4-Wire

Serial

Interface

INA

INB

SCLK

SDI

SDO

CS

INTB

VDD

CFA

CFB

Power

GND

VIO

DGND

CLDO

L

C

Frequency Counter

TBCLK

Threshold

Detector

Frequency

Counter Data

Register

Proximity Data

Register

LDC

LDC1041

SNOSCY1 – MARCH 2014

www.ti.com

7 Detailed Description

7.1 Overview

The LDC1041 is an Inductance-to-Digital Converter that simultaneously measures the impedance and resonant

frequency of an LC resonator. It accomplishes this task by regulating the oscillation amplitude in a closed loop

configuration to a constant level, while monitoring the energy dissipated by the resonator. By monitoring the

amount of power injected into the resonator, the LDC1041 can determine the value of Rp; it returns this as a

digital value which is inversely proportional to Rp. In addition, the LDC1041 also measure the oscillation

frequency of the LC circuit; this frequency is used to determine the inductance of the LC circuit. The device

outputs a digital value that is inversely proportional to frequency.

The threshold detector block provides a comparator with hysteresis, with the threshold registers programed and

comparator enabled, proximity data register is compared with threshold registers and INTB pin indicates the

output.

The device has a simple 4-wire SPI interface. The INTB pin provides multiple functions which are programmable

with SPI.

The device has separate supplies for Analog and I/O, with analog operating at 5V and I/O at 1.8-5V. The

integrated LDO needs a 56nF capacitor connected from CLDO pin to GND.

7.2 Functional Block Diagram

7.3 Feature Description

7.3.1 Inductive Sensing

An AC current flowing through a coil will generate an AC magnetic field. If a conductive material, such as a metal

target, is brought into the vicinity of the coil, this magnetic field will induce circulating currents (eddy currents) on

the surface of the target. These eddy currents are a function of the distance, size, and composition of the target.

The eddy currents then generate their own magnetic field, which opposes the original field generated by the coil.

This mechanism is best compared to a transformer, where the coil is the primary core and the eddy current is the

secondary core. The inductive coupling between both cores depends on distance and shape. Hence the

resistance and inductance of the secondary core (eddy current), shows up as a distant dependent resistive and

inductive component on the primary side (coil). Figure 5 to Figure 8 show a simplified circuit model.

8

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