The coil board contains the outer coils and inner coils. Each set of coils includes the excitation coils and receiver coils. Excitation coils have a spiral shape. Receiver coils have a sine or cosine shape.

The resolution of the inductive encoder mainly depends on the periods of coils. To get resolution higher than 16-bit ENOB, this reference design uses 16-periods outer coil and 15-periods inner coil. However, the PCB size also increases with higher periods of coils, the outer diameter of the coil is restricted to 58mm.

For the excitation coil, the inductance needs to be well controlled. The excitation coils perform as an inductor and are connected to the LCIN and LCOUT pins of the LDC5072 device. Two capacitors are also connected to these two pins which form the LC oscillator. The inductance and capacitance determine the excitation signal frequency. There is a balance between signal amplitude and power consumption. Higher signal amplitude means higher electromagnetic field which needs higher turns of excitation coils. To keep the inductance unchanged, the width of the coils needs to be reduced and leads to higher resistance and higher power consumption. This reference design controls the inductance of the excitation coils keeping around 5μH.

Both the excitation coil signal and receiver coil signal are high-frequency signals. To get better noise immunity, the reference design uses a six-layer PCB board and makes differential signal routing overlap in different layers.

Figure 3-2 Coil PCB