN using a piece-wise linear
(PWL) lookup table [11]. This table has been designed
to make the firmware simple and quick by using 64
lookup table rows. This has the added benefit of
producing a very accurate estimate of RH (better than
±0.01% error).
Overview of Errors
Table 3 includes all of the errors mentioned in this
section. These errors are at room temperature (+25°C).
It also shows how the errors propagate through the
circuit and the algorithm.
The dominant errors are:
VREF accuracy: (VRH – VRL) / (VDD/2)
RINT
The internal oscillator frequency
The op amp Gain-Bandwidth Product (fGBWP) for
very small CSEN values
• The nominal sensor (HS1101LF) error
•
•
•
•
Note that when we subtract VRL from VRH, the
comparator’s offset voltage is cancelled (because it is
constant).
© 2005 Microchip Technology Inc.
DS01016A-page 7
AN1016
TABLE 3:
ERROR ANALYSIS AT ROOM TEMPERATURE
Error Sources
Special
Conditions
Cause
Effects
Worst Case
Imbalance ε Error in k
Error
Error in CSEN Error in RH
(Note 2)
(Note 3)
(Note 1)
Current (IINT) Imbalance
RCM1 and RCM2
—
±1%
±2.0%
VOS_OA
—
±3 mV
±0.2%
IB_OA (Note 4)
—
≈ 20 pA
±0.004%
±0.05%
±0.05%
±0.3%
Average Count (k)
VRH – VRL (Note 5)
—
±4%
—
±4%
±4%
±24%
RINT
—
±1%
—
±1%
±1%
±6%
Comparator CMRR
—
±0.18%
—
±0.18%
±0.18%
±1.1%
Comparator VOS
—
±10 mV
—
0%
0%
0%
Oscillator
fGBWP
internal
±1%
—
±1%
±1%
±6%
external
±0.01%
—
±0.01%
±0.01%
±0.1%
for HS1101LF
–0.002%
—
–0.002%
–0.002%
–0.01%
at CSEN = 0.6 pF
–0.5%
—
–0.5%
–0.5%
N/A
not corrected
0.5 pF
—
—
0.5 pF
1.6%
Calculating CSEN
CPAR
corrected
±0.1 pF
—
—
±0.1 pF
±0.3%
B1
—
0.25%
—
—
0.25%
1.5%
Quantization Error
—
±0.05 pF
—
—
±0.05 pF
±0.3%
HS1101LF, Nominal Error —
±3%
—
—
—
±3%
HS1101LF, Drift per Year
—
±1%
—
—
—
±1%
PWL Lookup Table
—
±0.01%
—
—
—
±0.01%
Calculating RH
Note 1:
2:
3:
4:
5:
It is assumed that VDD is at its lowest value (3.0V for this design) when converting errors in mV to percentage errors.
The error in VCM is given as a percentage of VDD/2, which is the same as the relative error ε.
The error magnitudes are calculated one at a time, except when converting from ε to ε2 (“Error in k”
column) for the current imbalance. In the latter case, the relative errors are added together before
squaring.
This error becomes bigger as the ambient temperature increases. At +125°C, IB_OA’s contribution to the
imbalance ε is ±5 nA or ±1.1%, causing the RH error to be ±0.6%.
Both VRL and VRH have a maximum specified error of ±0.0104VDD. Since (VRH – VRL) has a nominal
value of 0.375VDD, each error becomes ±0.0278(VRH – VRL). The two errors were added in the RMS
sense to obtain the ±4% value shown above.
Possible Improvements
The VREF levels (VRH and VRL) can be corrected, in
some microcontrollers, by internally connecting an
ADC to the VREF output. Since the ADC has better
accuracy than the VREF ladder, the measurement can
be improved.
Most of the remaining errors can be corrected with
appropriate calibration procedures and calculations.
The calculated RH can be corrected for errors across
temperature; both the sensor and the circuit can be
calibrated at the same time. Once the decision is made
to calibrate the errors, there is no need to correct the
VREF levels using the internal ADC.
