Configuring Inductive-to-Digital-Converters for

Application Report
SNAA221A – April 2015 – Revised June 2015
Configuring Inductive-to-Digital-Converters for Parallel
Resistance (RP) Variation in L-C Tank Sensors
Natallia Holubeva .................................................................................................... Sensor Signal Path
ABSTRACT
This application note reviews sensor RP configuration for LDC devices. LDC1000, LDC1041, LDC1051,
LDC1312, LDC1314, LDC1612, LDC1614 are covered in this note. Clear understanding on how to set the
RP_MIN and RP_MAX registers is necessary for not only RP measurements, but also for optimum L
measurements.
The fundamental principle of RP measurements is that magnetic fields from an LC circuit generate eddy
currents on the surface of nearby conductive materials. These currents appear as additional parasitic
resistance in the LC circuit. The energy dissipated as heat due to this resistance will be lost, and LDC
devices can measure this loss. The amount of parasitic resistance generated by eddy currents is a
function of the inductor shape, distance between inductor and conductive target, temperature, and the
target composition.
RP configuration is quite different for LDC10xx devices and LDC13xx/16xx devices.
• While the LDC131x and LDC161x devices do not measure RP, they still need to be configured to
accommodate the change in RP as the target moves. Due to the different architecture between the
LDC10xx family of devices and the LDC131x and LDC161x devices, the process is different.
• LDC10xx devices use RP_MIN and RP_MAX settings to determine minimum and maximum amount of
energy inserted into the resonator. The converters are also able to measure the amount of energy
dissipated at each point of time. Therefore, resistive losses can be determined and proximity values
are used as the output.
LDC13xx/16xx devices use only one register setting, represented as drive current, to configure the
maximum amount of energy inserted into the resonator. The current drive is usually constant during
normal operation.
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Configuring Inductive-to-Digital-Converters for Parallel Resistance (RP)
Variation in L-C Tank Sensors
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What is RP and How it is Derived?
1
2
3
4
5
6
7
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Contents
What is RP and How it is Derived? ........................................................................................
Determining System RP ....................................................................................................
Meeting RP Boundary Conditions of LDC1000, LDC1041, and LDC1051............................................
Setting RP_MAX and RP_MIN for Proper Operation of LDC10xx Devices ...........................................
4.1
Setting RP_MAX and RP_MIN .....................................................................................
4.2
Limiting Cases.......................................................................................................
Meeting RP Boundary Conditions of LDC1312, LDC1314, LDC1612, and LDC1614...............................
Setting the Current Drive LDC1312, LDC1314, LDC1612, and LDC1614 ...........................................
Conclusion ....................................................................................................................
2
4
4
6
6
6
7
7
9
List of Figures
1
Series Electrical Model of an LC Tank .................................................................................... 2
2
Parallel Electrical Model of an LC Tank
3
RP [kΩ] vs Distance [mm] of a 14mm Diameter PCB Coil .............................................................. 4
4
LDC10xx Change of the Oscillation Amplitude set by RP_MIN, RP_MAX ............................................ 5
5
LDC13xx/LDC16xx Sensor Amplitude set by DRIVE_CURRENT_CHx .............................................. 7
..................................................................................
3
List of Tables
1
1
Current Drive as a Function of Parallel Resistance ..................................................................... 8
What is RP and How it is Derived?
TI’s Inductance to Digital Converters (LDCs) use an LC resonator to generate a magnetic field that is used
to sense nearby conductive objects. For many applications, the location of a specific conductive object,
generally referred to as the target, is desired. The sensor inductor can be a PCB coil, an unshielded wirewound SMD inductor, or a spring. Texas Instrument’s application note, LDC Sensor Design (SNOA930),
provides details on coil design and sensor parameters.
An electrical model of an LC resonator is shown in Figure 1. Ideally, an LC tank does not have a resistive
component and can oscillate forever with sustained excitation. For all real inductors, there is parasitic
series resistance based on the conductor profile, sensor operating frequency, and the sensor geometry.
The sensor can therefore be modeled as shown in Figure 1, where the inductor and its parasitic resistance
vary as a function of target proximity.
Figure 1. Series Electrical Model of an LC Tank
Figure 2 shows the Norton equivalent of the series electrical model at resonant frequency. In this model,
RP(d) stands for the equivalent parallel resistance of the sensor, or resonant impedance. RP(d) is a
function of the target position. The resonator can therefore be modeled as a distance dependent inductor,
a fixed capacitor, and a distance dependent resistor in parallel.
