seek a replacement for AD7671

Hi, emersom .

We'll look into this and get back to you.

Regards,
Jo

You’re looking to upgrade a circuit that originally used the AD7671, a 16-bit, 1 MSPS SAR analog-to-digital converter (ADC), by replacing it with a part that offers a higher Effective Number of Bits (ENOB) while maintaining the same 48-lead LQFP or LFCSP packaging and pin compatibility to avoid modifying the existing FPGA program. The AD7671 achieves an ENOB of approximately 14 bits across the 100 kHz to 500 kHz range, and you’d like a replacement with improved ENOB under similar conditions. Let’s analyze this based on available information.

### Key Requirements
- **Higher ENOB**: Target >14 bits in the 100 kHz to 500 kHz range.
- **Packaging**: Must match 48-lead LQFP or 48-lead LFCSP.
- **Pin Compatibility**: Same pin definitions to preserve FPGA interfacing.
- **Other Parameters**: Similar sampling rate (1 MSPS), input range, supply voltage (5V), and interface (parallel/serial).

### Analysis of AD7671
The AD7671 is a 16-bit SAR ADC with:
- ENOB ~14 bits (SNR ~86 dB) at 100 kHz to 500 kHz input frequencies, limited by noise and distortion.
- 1 MSPS maximum sampling rate.
- Single 5V supply.
- 48-lead LQFP or LFCSP package.
- Parallel and serial interface options.
The ENOB of 14 bits reflects real-world performance, falling short of the ideal 16 bits due to noise, jitter, and distortion, which is typical for SAR ADCs at higher frequencies.

### Potential Replacements
Finding an exact pin-compatible drop-in replacement with higher ENOB is challenging, as ADC technology has evolved, and manufacturers rarely guarantee identical pinouts across generations with significant performance improvements. However, let’s explore options within Analog Devices’ portfolio and critically assess their suitability:

1. **AD7675**:
- **Resolution**: 16 bits.
- **Sampling Rate**: 1 MSPS.
- **ENOB**: Typically 15 bits (SNR ~92 dB) at lower frequencies, slightly above 14 bits up to 500 kHz depending on conditions.
- **Package**: 48-lead LQFP or LFCSP.
- **Pin Compatibility**: Shares the same pinout as AD7671, designed as a successor with improved noise performance.
- **Supply Voltage**: 5V.
- **Considerations**: The AD7675 offers a modest ENOB increase due to enhanced internal architecture and lower noise. However, the improvement may be marginal at 500 kHz, and real-world ENOB could still hover near 14.5 bits. Testing is recommended to confirm performance.

2. **AD7676**:
- **Resolution**: 16 bits.
- **Sampling Rate**: 1 MSPS.
- **ENOB**: Up to 15.5 bits (SNR ~95 dB) at lower frequencies, potentially exceeding 14 bits at 500 kHz with optimal conditions.
- **Package**: 48-lead LQFP or LFCSP.
- **Pin Compatibility**: Pin-compatible with AD7671, offering a direct swap.
- **Supply Voltage**: 5V.
- **Considerations**: The AD7676 includes a differential input option and improved linearity, which could boost ENOB. Its performance edge may diminish at 500 kHz due to increased distortion, so validation with your specific input signal is advised.

3. **AD7656/AD7657**:
- **Resolution**: 16 bits.
- **Sampling Rate**: 1 MSPS (AD7656) or slightly higher variants.
- **ENOB**: Around 15 bits (SNR ~90-92 dB), potentially exceeding 14 bits up to 500 kHz.
- **Package**: 48-lead LQFP.
- **Pin Compatibility**: Similar pinout to AD7671, but minor differences in control signals may require FPGA register tweaks, risking your no-change requirement.
- **Supply Voltage**: 5V.
- **Considerations**: These are multichannel ADCs, which might complicate a single-channel replacement. Pin compatibility is close but not identical, so a detailed pin mapping is needed.

### Critical Examination
- **ENOB Limitations**: The AD7671’s ENOB of 14 bits at 500 kHz suggests noise and jitter dominate beyond its ideal resolution. Successors like AD7675 or AD7676 improve this through better process technology and design, but the gain may be limited unless your application benefits from their enhanced linearity or differential inputs. Higher ENOB (e.g., 16 bits) typically requires sigma-delta ADCs, which differ in architecture and pinout.
- **Pin Compatibility Risks**: While AD7675 and AD7676 are marketed as pin-compatible, subtle changes in timing or control signals (e.g., conversion start or clock) could affect FPGA synchronization, even if pins are physically identical. This needs bench testing.
- **Obsolescence**: The AD7671, being a 10-year-old part, may have been superseded, and newer designs might prioritize lower power or different packages, reducing exact matches.

### Recommendation
The **AD7676** appears to be the best candidate:
- It offers an ENOB potentially exceeding 14 bits (up to 15.5 bits) at lower frequencies, with a slight advantage at 500 kHz.
- It maintains the 48-lead LQFP/LFCSP package and pinout, aligning with your FPGA constraint.
- It operates at 5V and 1 MSPS, matching the AD7671’s core specs.

### Next Steps
- **Verify Performance**: Obtain samples of the AD7676 and test with your 100 kHz to 500 kHz input signals to confirm ENOB exceeds 14 bits under load.
- **Check Datasheet**: Review the AD7676 datasheet for exact pin definitions and timing to ensure FPGA compatibility.
- **Consult Support**: Contact Analog Devices (via their support portal) to confirm the AD7676 as a drop-in replacement and inquire about any hidden differences.
- **Prototype**: Run a small batch in your circuit to validate no FPGA code changes are needed.

If the ENOB improvement is insufficient, consider redesigning the FPGA interface for a higher-resolution ADC (e.g., 18-bit AD7641), but this would violate your pin constraint. For now, the AD7676 is the closest match with minimal risk. Let me know if you need help with testing protocols!