Components with tighter tolerance will directly improve
the RH accuracy. The resistor, RINT, and the oscillator
are two important examples.
DS01016A-page 8
© 2005 Microchip Technology Inc.
AN1016
MEASURED RESULTS
Fine Resolution Measurements
The basic circuits in Figure 3 and Figure 6 were
measured with different RINT values. First, known
capacitors were measured to validate the accuracy of
these designs. Then the HS1101LF relative humidity
sensor was measured and compared to another,
calibrated humidity sensor.
The circuit in Figure 6 was measured first; it has the
40 dB attenuation of the square wave. This made it
possible to measure the parasitic capacitance, CPAR,
and other small capacitances. The CSEN resolution is
0.001 pF / count.
The measurements were made on the Humidity Sensor
PICtail™ Demo Board, which is discussed in the user’s
guide (DS51594) [15]. This board is further described
by:
Figure 7 shows the VSEN waveform across time. T1G is
the Timer 1 Gate waveform; it shows when the
comparator decides that VSEN has reached either VRL
or VRH. CSEN was 166.0 pF and RINT was 6.65 MΩ.
• Order Number: PIC16F690DM-PCTLHS
• Assembly Number: 102-00084R1
VINT and VSEN Waveforms with CSEN = 166 pF.
Figure 8 shows the measurement discrepancy
between the values read from an HP4285A LCR meter
and the circuit in Figure 6. These measurements were
taken across a range of allowed CSEN values. The
parasitic capacitance CPAR was measured by leaving
CSEN open; the value for the Humidity Sensor PICtail™
Demo Board [15] turned out to be approximately
0.27 pF.
CSEN Measurement
Discrepancy
FIGURE 7:
10%
8% Modified
Circuit
6%
Not Corrected
4%
2%
0%
-2%
Corrected:
-4%
CPAR = 0.270 pF
-6%
B1 = 0.09499 pF / count
-8%
-10%
1p
10p
1.E+00
1.E+01
CSEN (F)
FIGURE 8:
Discrepancy.
© 2005 Microchip Technology Inc.
100p
1.E+02
CSEN Measurement
DS01016A-page 9
AN1016
LESSONS LEARNED
The circuit in Figure 3 does not attenuate the square
wave, making it possible to measure larger capacitors.
The CSEN resolution is 0.1 pF / count.
Several important lessons were learned in the process
of building, measuring and debugging this design.
Figure 9 shows the measurement discrepancy
between the values read from an HP4285A LCR meter
and the circuit in Figure 3. These measurements were
taken across a range of allowed CSEN values.
Ratiometric Design
CSEN Measurement
Discrepancy
Normal Resolution Measurements
10%
8%
6%
4%
2%
0%
-2%
-4%
-6%
-8%
-10%
1p
1.E+00
• The square wave VINT is ratiometric
• VCM is ratiometric
• VCM and RINT make the current IINT ratiometric,
so the voltage VSEN is ratiometric
• CVREF reference (used for VRH and VRL) is
ratiometric (the other internal reference is not)
Not Corrected
Corrected:
CPAR = 0.270 pF
B1 = 0.09987 pF / count
10p
1.E+01
FIGURE 9:
Discrepancy.
100p
1.E+02
CSEN (F)
1n
1.E+03
10n
1.E+04
HS1101LF Sensor Measurements
The circuit in Figure 3 was used to measure the
HS1101LF relative humidity sensor. The measurement
resolution is 0.1 pF / count (0.6% RH change per
count) and RINT is 6.65 MΩ. The curves in Figure 10
show how the sensor reacted when it was breathed on
for about half second; the result is the impulse
response of the sensor.
190
18%
185
16%
180
14%
RH
175
12%
CSEN
170
10%
165
8%
-5
0
5
10 15 20 25 30 35 40 45
Time (s)
FIGURE 10:
Response.
