Document order number: MC34940
Rev 2.0, 2/2006
Freescale Semiconductor
Technical Data
Freescale Semiconductor, Inc., 2006. All rights reserved.
Electric Field Imaging Device
The 34940 is intended for cost-sensitive applications where non-contact
sensing of objects is desired. When connected to external electrodes, an
electric field is created. The 34940 detects objects in this electric field. The IC
generates a low-frequency sine wave, which is adjustable by using an external
resistor and is optimized for 120 kHz. The sine wave has very low harmonic
content to reduce harmonic interference. The 34940 also contains support
circuits for a microcontroller unit (MCU) to allow the construction of a two-chip
E-field system.
Features
Supports up to 7 Electrodes
Shield Driver for Driving Remote Electrodes Through Coaxial
High-Purity Sine Wave Generator Tunable with External Resistor
Response Time Tunable with External Capacitor
Can support up to 28 touch pad sensors
Pb-Free and RoHS compliant
Typical Applications
Appliance Control Panels and Touch Sensors
Linear and Rotational Sliders
Spill Over Flow Sensing Measurement
Refrigeration Frost Sensing
Industrial Control and Safety Systems Security
Proximity Detection for Wake-Up Features
Touch Screens
Garage Door Safety Sensing
PC Peripherals
Patient Monitoring
Point of Sale Terminals
Size Detection
Liquid Level Sensing
ORDERING INFORMATION
Device Name
Temperature
Range
Drawing
Package
MC34940EG/R2
0 to 90C
98ASB42564B
SOICW-24
34940
ELECTRONIC FIELD
IMAGING DEVICE
24 LEAD (PB-FREE)
SOICW
98ASB42344B
Figure 1. Pin Connections
N/C
E7
E6
E5
E4
E3
E2
E1
TEST
GND
SHIELD
AGND
DGND
N/C
SHIELDEN
C
B
A
LEVEL
LPCAP
ROSC
VDDCAP
VPWR
VCCCAP
MC34940
Sensors
2
Freescale Semiconductor
Figure 2. Simplified Functional Block Diagram
Table 1. MAXIMUM RATINGS All voltages are with respect to ground unless otherwise noted. Exceeding these ratings may
cause a malfunction or permanent damage to the device.
Rating
Symbol
Value
Unit
Electrical Ratings
Peak V
PWR
Voltage
V
PWRPK
40
V
Double Battery
1 Minute Maximum T
A
= 30
C
V
DBLBAT
26.5
V
ESD Voltage
Human Body Model (C
ZAP
= 100 pF, R
ZAP
= 1500 W)
Machine Model (C
ZAP
= 200 pF, R
ZAP
= 0 W)
Charge Device Model (CDM), Robotic (C
ZAP
= 4.0pF)
V
ESD
2000
200
1200
V
Thermal Ratings
Storage Temperature
T
STG
-55 to 150
C
Operating Ambient Temperature
T
A
-0 to 90
C
Operating Junction Temperature
T
J
-0 to 150
C
Thermal Resistance
Junction-to-Ambient
(1)
Junction-to-Case
(2)
Junction-to-Board
(3)
R
JA
R
JC
R
JB
41
0.2
3.0
C/W
Soldering Temperature
(4)
T
SOLDER
260
C
OSC
700
700
LEVEL
LPCAP
VDDCAP
VCCCAP
VPWR
AGND
GND
ROSC
SHIELDEN
SHIELD
E1-E7
A,B,C
150
RECT
LPF
GAIN AND
OFFSET
MUX
IN
CONTROL
LOGIC
MUX
OUT
3
22 k
(Nominal)
2.8 k
2.8 k
V
CC
REG
V
DD
REG
MC34940
Sensors
Freescale Semiconductor
3
Notes
1.
Junction temperature is a function of on-chip power dissipation, package thermal resistance, mounting site (board) temperature, ambient
temperature, air flow, power dissipation of other components on the board, and board thermal resistance. In accordance with SEMI G38-
87 and JEDEC JESD51-2 with the single layer board horizontal.
2.
Indicates the average thermal resistance between the die and the case top surface as measured by the cold plate method (MILSPEC 883
Method 1012.1) with the cold plate temperature used for the case temperature.
3.
Thermal resistance between the die and the printed circuit board per JEDEC JESD51-8. Board temperature is measured on the top
surface of the board near the package.
4.
Terminal soldering temperature limit is for 10 seconds maximum duration. The device is not designed for immersion soldering. Exceeding
these limits may cause malfunction or permanent damage to the device
5.
Verified by design and characterization. Not tested in production.
6.
Current into grounded terminal under test = 1.0 mA.
