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Freescale Semiconductor Technical Data
Document number: MC34940 Rev 4, 11/2006
Electric Field Imaging Device
The MC34940 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 MC34940 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 MC34940 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
MC34940
ELECTRONIC FIELD IMAGING DEVICE
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 MC34940EG/R2 Temperature Range 0 to 90C Drawing CASE 751E-04 Package SOICW-24 EG SUFFIX (Pb-FREE) 24-TERMINAL SOICW CASE 751E-04
DGND N/C SHIELDEN C B A LEVEL LPCAP ROSC VDDCAP VPWR VCCCAP
N/C E7 E6 E5 E4 E3 E2 E1 TEST GND SHIELD AGND
Pin Connections
(c) Freescale Semiconductor, Inc., 2006. All rights reserved.
A,B,C
3
CONTROL LOGIC
2.8 k E1-E7 2.8 k MUX OUT
OSC
22 k (Nominal)
ROSC
SHIELDEN 150
700
MUX IN
SHIELD
700
RECT LPCAP
VCCCAP VDDCAP VCC REG VDD REG
LPF
GAIN AND OFFSET LEVEL
VPWR
AGND GND
Figure 1. Simplified Functional Block Diagram
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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 ELECTRICAL RATINGS Peak VPWR Voltage Double Battery 1 Minute Maximum TA = 30C ESD Voltage Human Body Model (CZAP = 100 pF, RZAP = 1500 ) Machine Model (CZAP = 200 pF, RZAP = 0 ) Charge Device Model (CDM), Robotic (CZAP = 4.0 pF) THERMAL RATINGS Storage Temperature Operating Ambient Temperature Operating Junction Temperature Thermal Resistance Junction-to-Ambient (1) Junction-to-Case (2) Junction-to-Board (3) Soldering Temperature (4) 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. 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. 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. 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. TSTG TA TJ RJA RJC RJB TSOLDER -55 to 150 -0 to 90 -0 to 150 41 0.2 3.0 260 C C C C C/W VPWRPK VDBLBAT VESD 40 26.5 V 2000 200 1200 V V Symbol Value Unit
2. 3. 4.
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Table 2. Static Electrical Characteristics Characteristics noted under conditions 5.5 V VSUP 18 V, 0C TA 90C, GND = 0 V unless otherwise noted. Typical values noted reflect the approximate parameter means at TA = 25C under nominal conditions unless otherwise noted.
Characteristic SUPPLY (VPWR) Supply Voltage IDD (VPWR = 14 V) (Quiescent supply current measured over temperature. Assumes that no external devices connected to internal voltage regulators) ELECTRODE SIGNALS (E1-E7) Total Variance Between Electrode Measurements (5) All CLOAD = 15 pF Electrode Maximum Harmonic Level Below Fundamental (5) 5.0 pF CLOAD 150 pF Electrode Transmit Output Range 5.0 pF CLOAD 150 pF Receive Input Voltage Range Grounding Switch on Voltage ISW = 1.0 mA LOGIC I/O (C, B, A) CMOS Logic Input Low Threshold Logic Input High Threshold Voltage Hysteresis Input Current VIN = VCC VIN = 0 V SIGNAL DETECTOR (LPCAP) Detector Output Resistance LPCAP to LEVEL Gain LPCAP to LEVEL Offset DETRO AREC VRECOFF - 3.6 -3.3 50 4.0 -3.0 - 4.4 -2.7 k AV V VTHL VTHH VHYS IIN 0.3 - - 10 -5.0 - - 0.06 - - - 0.7 - 50 5.0 VCC VCC VCC A
(6)
Symbol
Min
Typ
Max
Unit
VPWR IDD
V 9.0 6.0 12 7.0 18 mA 8.0
ELVVAR ELHARM ELTXV RXV SWVON
% - - 1.0 0 - - -20 - - - 3.0 dB - V 8.0 9.0 5.0 V V
Notes 5. Verified by design and characterization. Not tested in production. 6. Current into grounded terminal under test = 1.0 mA.
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Table 3. Dynamic Electrical Characteristics Characteristics noted under conditions 5.5 V VSUP 18 V, 0C TA 90C, GND = 0 V unless otherwise noted. Typical values noted reflect the approximate parameter means at TA = 25C under nominal conditions unless otherwise noted.