DS01016A-page 10
Relative Humidity
Sensor Capacitance (pF)
20%
breathed on sensor
• The lower CVREF range is more accurate than the
upper range (±1.04% vs. ±1.56%).
• The comparator’s VCMR range is VDD – 1.5V,
which is at its worst case value (VDD/2) when
VDD = 3.0V (it is not ratiometric).
• The op amp’s output should stay above 0.2V to
0.3V to maintain accuracy and avoid overdrive
recovery problems (about 0.1VDD when
VDD = 3.0V).
For these reasons, the design uses VRL = 0.125VDD
and VRH = 0.500VDD in the lower CVREF range.
22%
195
Reference Voltages Chosen
The reference voltages (VRH and VRL) need to be
selected carefully. The analog components need to
stay within their allowed ranges, but VRH and VRL need
to be as far apart as possible for accuracy reasons:
CSEN Measurement
200
This design assumes VDD ranges from 3.0V to 5.5V
(e.g., two lithium batteries). To avoid supply rejection
errors, and to make the design simpler to implement, a
ratiometric approach was very helpful. The implementation is as follows:
Choosing the Microcontroller
The PIC16F690 has several key features that help this
design:
• Comparator latch makes the firmware simpler and
avoids delay in a firmware loop.
• The comparator can be connected internally to
the reference (CVREF) and to the board.
• It has an accurate internal oscillator.
• It operates over the required supply range.
HS1101LF Impulse
© 2005 Microchip Technology Inc.
AN1016
Calibration
Capacitive relative humidity sensors are quite sensitive
to capacitive measurement errors. It can be costly to
use components with tighter tolerances. The designs in
this application note are a big help because they have
low parasitic capacitance in parallel with the sensor,
and they are immune to sensor case to ground
capacitances.
The most effective way to overcome the sensitivity
problem is to calibrate the errors in production. All of
the errors can be corrected in the same step.
Miscellaneous
Using dual slope integration minimizes errors due to
the VCM error (IINT imbalance).
The op amp’s gain-bandwidth product affects the
relative error for small CSEN values. Select an op amp
that gives an acceptable error at the smallest CSEN
value that will be measured.
RINT is selected last to make the code simpler to
implement. It is chosen so that B1 becomes a
convenient number (e.g., 0.1 pF / count).
It is important to use a greater number of bits for the
averaging (accumulation) of CSEN. The counter in this
design is a 16-bit value, and the accumulator is a 24-bit
value. This avoids truncation errors, and can prevent
counter results being interpreted as negative values.
SUMMARY
This application note shows hardware and firmware
design engineers how to use a PICmicro®
microcontroller and an op amp integrator to accurately
measure small capacitances. Simple firmware is
included that produces capacitance and relative
humidity values for the HS1101LF relative humidity
sensor.
Measurements verify the theory and design choices.
An error analysis points to possible improvements in
the design. Other alternatives and modifications to this
design are also covered. Key lessons learned help the
user focus on the important aspects of the given
design.
The references and appendices give detailed
information that supports the text of this application
note.
REFERENCES
Humidity Sensors
[1] Humirel, “Relative Humidity Sensor HS1100LF /
HS1101LF,” Data Sheet, Rev. C, Sept. 2004.
General Sensors
[2] “The OMEGA® Made in the USA Handbook™”,
Vol. 1, OMEGA Engineering, Inc., 2002.
[3] “The OMEGA® Made in the USA Handbook™”,
Vol. 2, OMEGA Engineering, Inc., 2002.
[4] AN866, “Designing Operational Amplifier Oscillator Circuits For Sensor Applications”, Jim
Lepkowski; Microchip Technology Inc., DS00866,
2003.
[5] AN990, “Analog Sensor Conditioning Circuits – An
Overview”, Kumen Blake, Microchip Technology
Inc., DS00990, 2005.