Table 2. STATIC ELECTRICAL CHARACTERISTICS
Characteristics noted under conditions 5.5 V
V
SUP
18 V, 0C T
A
90C, GND = 0 V unless otherwise noted. Typical
values noted reflect the approximate parameter means at T
A
= 25C under nominal conditions unless otherwise noted.
Characteristic
Symbol
Min
Typ
Max
Unit
SUPPLY (V
PWR
)
Supply Voltage
V
PWR
9.0
12
18
V
I
DD
(V
PWR
= 14V)
(Quiescent supply current measured over temperature. Assumes
that no external devices connected to internal voltage regulators)
I
DD
6.0
7.0
8.0
mA
ELECTRODE SIGNALS (E1E7)
Total Variance Between Electrode Measurements
(5)
All C
LOAD
= 15 pF
ELV
VAR
3.0
%
Electrode Maximum Harmonic Level Below Fundamental
(5)
5.0 pF
C
LOAD
150 pF
EL
HARM
-20
dB
Electrode Transmit Output Range
5.0 pF
C
LOAD
150 pF
EL
TXV
1.0
8.0
V
Receive Input Voltage Range
RX
V
0
9.0
V
Grounding Switch on Voltage
(6)
I
SW
= 1.0 mA
SW
VON
5.0
V
LOGIC I/O (C, B, A)
CMOS Logic Input Low Threshold
V
THL
0.3
V
CC
Logic Input High Threshold
V
THH
0.7
V
CC
Voltage Hysteresis
V
HYS
0.06
V
CC
Input Current
V
IN
= V
CC
V
IN
= 0 V
I
IN
10
-5.0
50
5.0
A
SIGNAL DETECTOR (LPCAP)
Detector Output Resistance
DET
RO
50
k
LPCAP to LEVEL Gain
A
REC
3.6
4.0
4.4
A
V
LPCAP to LEVEL Offset
V
RECOFF
-3.3
-3.0
-2.7
V
MC34940
Sensors
4
Freescale Semiconductor
Table 3. DYNAMIC ELECTRICAL CHARACTERISTICS
Characteristics noted under conditions 5.5 V
V
SUP
18 V, 0C T
A
90C, GND = 0 V unless otherwise noted. Typical
values noted reflect the approximate parameter means at T
A
= 25C under nominal conditions unless otherwise noted.
Characteristic
Symbol
Min
Typ
Max
Unit
OSC (ROSC)
OSC Frequency Stability
f
STAB
10
%
OSC Center Frequency
ROSC = 39 k
ROSC = 20 k
ROSC = 82 k
f
OSC
120
240
60
kHz
Harmonic Content
2nd through 4th Harmonic Level
5th and Higher
OSCH
ARM
-20
-60
dB
SHIELD DRIVER (SHIELD)
Shield Driver Maximum Harmonic level below Fundamental
10 pF
C
LOAD
500 pF
SD
HARM
-20
dB
Shield Driver Gain Bandwidth Product
Measured at 120 kHz
SD
GBW
4.5
MHz
MC34940
Sensors
Freescale Semiconductor
5
PRINCIPLE OF OPERATION
The 34940 generates a low radio frequency sine wave with
nominal 5.0 V peak-to-peak amplitude. The frequency is set
by and external resistor and is optimized for 120 kHz. An
internal multiplexer routes the signal to one of the 7 terminals
under control of the ABC input terminals. A receiver
multiplexer simultaneously connected to the selected
electrode routes its signal to a detector, which converts the
sine wave to a DC level. The DC level is filtered by and
external capacitor, is multiplied and offset to increase
sensitivity. All electrode outputs are grounded internally by
the device when not selected.
The amplitude and phase of the sinusoidal wave at the
electrode are affected by objects in proximity. A "capacitor" is
formed between the driving electrode and the object, each
forming a "plate" that holds the electric charge. The voltage
measured is an inverse function of the capacitance between
the electrode being measured, the surrounding electrodes
and other objects in the electric field surrounding the
electrode. Increasing capacitance results in decreasing
voltage. The value of the series resistor (22kohm) was
chosen to provide a near linear relationship at 120 kHz over
a range of 10pF to 70pF.
While exploring applications using the E-Field chip, it is
always useful to approach the problem using the capacitor
model.