Characteristic OSC (ROSC) OSC Frequency Stability OSC Center Frequency ROSC = 39 k ROSC = 20 k ROSC = 82 k Harmonic Content 2nd through 4th Harmonic Level 5th and Higher SHIELD DRIVER (SHIELD) Shield Driver Maximum Harmonic level below Fundamental 10 pF CLOAD 500 pF Shield Driver Gain Bandwidth Product Measured at 120 kHz SDHARM SDGBW dB - - -20 4.5 - MHz - f STAB f OSC - - - - - - - 120 240 60 - - 10 - - - dB -20 -60 % kHz Symbol Min Typ Max Unit
OSCHARM
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PRINCIPLE OF OPERATION
The MC34940 generates a low radio frequency sine wave with nominal 5.0 V peak-to-peak amplitude. The frequency is set by an 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 an 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
Drive level ~ 5 V p-p
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 (22 k) was chosen to provide a near linear relationship at 120 kHz over a range of 10 pF to 70 pF. While exploring applications using the E-Field chip, it is always useful to approach the problem using the capacitor model.
Voltage Level Proportional to 1/C (voltage divider)
Load Resistor (22 k)
Detector
Low Pass Filter
Stray Variable Capacitance Electrodes
Object Sine Generator (120 kHz) Capacitance increases as electrodes move closer together Virtual Ground
Detected Signal Level Decreases with Increasing Capacitance
Capacitor Model
Figure 2. 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 * * Table 4. Dielectric Constants of Various Materials
Dielectric Material Acrylic Glass Nylon Plastic Polyester Film Thickness (mil) 84.5 74.5 68 10 9 k 2.4-4.5 7.5 3.0-5.0 3.2 2.8-4.5 1.0 80 3.2 2.1
k A C= 0 d
Flexible Vinyl Film
C
k
d
Air Water Ice
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)
Automotive Oil
Figure 3. Capacitor Model
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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 MC34940 offers 3 operating frequencies. In addition to the default frequency of 120 kHz, the MC34940 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 150 pF as seen on Figure 4. 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 an 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 10 nF and in practice it will have a response time of 2.5 ms. If faster response time is required a 1.0 nF capacitor can be used and it will have response times around 500 s. Please note that reducing the LP_CAP capacitor value increases noise accordingly.
Output Voltage vs Capacitance at 3 Discrete Frequencies
4 3.5 Voltage Output (Volts) 3 2.5 2 1.5 1 0.5 0 0 50 100 Capacitance (pF) 150 200 120 kHz 240 kHz 60 kHz
Figure 4. Output Voltage vs. Capacitance at 3 Discrete Frequencies
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BASIC CONNECTIONS
PIN DESCRIPTIONS Table 6. Pin Description
Pin Number
DGND N/C SHIELDEN C B A LEVEL LPCAP ROSC VDDCAP VPWR VCCCAP N/C E7 E6 E5 E4 E3 E2 E1 TEST GND SHIELD AGND
Pin Name DGND N/C SHIELDEN C, B, A LEVEL
Definition Connected to the ground return These pins should be left open. Used to enable the shield signal Controls electrode or reference activity This is the detected, amplified, and offset representation of the signal voltage on the selected electrode A capacitor on this pin forms a low pass filter with the internal series resistance from the detector to this pin A resistor from this pin to circuit ground determines the operating frequency of the oscillator A 47 F capacitor is connected to this pin to filter the internal analog regulated supply 12 V power applied to this pin will be converted to the internal regulated voltages needed to operate the part A 47 F capacitor is connected to this pin to filter the internal digital regulated supply Connected to the ground return of the analog circuitry Connects to cable shields to cancel cable capacitance. Main IC ground Connect to circuit ground Electrode pins
1 2, 24 3 4,5,6 7
8
LPCAP
Figure 5. Pin Descriptions Table 5. Electrode Selection
Terminal/SIGNAL No electrodes selected E1 E2 E3 E4 E5 E6 E7 C 0 0 0 0 1 1 1 1 B 0 0 1 1 0 0 1 1 A 0 1 0 1 0 1 0 1
9
ROSC
10
VDDCAP
11
VPWR
12
VCCCAP
13 14 15 16 17-23
AGND SHIELD GND TEST E1-E7
MC34940
47 F 47 F
VCCCAP
ROSC
39k
VDDCAP LPCAP
10 nF
MCU Analog In
LEVEL
3
Electrode Select Shield Enable
+12 V
A, B, C SHIELDEN
E1 Field Electrodes (E1 through E7) E7 SHIELD
VPWR TEST AGND GND
Figure 6. Simplified Application Diagram
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PACKAGE DIMENSIONS
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EG SUFFIX CASE 751E-04 ISSUE F
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PACKAGE DIMENSIONS
PAGE 2 OF 2 PAGE 2 OF 2
EG SUFFIX CASE 751E-04 ISSUE F
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MC34940 Rev 4 11/2006

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