[6] AN512, “Implementing Ohmmeter/Temperature
Sensor”, Doug Cox; Microchip Technology Inc.,
DS00512, 1997.
[7] AN895 “Oscillator Circuits For RTD Temperature
Sensors”, Ezana Haile and Jim Lepkowski;
Microchip Technology Inc., DS00895, 2004.
Capacitive Sensors
[8] AN611, “Resistance and Capacitance Meter
Using a PIC16C622”, Rodger Richey; Microchip
Technology Inc., DS00611, 1997.
[9] Ricardo Jimenez and Claudia Angulo, “Measure
Relative Humidity With a PIC® MCU”, Design
Briefs, Electronic Design, March 17, 2005, p. 7980.
[10] AN1014, “Measuring Small Changes in Capacitive Sensors”, Kumen Blake and Steven Bible;
Microchip Technology Inc., DS01014, 2005.
Related Literature
[11] AN942, “Piecewise Linear Interpolation on PIC12/
14/16 Series Microcontrollers”, John Day and
Steven Bible; Microchip Technology Inc., 2004.
[12] MCP6291/2/3/4/5 Data Sheet, “1.0 mA, 10 MHz
Rail-to-Rail Op Amp,” Microchip Technology Inc.,
DS21812, 2004.
[13] PIC16F685/687/689/690 Data Sheet, “20-Pin
Flash-Based, 8-Bit CMOS Microcontrollers with
nanoWatt Technology”, Microchip Technology Inc.,
DS41262, 2005.
[14] PIC16C745/765 Data Sheet, “8-Bit CMOS Microcontrollers with USB”, Microchip Technology Inc.,
DS41124, 2000.
[15] “Humidity Sensor PICtail™ Demo Board User’s
Guide”, Microchip Technology Inc., DS51594,
2005.
© 2005 Microchip Technology Inc.
DS01016A-page 11
AN1016
APPENDIX A:
PIECEWISE LINEAR
INTERPOLATION
TABLE
In order to convert CSEN to RH, the firmware uses a
piecewise linear interpolation table [11]. This table is
designed specifically for Humirel’s HS1101LF relative
humidity sensor [1]. First, the calculated CSEN value is
limited in range and converted to a 10-bit integer:
EQUATION A-1:
CSN = 0,
= 1023,
C SEN < 130.0 pF
C SEN > 232.3 pF
C SEN – 130.0 pF
= ----------------------------------------, otherwise
0.1 pF
The top 6 bits of CSN are used as a pointer (INTRVL)
to the correct row of the table. The bottom 4 bits of CSN
(CSN – ACSN, interpreted as integers between 0 and
15) are the input to the linear interpolation formula
using the coefficients for the current row. The entries in
the table give errors less than ±0.01%.
EQUATION A-2:
X = A RHO + A RH1 ( CSN – ACSN )
RH = max { min { X, 100% }, 0% }
DS01016A-page 12
TABLE A-1:
HS1101LF COEFFICIENT
LOOKUP TABLE
ACSN
ARH0
(%)
ARH1
(% / LSb)
0
0
0
0
1
16
0
0
···
···
···
···
17
272
0
0
18
288
0
0
19
304
-3.08
0.250
20
320
0.92
0.259
21
336
5.06
0.267
22
352
9.33
0.276
23
368
13.75
0.285
24
384
18.31
0.294
25
400
23.00
0.302
26
416
27.84
0.310
27
432
32.80
0.318
28
448
37.89
0.325
29
464
43.09
0.332
30
480
48.40
0.337
31
496
53.79
0.341
32
512
59.26
0.345
33
528
64.77
0.347
34
544
70.32
0.347
35
560
75.88
0.346
36
576
81.41
0.344
37
592
86.91
0.339
38
608
92.33
0.332
39
624
97.64
0.325
40
640
100
0
41
656
100
0
···
···
···
···
INTRVL
62
992
100
0
63
1008
100
0
© 2005 Microchip Technology Inc.
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