I. Overview: The TC7129 IC

️· Replacement Part: Designed as a direct replacement for the ICL7129.
️· Function: It's an integrated circuit (IC) used in digital multimeters (DMMs) and similar instruments, primarily for performing analog-to-digital conversions. It handles the integration and processing of input signals.
️· Important Note: Unlike the ICL7129, the TC7129 has internal phase compensation and *does not* require external components (capacitors and resistors) connected between pins 26 and 28.

II. Circuit Components & Selection Guidelines

️· Integrating Resistor (RINT):
- Sets the charging current for the integrating capacitor.
- Value should be between 5µA and 20µA at 2V (full-scale input).
- Typical value: ~150 kΩ
- Lower values are better, but not too low to prevent leakage.
️· Integrating Capacitor (CINT):
- Important to have low dielectric absorption (polypropylene or Teflon recommended).
- Formula for determining value: *see Equations 3-1 & 3-2 in the original document*
️· Reference Capacitor (CREF):
- Primary selection criterion: Low leakage.
- Value: At least 1µF
️· Voltage Reference (DREF, RREF, RBIAS, CRF):
- RREF: > 20 kΩ
- RBIAS: Limits current through DREF to < 150µA
- CRF: Forms an RC filter to reduce noise.
️· Input Filter (RIF, CIF):
- RIF: ≤ 100 kΩ
- CIF: Should have low leakage.
️· Powering Options:
- Typical: 9V battery
- Alternative 1: ±5V power supply (DGND is internally set to ~5V less than V+ and is NOT a power supply input)
- Alternative 2: Low-voltage battery (3.8V - 6V) using a DC-to-DC voltage doubler (TC7660 and external capacitors).

III. Typical Applications & Considerations

️· Direct Replacement: For ICL7129 applications, remove any external components (capacitors and resistors) between pins 26 and 28.
️· External Logic Connection: Pin 36 (DGND) can be used as a reference voltage (~5V) for interfacing with external logic circuits.
️· Oscillator Frequency: The oscillator frequency must be calculated correctly for proper function and noise rejection, based on the desired frequency of operation.

Key Equations (from the document)
️· Charging Current Formula: *see Equations 3-1 & 3-2 in the original document*
️· Calculating CINT: *see Equations 3-1 & 3-2 in the original document*