Figure 3. Conceptual Block Diagram
CAPACITOR MODEL
The capacitance measured by the E-Field IC is:
Proportional to the area of the electrode
Proportional to the dielectric constant of the material
between the electrodes
Inversely proportional to the distance between the objects
Figure 4. Capacitor Model
Detector
Low Pass Filter
Voltage Level Proportional to 1/C (voltage divider)
Drive level ~ 5 v p-p
Load Resistor
(22 K ohms)
Sine Generator
(120 KHz)
Detected Signal
Level Decreases
with Increasing
Capacitance
Electrodes
Capacitance
increases as
electrodes move
closer together
Capacitor Model
Virtual Ground
Stray Variable
Capacitance
Object
Detector
Low Pass Filter
Voltage Level Proportional to 1/C (voltage divider)
Drive level ~ 5 v p-p
Load Resistor
(22 K ohms)
Sine Generator
(120 KHz)
Detected Signal
Level Decreases
with Increasing
Capacitance
Electrodes
Capacitance
increases as
electrodes move
closer together
Capacitor Model
Virtual Ground
Stray Variable
Capacitance
Object
d
A
k
C
0
=
C
k
d
C=The Capacitance in Farads (F)
A=The area of the plates in square meters (m2)
d=The distance between the plates in meters (m)
k=The dielectric constant of the material separating the plates
0=Is the permittivity of free space (8.85 x 10-12 F/m)
Table 4 Dielectric Constants of Various Materials
Dielectric Material
Thickness (mil)
k
Acrylic
84.5
2.4-4.5
Glass
74.5
7.5
Nylon Plastic
68
3.0-5.0
Polyester Film
10
3.2
Flexible Vinyl Film
9
2.8-4.5
Air
-
1
Water
-
80
Ice
-
3.2
Automotive Oil
-
2.1
MC34940
Sensors
6
Freescale Semiconductor
FEATURES
SHIELD DRIVER
A shield driver is included to minimize the electrode signal
along wires. This circuit provides a buffered version of the
returned AC signal from the electrode. Since it has nearly the
same amplitude and phase as the electrode signal, there is
little or no potential difference between the two signals
thereby canceling out any electric field. In effect, the shield
drive isolates the electrode signal from external virtual
grounds. A common application is to connect the Shield
Driver to the shield of a coax cable used to connect an
electrode to the corresponding electrode terminal. Another
typical use is to drive a ground plane that is used behind an
array of touch sensor electrodes in order to cancel out any
virtual grounds that could attenuate the AC signal.
TUNABLE FREQUENCY
The 34940 offers 3 operating frequencies. In addition to
the default frequency of 120 kHz, the 34940 has also been
characterized to work in two other frequencies (240 kHz and
60 kHz) for applications with specific needs. These
frequencies are tunable by attaching a 20k and 82k resistor
at ROSC respectively. If a wider capacitance range is
needed, simply change the ROSC resistor value to 82k to
have the signal generator operate at 60 kHz which will extend
the capacitance range to 150pF as seen on
Figure 5
. The
figure also shows that one can achieve higher sensitivity at
lower capacitances by setting the ROSC resistor value to
20k. All resistor values listed above are for 5% tolerance
resistors.
ADJUSTABLE RESPONSE TIME
The rectified sine wave is filtered by a Low Pass Filter
formed by and internal resistor and an external capacitor
attached to LP_CAP. The value of the external capacitor is
selected to allow the designer to optimize the balance
between noise and settling time. A typical value for the
external capacitor is 10nF and in practice it will have a
response time of 2.5ms. If faster response time is required a
1nF capacitor can be used and it will have response times
around 500uS. Please note that reducing the LP_CAP
capacitor value increases noise accordingly.
Figure 5 Output Voltage vs. Capacitance at 3 Discrete Frequencies
Output Voltage vs Capacitance at 3 Discrete Frequencies
0
0.5
1
1.5
2
2.5
3
3.5
4
0
20
40
60
80
100
120
140
160
Capacitance (pF)
V
o
l
t
a
g
e
O
u
t
put
(
V
ol
t
s
)
120 kHz
240 kHz
60 kHz
MC34940
Sensors
Freescale Semiconductor
7
BASIC CONNECTIONS
Pin Descriptions
Figure 6 Pin Descriptions
Figure 7 Simplified Application Diagram
Table 5. Electrode Selection
Terminal/SIGNAL
C
B
A
No electrodes selected
0
0
0
E1
0
0
1
E2
0
1
0
E3
0
1
1
E4
1
0
0
E5
1
0
1
E6
1
1
0
E7
1
1
1
N/C
E7
E6
E5
E4
E3
E2
E1
TEST
GND
SHIELD
AGND
DGND
N/C
SHIELDEN
C
B
A
LEVEL
LPCAP
ROSC
VDDCAP
VPWR
VCCCAP
Table 6. Pin Description
Pin
Number
Pin Name
Definition
1
DGND
Connected to the ground return
2, 24
N/C
These pins should be left open.