2
Configuring Inductive-to-Digital-Converters for Parallel Resistance (RP)
Variation in L-C Tank Sensors
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What is RP and How it is Derived?
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Note that in either model, RS and RP are the AC resistances of the sensor, not the DC resistances. The
AC resistance of an inductor increases with oscillation frequency because of skin effect, which is the
tendency for AC signals to propagate on the surface of conductors.
Figure 2. Parallel Electrical Model of an LC Tank
In the parallel model, the value of RP can be calculated with:
L
Rp =
C ´ Rs
(1)
Parallel Resistance as a Function of Series Resistance, Inductance, and Capacitance is shown in
Equation 1.
The variation of L(d) and RP(d) as a function of distance (d) or conductor composition occurs due to the
interaction of the sensor’s magnetic field with the conductive target object. The magnetic field generates
eddy currents on the surface of the target, which in turn generate their own magnetic field. The magnetic
field generated by eddy currents opposes the original field generated by the sensor. As the sensor and the
target move closer together, the eddy currents in the target increase in intensity and the opposing
magnetic field strength increases. The result is that the RP of the resonator and the observed inductance
of the sensor decrease as the target moves closer to the sensor. This decrease in sensor inductance is
apparent as an increase in the resonant frequency.
1
Fsensor (d) =
2 ´ p L (d)´ C
(2)
Sensor Frequency as a function of Inductance and Capacitance is shown in Equation 2.
With the LDC10xx family of devices, when RP decreases, the LDC injects more energy into the resonator
to maintain the oscillation. The additional energy dissipation is detected by the LDC and is reflected as a
change of the output code. The change in RP is a function of the shape, size, and composition of the
conductive target. By measuring both the RP change and the change in inductance, it is possible to
determine metal composition.
Figure 3 shows the variation in RP as a function of target distance for a 14mm diameter PCB coil (23
turns, 4 mil trace width, 4mil spacing between trace,1oz Cu thickness, FR4). The target is composed of
2mm thick Stainless steel.
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Determining System RP
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Figure 3. RP [kΩ] vs Distance [mm] of a 14mm Diameter PCB Coil
2
Determining System RP
1. For an air coil purchased from a 3rd party manufacturer – obtain the following specifications at
operating frequency: Q, LS, and RS from the datasheet of the coil. Then use Equation 1, along with the
capacitance value, to calculate RP. This value represents the highest expected RP value, with no target.
2. For a PCB coil designed using WEBENCH, the parameters are provided by WEBENCH. Again, the RP
value reported in WEBENCH represents the maximum expected RP.
3. For a PCB coil designed outside of WEBENCH, an impedance analyzer can be used to measure Q,
LS, and RS of the sensor.
3
Meeting RP Boundary Conditions of LDC1000, LDC1041, and LDC1051
LDC10xx devices are able to determine the amount of energy lost to eddy currents by regulating the
oscillation amplitude. The LDC10xx devices have two programmable current drive values: RP_MIN and
RP_MAX. Appropriate values should be set into registers 0x01 (RP_MIN) and 0x02 (RP_MAX).
When the LDC10xx drives RP_MIN into the oscillator, it injects a higher current, and the energy in the
oscillator increases. This additional energy is apparent as higher sensor amplitude. When the sensor
amplitude exceeds the programmed threshold of 0x02 register, the LDC then drives a lower current into
the sensor, and so the amplitude of the oscillator decreases.
Figure 4 shows the sensor oscillation across the LDC10xx sensor pins, INA and INB. The current drive
mode is overlaid onto the trace; notice the increasing amplitude as RP_MIN is driven into the sensor.
4
Configuring Inductive-to-Digital-Converters for Parallel Resistance (RP)
Variation in L-C Tank Sensors
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Meeting RP Boundary Conditions of LDC1000, LDC1041, and LDC1051
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Figure 4. LDC10xx Change of the Oscillation Amplitude set by RP_MIN, RP_MAX
The principle of RP conversion used by LDC10xx devices can be described as following.
1. Voltage amplitude may be set to a constant level of 1Vpp, 2Vpp, or 4Vpp. In general, the 4Vpp is
recommended for the majority of applications, as improved ENOB can be realized. Lower amplitude
settings can be used to reduce the LDC10xx current consumption.
2. Device registers that are used to configure RP_MIN and RP_MAX settings are set to minimum and
maximum amount of energy that LDC will input into the resonator.
