Datasheet

LDC1000
SNOSCX2 –SEPTEMBER 2013 www.ti.com
For example, if Rp at 8mm is measured to be 18kΩ, 18000×2 = 36000. In Table 6, then 38.785kΩ is the smallest
value larger than 36kΩ; this corresponds to Rp_MAX value of 0x11.
Note that setting Rp_MAX to a value not listed in Table 6 can result in indeterminate behavior.
Rp_MIN
Rp_MIN sets the lower limit of the LDC1000 resonant impedance input range.
• Configure the sensor such that the eddy current losses are maximized. As an example, for a proximity
sensing application, set the distance between the sensor and the metal target to the minimum sensing
distance.
• Measure the resonant impedance Rp using an impedance analyzer.
• Divide the Rp value by 2 and then select the next lower Rp value from Table 8.
For example, if Rp at 1mm is measured to be 5kΩ, 5000/2 = 2500. In Table 8 , 2.394kΩ is the smallest value
lower than 2.5kΩ; this corresponds to Rp_MIN value of 0x3B.
Note that setting Rp_MIN to a value not listed on Table 8 can result in indeterminate behavior. In addition,
Rp_MIN powers on with a default value of 0x14 which must be set to a value from Table 8 prior to powering on
the LDC.
Measuring Inductance
LDC1000 measures the sensor’s frequency of oscillation by a frequency counter. The frequency counter timing is
set by an external clock or crystal. Either the external clock(8MHz typical) from microcontroller can be provided
on the TBCLK pin or a crystal can be connected on the XTALIN and XTALOUT pins. The clock mode is
controlled through Clock Configuration register (address 0x05). The sensor resonance frequency is derived from
the frequency counter registers value (see registers 0x23 through 0x25) as follows:
Sensor frequency, f
sensor
= (1/3)*(Fext/Fcount)*(Response Time)
where Fext is the frequency of the external clock or crystal, Fcount is the value obtained from the Frequency
Counter Data register(address 0x23,0x24,0x25), and Response Time is the programmed response time (see
LDC configuration register, address 0x04).
The Inductance can be calculated as follows:
L=1/[C*(2*π*f
sensor
)
2
], (2)
where C is the parallel capacitance of the resonator.
Example: If Fext=6Mhz, Response time=6144, C=100pF and measured Fcount= 3000 (dec) (address 0x23
through 0x25)
f
sensor
=(1/3)*(6000000/3000)*(6144)= 4.096Mhz
Now using, L=1/[C*(2*π*f
sensor
)
2
]
Inductance, L = 15.098uH
The accuracy of measurement largely depends upon the choice of the external time-base clock (TBCLK) or the
crystal oscillator (XIN/XOUT).
Output Data Rate
Output data rate of LDC1000 depends on the sensor frequency, f
sensor
and 'Response Time' field in LDC
Configuration register(Address:0x04).
Output data rate=( f
sensor
)/(Response Time/3), Sample per second(SPS)
Example: If f
sensor
=5Mhz and Response Time=192
Output data rate= (5M)/(192/3)= 78.125 KSPS
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