3
SHIELDEN
Used to enable the shield signal
4,5,6
C, B, A
Controls electrode or reference activity
7
LEVEL
This is the detected, amplified, and
offset representation of the signal
voltage on the selected electrode
8
LPCAP
A capacitor on this pin forms a low pass
filter with the internal series resistance
from the detector to this pin
9
ROSC
A resistor from this pin to circuit ground
determines the operating frequency of
the oscillator
10
VDDCAP
A 47
F capacitor is connected to this
pin to filter the internal analog regulated
supply
11
VPWR
12 V power applied to this pin will be
converted to the internal regulated
voltages needed to operate the part
12
VCCCAP
A 47
F capacitor is connected to this
pin to filter the internal digital regulated
supply
13
AGND
Connected to the ground return of the
analog circuitry
14
SHIELD
Connects to cable shields to cancel
cable capacitance.
15
GND
Main IC ground
16
TEST
Connect to circuit ground
17-23
E1E7
Electrode pins
MC34940
Field Electrodes
(E1 through E7)
MCU
SHIELDEN
AGND
TEST
ROSC
GND
VDDCAP
E1
E7
SHIELD
VPWR
Analog In
Electrode Select
Shield Enable
LPCAP
LEVEL
A, B, C
VCCCAP
3
+12V
39k
10nF
47uF
47uF
MC34940
Sensors
8
Freescale Semiconductor
PACKAGING DIMENSIONS
EG SUFFIX
24-TERMINAL SOICW
98ASB42344B
ISSUE F
MC34940
Rev 2.0
2/2006
RoHS-compliant and/or Pb-free versions of Freescale products have the functionality
and electrical characteristics of their non-RoHS-compliant and/or non-Pb-free
counterparts. For further information, see
http://www.freescale.com
or contact your
Freescale sales representative.
For information on Freescale's Environmental Products program, go to
http://
www.freescale.com/epp
.
Information in this document is provided solely to enable system and software
implementers to use Freescale Semiconductor products. There are no express or
implied copyright licenses granted hereunder to design or fabricate any integrated
circuits or integrated circuits based on the information in this document.
Freescale Semiconductor reserves the right to make changes without further notice to
any products herein. Freescale Semiconductor makes no warranty, representation or
guarantee regarding the suitability of its products for any particular purpose, nor does
Freescale Semiconductor assume any liability arising out of the application or use of any
product or circuit, and specifically disclaims any and all liability, including without
limitation consequential or incidental damages. "Typical" parameters that may be
provided in Freescale Semiconductor data sheets and/or specifications can and do vary
in different applications and actual performance may vary over time. All operating
parameters, including "Typicals", must be validated for each customer application by
customer's technical experts. Freescale Semiconductor does not convey any license
under its patent rights nor the rights of others. Freescale Semiconductor products are
not designed, intended, or authorized for use as components in systems intended for
surgical implant into the body, or other applications intended to support or sustain life,
or for any other application in which the failure of the Freescale Semiconductor product
could create a situation where personal injury or death may occur. Should a Buyer
purchase or use Freescale Semiconductor products for any such unintended or
unauthorized application, the Buyer shall indemnify and hold Freescale Semiconductor
and its officers, employees, subsidiaries, affiliates, and distributors harmless against all
claims, costs, damages, and expenses, and reasonable attorney fees arising out of,
directly or indirectly, any claim of personal injury or death associated with such
unintended or unauthorized use, even if such claim alleges that Freescale
Semiconductor was negligent regarding the design or manufacture of the part.
FreescaleTM and the Freescale logo are trademarks of Freescale Semiconductor, Inc.
All other product or service names are the property of their respective owners.
Freescale Semiconductor, Inc., 2006. All rights reserved.
How to Reach Us:
Home Page:
www.freescale.com
E-mail:
support@freescale.com
USA/Europe or Locations Not Listed:
Freescale Semiconductor
Technical Information Center, CH370
1300 N. Alma School Road
Chandler, Arizona 85224
+1-800-521-6274 or +1-480-768-2130
support@freescale.com
Europe, Middle East, and Africa:
Freescale Halbleiter Deutschland GmbH
Technical Information Center
Schatzbogen 7
81829 Muenchen, Germany
+44 1296 380 456 (English)
+46 8 52200080 (English)
+49 89 92103 559 (German)
+33 1 69 35 48 48 (French)
support@freescale.com
Japan:
Freescale Semiconductor Japan Ltd.
Headquarters
ARCO Tower 15F
1-8-1, Shimo-Meguro, Meguro-ku,
Tokyo 153-0064
Japan
0120 191014 or +81 3 5437 9125
support.japan@freescale.com
Asia/Pacific:
Freescale Semiconductor Hong Kong Ltd.
Technical Information Center
2 Dai King Street
Tai Po Industrial Estate
Tai Po, N.T., Hong Kong
+800 2666 8080
support.asia@freescale.com
For Literature Requests Only:
Freescale Semiconductor Literature Distribution Center
P.O. Box 5405
Denver, Colorado 80217
1-800-441-2447 or 303-675-2140
Fax: 303-675-2150
LDCForFreescaleSemiconductor@hibbertgroup.com
PDF 
ZIP