Hi, I am wondering what drives the sensor design curve shown in "Figure 10. Sensor Design Space for VDD = 3.3 V" on page 8. For example, I have used this device down below 300 khz (about 200 kHz) and still saw "good" clock pulses as-shown in Figure 6 of the "LDC0851 - Troubleshooting" document and still was able to detect my target. My inductances so far have been between 200 uH and 2.5 mH, with Rs below 26 ohm. Is there some calculation that can be formed to understand the relationship between sensor L and frequency, and how low in frequency I can go?
Basically, with frequencies around 300 kHz and inductance's up to 2.5 mH, I cannot seem to sense a ferromagnetic target greater than 1 mm from the sensing face. I have a shielded configuration where the sense coil is sitting up against a 0.006" stainless steel face (and full-enclosed in stainless steel) and the reference is sitting spaced behind it. In the steel enclosure I have matched inductaces of the sense and the reference less than 0.5%.
The diameter of my coil is maxed out at about 0.5" with din/dout at 0.68. I understand that in air I would theoretically be able to sense about 40% of this, or about 5 mm. Since I am sensing through metal I wold expect to loose about 50% of my sense distance, but this still puts me 2X what I am getting now. For now, I feel like I have done everything to optimize sensing with this chip, but I am wondering if there is anything that can be done to improve (coil design, better coil matching, etc).
Also, when I had both coils in the stainless steel enclosure, I was measuring inductance with an LCR meter at 10 kHz. As I brought my target closer, I saw the sense coil change at a much farther distance that what I am seeing using LDC0851. Is this evidence that I need to lower my frequency further?