3. The LDC10xx will alternate current drive as a function of target position. The LDC converts RP change
that falls within the limits to a digital value, shown as Proximity Data in the EVM GUI. Note that the RP
change must lie within the RP_MIN and RP_MAX limits.
The amount of power is determined by P = VI or P = V2/R. Due to the inverse relationship, RP_MIN sets
the maximum amount of power, while RP_MAX determines the minimum amount. One might also think of
IMAX = V/RMIN and vice versa. Since the amount of current injected is a function of distance and conductor
composition, this parameter can be used to measure distance and metal composition.
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Setting RP_MAX and RP_MIN for Proper Operation of LDC10xx Devices
4
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Setting RP_MAX and RP_MIN for Proper Operation of LDC10xx Devices
Various sensing applications may have different ranges of the resonance impedance RP due to system
design and implementation. The LDC10xx measurement range of RP is controlled by setting 2 registers –
RP_MIN and RP_MAX. For a given application, RP must never be outside the range set by these register
values, otherwise the measured value will be clipped. For optimal sensor resolution, the range of Rp_MIN
to Rp_MAX should be set to the minimum range that will cover the maximum expected range of RP.
Properly setting the RP range is necessary for effective proximity measurement and highest conversion
resolution.
First, the RP_MIN and RP_MAX settings must meet sensor boundary conditions (i.e. the range of RP
variation that the sensor experiences due to target movement). A detailed procedure on how to determine
RP_MIN and RP_MAX is described below; refer to the appropriate LDC10xx datasheet for additional
information.
4.1
Setting RP_MAX and RP_MIN
Use the following procedure:
1. Set RP_MIN to 0x3F, RP_MAX to 0x00.
2. Expose the coil to the maximum metal coverage for the application (closest target position, thickest
part etc.)
3. Start reducing RP_MIN setting 1 code at a time, and take RP Measurements (they will go up with each
change).To speed up tuning, it is also possible to change the value by more than 1 code at a time, for
example by using binary search.
4. When RP gets in the range of 20,000–30,000 codes, it is the optimal RP_MIN setting.
5. Move the target to a position where it's exposed the least (farthest position, thinnest part etc.)
6. Start increasing RP_MAX setting 1 code at a time, and take RP Measurements (they will go down with
each change).
7. When RP gets in the range of 2,000–3,000 codes, or the difference between RP_MIN and RP_MAX
values reaches 25x to 26x, that is the optimal RP_MAX setting. If too much noise, then back off 1 to 2
RP_MAX codes.
4.2
Limiting Cases
There are two limiting cases for sensor range: low RP and saturated RP.
1. For the case of low value of RP, system accuracy will be affected. The datasheet of the LDC10xx
indicates that the minimum acceptable RP is 798Ω. In the case when RP is of a lower value, a series
inductor should be added to the sensor network. The quality factor of the coil is a function of L, C, and
RS. If RP is low, the quality factor is also low. System accuracy is affected by it because low Q coils are
less immune to noise interference.
Q = Rp ´
C
1
L
=
´
L RS
C (3)
Quality Factor as a Function of Series Resistance, Inductance, and Capacitance as shown in
Equation 3.
2. When the sensor RP exceeds the programmed RP_MAX value, the sensor amplitude will exceed the
maximum acceptable range and the output will saturate. The LDC10xx will enter an invalid operating
state. To avoid this scenario, the first option is to adjust Rp_MAX to a larger value. If Rp_MAX cannot
be increased further, the sensor may need to be redesigned with a lower RP. This can be done by
either lowering L or increasing C.
6
Configuring Inductive-to-Digital-Converters for Parallel Resistance (RP)
Variation in L-C Tank Sensors
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Meeting RP Boundary Conditions of LDC1312, LDC1314, LDC1612, and LDC1614
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5
Meeting RP Boundary Conditions of LDC1312, LDC1314, LDC1612, and LDC1614
Although the LDC131x/161x family of devices does not measure RP, the optimal configuration of these
devices still requires attention to the effect of RP on sensor performance. The current drive that these
devices provide can accommodate RP values in the range from 1kΩ to 100kΩ. The LDC131x/LDC161x
devices do not vary the current drive, but instead are programmed to use a constant current drive. The
current drive for each channel can be programmed to fall within a certain range of values. The channels
are individually configured using their respective DRIVE_CURRENT_CHx register. Because the current
drive is constant, it must be set so that as the RP of the sensor varies over the operating range, the sensor
oscillation amplitude remains with a useable range. The maximum allowable sensor amplitude is 1.8V,
while the minimum is determined by the requirements of the application, Note that as the sensor amplitude
decreases to a few hundred millivolts in amplitude, the output SNR will degrade. It is possible that as the
target-to-sensor distance approaches zero, the oscillations will completely stop. Figure 5 shows the sensor
oscillation across the LDC13xx/16xx sensor INA pin. For any specific target position the sensor amplitude
is constant.
