PIC16F690DM-PCTLHS Microchip Technology, PIC16F690DM-PCTLHS Datasheet - Page 7

• Manufacturer Part Number: PIC16F690DM-PCTLHS
• Description: BOARD DEMO PICTAIL HUMIDITY SNSR
• Manufacturer: Microchip Technology
• Series: PICtail™r

Specifications of PIC16F690DM-PCTLHS

• Sensor Type: Humidity
• Sensing Range: 1 ~ 99% RH
• Interface: Analog
• Voltage - Supply: 5V
• Embedded: Yes, MCU, 8-Bit
• Utilized Ic / Part: MCP6291, PIC16F690
• Processor To Be Evaluated: MCP6291 and PIC16F690
• Interface Type: ICSP
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• For Use With: AC162061 - HEADER INTRFC MPLAB ICD2 20PIN
• Sensitivity: -
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant, Lead free / RoHS Compliant

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 I
proportional to V
Using a ratiometric design makes the measurement
times independent of power supply voltage (V
eliminating one source of measurement error.
Current (I
When I
k
calculated counts k
tions” discusses this phenomenon in detail.
Errors in V
the current imbalance. The common mode voltage
setting resistors (R
dominate the V
is 1% high, then the relative error ( ) would would be
+2%. If op amp U
+4.5 mV and V
The current I
bias current (I
amps, at high temperatures.
The attenuator in Figure 6 also causes a current (I
imbalance. A mismatch between R
produces this current mismatch.
Errors in Average Count (k)
The relative error ( ) in k
of
Extraction Equations”.
Errors in the average count, k, are produced by the rel-
ative error in the following:
• V
• R
• Comparator CMRR (change in offset vs. V
• Oscillator frequency
Note that when we subtract V
comparator’s offset voltage is cancelled (because it is
constant).
© 2005 Microchip Technology Inc.
1
= I
and k
2
REF
INT
B_OA
in the average count, k = (k
INTP
levels (V
2,
/ I
are not equal. This causes an error ( ) in the
CM
INT
and I
INT
INT
. This error is largest, for CMOS input op
DD
B_OA
(V
CM
) Imbalance
is also imbalanced by the op amp input
DD
RH
INTM
CM_Error
is 5.0V, then would be +0.2%.
CM1
1
errors. If R
.
1
’s input offset voltage (V
). This produces a relative error
– V
and k
are not equal, the timer counts,
and R
RL
1
and k
)
2
in Equation 6) contribute to
. “CSEN Extraction Equa-
INT
CM2
CM1
2
, V
causes a smaller error
) and the op amp (U
1
is 1% low and R
+ k
RL
CM
2
, V
)/2; see “CSEN
from V
A2
RL
SEN
OS_OA
and R
and V
RH
)
, the
DD
CM2
INT
) is
RL
A3
1
),
)
)
The op amp’s gain-bandwidth product can have a
significant effect on the errors for small C
see Equation 4. The smaller C
error is.
Errors in Calculating C
The parasitic capacitance C
about 0.5 pF if no correction is made, and about
±0.1 pF if the correction is made.
The nominal value of B
is
(+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 C
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 C
(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:
• V
• R
• The internal oscillator frequency
• The op amp Gain-Bandwidth Product (f
• The nominal sensor (HS1101LF) error
very small C
REF
INT
approximately
accuracy: (V
SEN
values
RH
0.10025 pF / count.
1
– V
SEN
SEN
is not exactly 0.1 pF / count; it
RL
PAR
produces a 6% RH error.
using a piece-wise linear
) / (V
SEN
SEN
will cause an error of
AN1016
DD
is, the larger this
/2)
DS01016A-page 7
GBWP
SEN
This
) for
values;
error