## Overview of the LTC1749 The **LTC1749** is a high-performance, high-speed 12-bit Analog-to-Digital Converter (ADC) manufactured by Analog Devices (formerly Linear Technology). It is specifically designed for digitizing high-frequency, wide-dynamic-range signals. --- ### Key Technical Specifications | Parameter | Specification | | :--- | :--- | | **Resolution** | 12-Bit | | **Sample Rate** | 80 Msps (Mega-samples per second) | | **Signal-to-Noise Ratio (SNR)** | 71.8 dB (at 30MHz input) | | **Spurious Free Dynamic Range (SFDR)** | 87 dB (at 30MHz input) | | **Input Bandwidth** | 500 MHz (Full Power) | | **Supply Voltage** | 5V (Analog), 3V/5V (Digital I/O) | | **Power Dissipation** | 1.45W (Typical) | --- ### Internal Functional Blocks The LTC1749 consists of several critical electronic sections: 1. **Pipelined Architecture:** It utilizes a multi-stage pipelined architecture with error correction logic. This allows the device to process high-speed conversions while maintaining 12-bit accuracy. 2. **Sample-and-Hold (S/H) Circuit:** The wideband differential S/H circuit allows the ADC to capture signals with frequencies up to 500 MHz, making it suitable for undersampling applications. 3. **Voltage Reference:** Includes an on-chip, low-drift bandgap reference. It can be used as a 2.5V reference or bypassed to use an external reference for higher precision. 4. **Digital Outputs:** The output data is provided in a parallel format. It features a separate digital output supply pin ($OV_{DD}$), allowing the output logic levels to interface directly with 3V or 5V CMOS logic (FPGAs or DSPs). --- ### Typical Pinout Configuration | Pin Category | Function | | :--- | :--- | | **$A_{IN}^+ / A_{IN}^-$** | Differential analog inputs. | | **$V_{REF}$** | Reference voltage input/output. | | **CLK** | Differential or single-ended clock input. | | **$D_0 - D_{11}$** | 12-bit parallel digital data output bits. | | **OE** | Output Enable; controls the high-impedance state of digital outputs. | | **$OF$** | Overflow bit; indicates if the input signal exceeds the full-scale range. | --- ### Application Circuit Considerations * **Clock Jitter:** At 80 Msps, the clock source must have extremely low jitter. Even a few picoseconds of jitter can degrade the SNR significantly at high input frequencies. * **Input Driving:** For best performance, the differential inputs should be driven using a transformer or a high-speed differential amplifier (like the LT1993). * **Bypassing:** Requires high-frequency ceramic bypass capacitors (0.1µF and 10µF) placed as close as possible to the supply pins to minimize noise.