15 SoilZ v1 Lock-In Impedance Analyzer
15 : SoilZ v1 Lock-In Impedance Analyzer
- Author: TechHU
- Description: 1-bit IQ lock-in sigma-delta impedance analyzer for soil moisture, IHP SG13G2 130nm
- GitHub repository
- Open in 3D viewer
- Clock: 0 Hz
How it works
SoilZ v1 is a 1-bit IQ lock-in sigma-delta impedance analyzer for soil moisture sensing, implemented in IHP SG13G2 130nm BiCMOS.
Signal chain: PTAT current source → cascode mirror → H-bridge excitation → external soil probe → chopper demodulator → CT-ΣΔ integrator (OTA + C_fb + R_in) → Strong-arm comparator → SR latch → TG DAC feedback → digital divider chain (VCO ÷2/÷4/÷8/÷16 with I/Q) → bitstream output.
Key blocks:
- 5-stage ring VCO (~9 MHz) with PTAT bias for temperature compensation
- Programmable current source (3-bit binary weighted, 3.7-24 µA)
- Chopper-based lock-in demodulation at excitation frequency
- First-order CT-ΣΔ modulator with 1 pF MIM feedback capacitor
- Digital frequency divider with quadrature output selection
How to test
- Connect a resistive load (1-27 kOhm) between ua[0] (probe_p) and ua[1] (probe_n).
- The internal VCO generates the excitation clock; the digital divider selects the measurement frequency.
- The bitstream density output on uo[] is proportional to the probe impedance.
- Use an ESP32 or similar to count the bitstream density over a measurement window.
External hardware
- Soil moisture probe or resistive test load connected to ua[0] and ua[1]
- MCU (e.g., ESP32) to read bitstream density from digital outputs
IO
| # | Input | Output | Bidirectional |
|---|---|---|---|
| 0 | |||
| 1 | |||
| 2 | |||
| 3 | |||
| 4 | |||
| 5 | |||
| 6 | |||
| 7 |
Analog pins
ua | PCB Pin | Internal index | Description |
|---|---|---|---|
| 0 | B9 | 15 | Soil probe positive (H-bridge excitation + chopper sensing) |
| 1 | B8 | 14 | Soil probe negative (H-bridge excitation + chopper sensing) |