MCP2036
Inductive Sensor Analog Front End Device
Features
• Complete Inductance Measurement System:
- Low-Impedance Current Driver
- Sensor/Reference Coil Multiplexer
- High-Frequency Detector
• Operating Voltage: 2.7 to 5.5V
• Low-Power Standby Mode
• Gain and Frequency set by external passive
components
Description
The MCP2036 Inductive Sensor Analog Front End
(AFE) combines all the necessary analog functions for
a complete inductance measurement system.
The device includes :
• High-frequency, current-mode coil driver for
exciting the sensor coil.
• Synchronous detector for converting AC sense
voltages into DC levels.
• Output amplifier/filter to improve resolution and
limit noise.
• Virtual ground reference generator for single
supply operation.
The device is available in 14-pin PDIP, SOIC and
16-pin QFN packages:
Typical Applications
•
•
•
•
Harsh environment inductive keyboards
Inductive rotational sensor interface
Inductive displacement sensor interface
Inductive force sensor interface
Package Types
MCP2036
14-pin PDIP, SOIC
V
REF
1
LREF 2
LBTN 3
V
DD
4
DRVOUT 5
DRVIN 6
CLK 7
MCP2036
16-pin QFN
V
DET+
13
V
REF
16
14 V
DET+
13 V
DET-
12 V
DETOUT
11 V
SS
10 Reserved
9 CS
8 REFSEL
LREF 1
LBTN 2
V
DD
3
DRVOUT 4
REFSEL 7
DRVIN 5
©
2009 Microchip Technology Inc.
CLK 6
CS 8
NC 15
NC 14
12 V
DET-
11 V
DETOUT
10 V
SS
9 Reserved
DS22186A-page 1
MCP2036
1.0
FUNCTIONAL DESCRIPTION
The MCP2036 measures a sensor coil’s impedance by
exciting the coil with a pulsed DC current and
measuring the amplitude of the resulting AC voltage
waveform. The drive current is generated by the
on-chip current amplifier/driver which takes the
high-frequency triangular waveform present on the
DRVIN input, and amplifies it into the pulsed DC current
for exciting the series combination of the sensor coils.
The AC voltages generated across the coils, are then
capacitively coupled into the LBTN and LREF inputs.
An input resistance of 2K between the inputs and the
virtual ground offsets the AC input voltages up to the
signal ground generated by the reference voltage
generator, as shown in
Figure 1-1.
LREF
LBTN
CLK
Input MUX
REFSEL
1
0
10K
Op. Amp. Block
V
DET+
+
V
SS
Mixer
V
DET-
V
DD
Key Inductor Driver
Voltage
Reference
V
REF
10K
V
DETOUT
-
CS
DRVIN
DRVOUT
FIGURE 1-1:
MCP2036 Block Diagram
DS22186A-page 2
©
2009 Microchip Technology Inc.
MCP2036
CD4052
0
1
2
3
10Ω
MCP2036
DRVOUT
V
DET-
V
DETOUT
0
1
2
3
10nF
Key Coils
LREF
LREF
REFSEL
I/O
I/O
I/O
I/O
10nF
LBTN
CS
V
REF
DRVIN
CLK
R
IN
C
IN
PWM
ADC
C
ADC
R
ADC
C
FILTER
V
DET+
R
GAIN
C
FILTER
R
GAIN
C
RGND
PIC
®
Microcontroller
FIGURE 1-2:
MCP2036 Typical Application
The gain of the detector is set by two pairs of resistors;
one pair are the internal fixed series resistors between
the frequency mixer and the amplifier. The second
resistor pair are the two external gain set resistors
(R
GAIN
). The two capacitors (C
FILTER
) in parallel with
the external gain setting resistors form a low pass filter
which converts the pulsed DC output signal into a
smooth DC voltage which is proportional to the AC
sensor voltage input. The output of the system is
present on the V
DETOUT
pin, which drives the
microcontroller’s ADC input for conversion into a digital
value.
The virtual ground reference for the detector/amplifier
is generated by a second internal op amp which
produces a virtual ground equal to ½ the supply
voltage. The virtual ground is available externally at the
V
REF
output and used internally throughout the
detector circuit, allowing single supply operation. A
small external capacitance is required to stabilize this
output and limit noise.
The coil voltages are then multiplexed into the
Synchronous Detector section by the LBTN/LREF
multiplexer. This allows the microcontroller to select
which signal is sampled by the detector. The detector
converts the coil voltages into a DC level using a
frequency mixer, amplifier, and filter.
The mixer is composed of two switches driven by the
clock present on the CLK signal input. The switches
toggle the amplifier/filter between an inverting and
non-inverting topology, at a rate equal to the clock input
frequency. This inverts and amplifies the negative side
of the signal, while amplifying the positive side. The
result is a pulsed DC signal with a peak voltage,
proportional to the amplitude of the AC coil voltage.
©
2009 Microchip Technology Inc.
DS22186A-page 3
MCP2036
1.1
Coil Driver
1.2
The coil driver produces the excitation current for the
sensor coils.