Figure 5. LDC13xx/LDC16xx Sensor Amplitude set by DRIVE_CURRENT_CHx
6
Setting the Current Drive LDC1312, LDC1314, LDC1612, and LDC1614
Current drive for LDC13xx/LDC16xx family of devices should be set according to the sensor RP. If the
value of RP is known, the corresponding current drive can be found in Table 1. If the known RP falls
between two table values, approximate the sensor RP to the lower value. The values in the 2nd column
represent the current drive value that will give oscillation amplitude of approximately 1.65V for the given
RP. The hexadecimal equivalent should be written to the DRIVE_CURRENT_CHx register, in the
CHx_IDRIVE field.
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Setting the Current Drive LDC1312, LDC1314, LDC1612, and LDC1614
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Table 1. Current Drive as a Function of Parallel
Resistance
Rp, kΩ
Corresponding Current Drive, decimal
89.99
0
77.59
1
66.87
2
57.63
3
49.67
4
42.83
5
36.91
6
31.81
7
27.42
8
23.64
9
20.37
10
17.56
11
15.14
12
13.05
13
11.25
14
9.69
15
8.36
16
7.2
17
6.21
18
5.35
19
4.61
20
3.98
21
3.43
22
2.95
23
2.55
24
2.19
25
1.89
26
1.63
27
1.4
28
1.21
29
1.05
30
0.9
31
If the RP is not known, the following steps for auto-calibration can be used to configure the needed drive
current, either during system prototyping, or during normal startup, if feasible:
1. Set target at the maximum planned operating distance from the sensor
2. Place the device into SLEEP mode by setting CONFIG.SLEEP_MODE_EN to b1.
3. Program the desired values of SETTLECOUNT and RCOUNT values for the channel.
4. Enable auto-calibration by setting RP_OVERRIDE_EN to b0
5. Take the device out of SLEEP mode by setting CONFIG.SLEEP_MODE_EN to b0.
6. Allow the device to perform at least one measurement, with the target stable (fixed) at the maximum
operating range.
7. Read the channel current drive value from the appropriate DRIVE_CURRENT_CHx register
(addresses 0x1e, 0x1f, 0x20, or 0x21), in the CHx_INIT_DRIVE field (bits 10:6). Save this value.
8. During startup for normal operating mode, write the value saved from the CHx_INIT_DRIVE bit field
into the CHx_IDRIVE bit field (bits 15:11).
9. During normal operating mode, the RP_OVERRIDE_EN must set to b1 to force the fixed current drive.
8
Configuring Inductive-to-Digital-Converters for Parallel Resistance (RP)
Variation in L-C Tank Sensors
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Conclusion
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If the current drive results in the oscillation amplitude greater than 1.8V, the internal ESD clamping circuit
will become active. This may cause the sensor frequency to shift so that the output values no longer
represent a valid system state. If the current drive is set at a lower value, the SNR performance of the
system will decrease, and at near zero target range, oscillations may completely stop, and the output
sample values will be all zeros.
7
Conclusion
TI’s LDC devices require configuration based on the sensor's equivalent parallel resistance, RP.
For LDC1000, LDC1041, and LDC1051 devices the change of energy can be detected and transformed
into proximity values. This can be used for various sensing applications, for example distance detection.
RP data is necessary for applications sensing the composition of the metal targets.
The inductance-only LDC1312, LDC1314, LDC1612, and LDC1614 use the maximum RP value to set the
sensor drive current, which determines amplitude level. RP values set the current limits and therefore the
energy the IC inserts into the resonator.
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Revision History
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Revision History
Changes from Revision (April 2015) to A Revision ........................................................................................................ Page
•
•
Changed from b0 to b1 ................................................................................................................... 8
Changed from b1 to b0 ................................................................................................................... 8
NOTE: Page numbers for previous revisions may differ from page numbers in the current version.
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Revision History
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