1. Overview & Architecture

️· What is it? The SPT7937 is a CMOS Analog-to-Digital Converter (ADC).
️· High-Speed: Achieves its high sample rate by using multiple (18) SAR (Successive Approximation Register) ADC sections operating in parallel.
️· Clocking: Uses an 18-phase clock, with each ADC section sampling the input signal in sequence. The latency from analog input sample to digital output is 14 clock cycles.
️· Calibration: Uses a user-transparent, auto-calibration scheme to maintain accuracy over time and temperature.

2. Power Supplies & Grounding

️· Configuration: Suggests using a single analog supply and a separate digital supply (for the digital output driver and interface circuitry) to prevent potential latch-up.
️· Power-up Sequence: Power supplies *must* be powered up *before* applying the analog input signal.
️· Grounding: AGND (0.0 V) is a common ground, but can be run up to 2.0 V with a second reference.

3. Voltage Reference

️· External Requirement: Needs a single external voltage reference.
️· Range: Reference voltage should be between 3V and 5V.
️· Full-Scale Range: The voltage difference between the ladder sense lines (V RHS – V RLS) should ideally be between 3V and 5V for optimal performance.
️· Simplified Configuration (Optional): Can tie V Ref directly to V RHF and AGND to V RLF, but this introduces voltage drops that need to be considered.

4. Analog Input

️· Range: Input voltage range is from V RLS to V RHS (typically 4.0V).
️· Minimal Drive Requirements: Very low input capacitance (5pF) and high resistance (35kΩ) reduce drive requirements.
️· Protection: Requires a series resistor and diode clamping circuit for input protection.

5. Calibration Details

️· Auto-Calibration: Continually adjusts gain and offset errors.
️· Calibration Time: Requires a minimum of 10,000 clock cycles (approx. 357 µsec at 28 MHz) to initially calibrate.
️· Continuous Operation: Clock must be continuously applied for the device to remain calibrated.

6. Table II: Clock Cycles (Highlights)

️· Clock 1: Reference Zero Sampling
️· Clock 2: Auto-Zero Comparison
️· Clock 3: Auto-Calibrate Comparison
️· Clock 4: Input Sample
️· Clock 35-47: 13-bit SAR Conversion (across 13 steps)
️· Clock 48: Data Transfer

Key Takeaways / Considerations for Use:

️· Power-Up Sequence is Critical: Get the power supply sequence right.
️· Reference Voltage: Choose a suitable reference voltage within the specified range and consider the force/sense circuit implications.
️· Input Protection: Use the recommended input protection circuit.
️· Calibration Time: Account for the initial calibration time at power-up.
️· Continuous Clock: Ensure a continuous clock signal to maintain calibration.