The coil driver input is derived from the digital clock
supplied to the CLK input. The digital signal is first
filtered through a low-pass filter, composed of R
IN
and
C
IN
, and passed to the DRVIN input. The driver will
create a triangular current in phase and proportional
with the input voltage. Because the digital drive into the
R
IN
-C
IN
filter has a 50% duty cycle, the voltage on the
DRVIN input will be centered at V
DD
/2. The relationship
between voltage, current, inductance and frequency is
shown in
Equation 1-1.
Synchronous Detector and Output
Amplifier
The Synchronous Detector has two inputs, LREF and
LBTN, selectable by REFSEL. This routes either signal
into the frequency mixer of the detector. The frequency
mixer then converts the AC waveform into a pulsed DC
signal which is amplified and filtered.
The gain of the amplifier is user-settable, using an
external resistor, R
GAIN
(see
Equation 1-2).
EQUATION 1-2:
Gain
∼
R
GAIN
⁄
10kOhm
An ADC plus firmware algorithm then digitizes the
detector output voltage and uses the resulting data to
detect a key press event..
Note:
The output amplifier/filter uses a
differential connection, so its output is
centered to V
REF
(V
DD
/2). The amplitude
of the detected signal should be calculated
as the difference between voltages at the
output of the detector and the reference
voltage.
EQUATION 1-1:
ΔV
OUT
=
( ΔI
DRV
•
L
COIL
•
2
•
F
DRV
)
V
OUT
= Pulsed Output Voltage
ΔI
DRV
= AC Drive Current Amplitude
F
DRV
= AC Drive Current Frequency
L
COIL
= Inductance of the Sensor Coil
Note:
These equations assume a 50% duty cycle.
1.3
Virtual Ground Voltage Reference
Circuit
To create both an inverting and non-inverting amplifier
topology, a pseudo split supply design is required. To
generate the dual supplies required, a rail splitter is
included, which generates the virtual ground by creat-
ing a voltage output at V
DD
/2. The output is used by the
external passive network of the Detector/Amplifier sec-
tion as a reference on the non-inverting input. A bypass
capacitor of 0.1uF is required to ensure the stability of
the output. For reference accuracy, no more than 3mA
should be supplied to, or drawn from the reference
output pin.
DS22186A-page 4
©
2009 Microchip Technology Inc.
MCP2036
2.0
PIN DESCRIPTION
Descriptions of the pins are listed in
Table 2-1.
TABLE 2-1:
Pad Name
PIN FUNCTION TABLE
Pin Number
14 Pins
16 Pins
16
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
OUT
IN
IN
PWR
OUT
IN
IN
IN
IN
—
PWR
OUT
IN
IN
—
—
AN
AN
AN
AN
AN
AN
CMOS
CMOS
CMOS
—
AN
AN
AN
AN
—
—
Voltage Reference
Reference Inductor Input
Active Inductor Input
Power Supply
Current Driver Output for Inductors
Current Driver Input
Clock Signal
Detector Select Input
Chip Select, Active low
Must be tied to GND for proper
operation.
Power Supply Return
Detector Output Voltage
Negative Input for Output Detector
Positive Input for Output Detector
No connect
No connect
I/O
Type
Description
V
REF
LREF
LBTN
V
DD
DRVOUT
DRVIN
CLK
REFSEL
CS
Reserved
V
SS
V
DETOUT
V
DET-
V
DET+
NC
NC
1
2
3
4
5
6
7
8
9
10
11
12
13
14
2.1
Chip Select (CS)
2.4
Inductor Inputs (LREF, LBTN)
The circuit is fully enabled when a logic-low is applied
to the CS input. The circuit enters in Low-Power mode
when a logic-high is applied to this input. During
Low-Power mode, the detector output voltage falls to
V
REF
and the supply current is reduced to 0.5
μA
(typ.).
This pin has an internal pull-up resistor to ensure
proper selection of the circuit.
These pins are inputs for the external coils (reference
and sensor). The inputs should be AC coupled to the
coils by a 10nF ceramic capacitor.
2.5
Input Selection (REFSEL)
Digital input that is used to select between coil inputs
(reference and sensor).
2.2
Voltage Reference (
V
REF
)
2.6
Clock (CLK)
V
REF
is a mid-scale reference output. It can source and
sink small currents and has low output impedance. A
load capacitor between 100nF and 1μF needs to be
located close to this pin.
The external clock input is used for synchronous
detection of the AC waveforms on the coils. The clock
signal is also used to generate a triangular waveform
applied to coil driver input.
2.3
Power Supply (V
DD
, V
SS
)
2.7
Inductor Driver Input (DRVIN)
The V
DD
pin is the power supply pin for the analog and
digital circuitry within the MCP2036. This pin requires
an appropriate bypass capacitor of 100nF. The voltage
on this pin should be maintained in the 2.7V-5.5V range
for specified operation.
The V
SS
pin is the ground pin and the current return
path for both analog and digital circuitry of the
MCP2036. If an analog ground plane is available, it is
recommended that this device be tied to the analog
ground plane of the PCB.
The analog input to the coil driver. The triangular
waveform applied to this input should be in phase with
the clock signal for best performance.
2.8
Inductor Driver Output (DRVOUT)
Driver output used to excite the sensor coils. It is a
current-mode output designed to drive small inductive
loads.
©
2009 Microchip Technology Inc.
DS22186A-page 5
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