LDC1000 Designing of Sensor

Hello Kuramochi-san,

Your diagram looks good. However, I would need more information to assist.

I assume you are planning to use L measurement only. What is the desired oscillating frequency of the sensor? How many PCB layers are you planning to use?  

Best Regards,

Natallia Holubeva

Natallia-san,

Thank you for your quick response.

How should they decide the oscillating frequency?

They tell me not to have the desired oscillating frequency.

(I can't understand Fcount. Should they obtain Fcount(0x23~25) with EVM?)

They plan to use 2 layers of the board.

Thanks and Regards.

Kuramochi

Hello Kuramochi-san,

The oscillating frequency of the sensor is determined by the following formula: f = 1/(2*PI*SQRT(L*C)). The range is 5kHz- 5MHz, described by the D/S. All I would worry about here is to stay away from the existing frequencies in the system (for example, if you have another signal at 1MHz nearby you would not want to operate at 1MHz).

Here is what I would recommend regarding the number of the turns in the coil: the more turns you have, the larger the inductance will be. Since the diameter is only 20mm, you want to use as many turns  as you can. Your limiting factor here is determinted by the price of PCB manufacturning. Thus, I would advise your customer to do as many turn as possible in his pricerange.

Regarding the Fcount, please, refer to the following paragraph on the page 12 of the D/S:

fsensor = (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).

Based on the above, you know your sensor frequency (set by L and C components). You know your external frequency (set by the oscillator). You also know your response time since you set it. Thus, you can either calculate Fcount or read it from the registers.

Best Regards,

Natallia Holubeva

Natallia-san,

Thank you for your advice.

I tell it to my customer.

Is it difficult to show the minimum number of the turns in the coil?

************************

The coil moves at 1m/s.

Is there the point that they should take care?

Best Regards,

Kuramochi

Hello Kuramochi-san,

Let me give you an example for your geometry: with 4milx4mil (spacing between traces and width of trace), 2 layers, and 6 turns per layer you get ~ 5.437 uH of inductance. 5uH inductance is border line sufficient to satisfy LDC1000 constrains (f sensor range and Rp range).  If you can have more turns, your inductance will go up, which gives you more freedom of selecting the capacitor. We generally recommend >5uH inductance (please note that it is possible to add a descrete inductor in series, but the % change will be minimized then).

Best Regards,

Natallia Holubeva

Hello-Nattalia-san,

Thank you for your information.

Is it correct that 4milx4mil = 0.1mmx0.1mm?

I think that this is very small for the coil of sensor.

And, is border line always 5uH regardless of  detecting condition?

Thanks,

Kuramochi

Kuramochi - san,

4x4 mil refers to the width trace and space between traces of the coil. These specs you enter in Altium when you are designing the coils. The PCB coil on LDC1000EVM uses 4x4 mil specs. You can choose otherwise - I just gave you an example.

You need to stay in the range of 2 parameters for LDC1000: Fsensor and Rp. Low inductance wont have enough parrallel resistance; thats why you need at least 5uH regardless of the test conditions.

Best Regards,

Natallia Holubeva

Natallia-san,

Thank you for your information.

I understood them.

************************

My customer needs the resolution of ±0.1mm and linerity ±0.1mm.

Please refer the following diagram.

The oscillating frequency of sensor is about 100kHz.

How should we think about the optimum inductance or number of the turns in the coil?

Best Regards,

Kuramochi

Hello Natallia-san,

I appreciate your advice.

I tell your example to my customer.

Thanks and Regards,

Kuramochi

Hello Kuramochi-san,

You are always welcome. Please, let me know if you have any other questions.

Best Regards,

Natallia Holubeva

Natallia-san,

Wishing you a happy New Year!

I ask you the other case.

I hope to detect angle of a rotator and to get the sin-wave(and cos-wave).

I am thinking the following pattern for this condition.

If you have the better idea of the pattern, please let me know.

And, I also ask you spec of LDC1000.

How much is ENOB of LDC1000?

Thank and Regards,

Kuramochi

Natallia-san,

Thank you for your support.
I understood each advantage.
I will use L measurement.

And, I'm examining to use LCD1312/LDC1612.
These IC has 2 input, but 1 ADC.
I have been reading the datasheet of LCD1312/LDC1612.
Please wait a moment.

Thanks and Regards,
Kuramochi

Hello Natallia-san,

I have questions.

The maximum Channel Sample Rate is 4.08kSPS in the datasheet of LDC1612/1614 .
If the number of using channel is 2, is this value 2.04kSPS?

What is the unit of the response time in LDC1000?

If it is "sec", is the maximum sample Rate 8.68kSPS?

⇒ 5MHz/(192*3)=8680

Thanks and Regards,
Kuramochi