Texas Instruments LMK04821NKDT

Overview

LMK04821NKDT from Texas Instruments is an ultra-low-noise JESD204B-compliant clock jitter cleaner with dual-loop PLL architecture, delivering 88 fs RMS jitter (12 kHz–20 MHz), 3.1 GHz maximum output frequency, and 14 programmable differential clock outputs. It serves as a high-precision timing engine for data converter synchronization in RF sampling systems.

For engineers reviewing the LMK04821NKDT datasheet, LMK04821NKDT pinout, LMK04821NKDT application, or LMK04821NKDT equivalent, key selection criteria include JESD204B SYSREF alignment capability, dual-VCO phase noise floor of –227 dBc/Hz (normalized to 1 Hz), hitless input clock switchover, and support for LVPECL/LVDS/HSDS/LCPECL output formats.

Technical Context

The LMK04821NKDT implements a two-stage PLLatinum™ architecture: PLL1 accepts up to three redundant reference inputs (with automatic LOS detection and holdover), integrates a low-noise crystal oscillator circuit, and provides a buffered VCXO/crystal output; PLL2 uses two integrated VCOs (1930–2075 MHz and 2920–3080 MHz) and supports phase detector rates up to 155 MHz.

It enables precise multi-clock synchronization via dynamically adjustable digital delay (1–32× integer division), 25-ps step analog delay, and independent SYSREF generation with DC/AC coupling options - all configurable via SPI interface while maintaining <50% duty cycle accuracy across output dividers.

Key Specifications

Pinout & Package

Package: 64-pin QFN (9.0 mm × 9.0 mm × 0.8 mm), thermally enhanced with exposed thermal pad (pin 64). RoHS-compliant, lead-free finish.

Key Features

Applications

Equivalent & Alternatives

The following parts are listed as comparable options for similar clock jitter cleaner applications.

Compared with LMK04832RTAR and AD9545BCPZ, the LMK04821NKDT uniquely combines triple-redundant reference handling, 3.1 GHz output, and JESD204B-native SYSREF generation - making it the only option for high-channel-count, high-speed converter systems demanding both jitter performance and fault tolerance.

Availability

LMK04821NKDT is available at Aetrix Electronics and suitable for wireless infrastructure, high-speed data acquisition, and aerospace phased array radar applications requiring stable component supply, long-term lifecycle assurance, and traceable sourcing.

Supply support for LMK04821NKDT includes scheduled delivery planning, volume procurement assistance, BOM continuity management, traceable sourcing, and lifecycle availability coordination for OEM customers, industrial embedded developers, connected-device designers, and electronics production programs.

Manufacturer

Texas Instruments is a global semiconductor company specializing in analog, embedded processing, and high-performance timing solutions, with over 50 years of leadership in clock technology and signal chain innovation.

The LMK0482x product line was designed specifically for JESD204B-compliant high-speed data converter clocking, addressing the need for ultra-low jitter, deterministic SYSREF alignment, and robust reference redundancy in 5G, radar, and test equipment.

FAQ

What is the primary function of the LMK04821NKDT in a JESD204B system?

The LMK04821NKDT serves as a jitter-cleaning clock generator that produces both low-jitter device clocks and precisely aligned SYSREF signals for JESD204B subclass 1 or 2 links. Its dual-loop architecture separates reference conditioning (PLL1) from high-frequency synthesis (PLL2), enabling independent optimization of input clean-up and output generation - a critical capability for maintaining deterministic latency across multi-converter systems. The LMK04821NKDT supports AC- and DC-coupled SYSREF, dynamic digital delay, and 25-ps analog delay steps to achieve sub-100-ps skew calibration.

Does the LMK04821NKDT support holdover operation, and how long does it maintain accuracy?

Yes, the LMK04821NKDT supports holdover mode when all reference inputs are lost. It uses an internal DAC and integrated crystal oscillator circuit to sustain stable output frequency based on prior calibration data. While exact holdover duration depends on crystal aging and temperature drift, the LMK04821NKDT is specified to maintain ±50 ppm accuracy for several hours under typical conditions - sufficient for telecom-grade failover scenarios per GR-1244-CORE. The LMK04821NKDT's holdover performance is enabled by its dedicated PLL1 oscillator block and non-volatile calibration storage.

What output formats does the LMK04821NKDT support, and which are used for device clock vs. SYSREF?

The LMK04821NKDT supports LVPECL, LVDS, HSDS, and LCPECL differential output formats across its 14 clock outputs - seven DCLKout pairs for device clocks and seven SDCLKout pairs for SYSREF. Each output is individually programmable via SPI register settings. LVDS and HSDS are commonly selected for SYSREF due to lower power and better noise immunity at lower frequencies, while LVPECL and LCPECL are preferred for high-frequency device clocks (>1.5 GHz). The LMK04821NKDT's output driver strength and common-mode voltage are configured per-output to match target receiver specifications.

How does the LMK04821NKDT achieve 88 fs RMS jitter, and what measurement bandwidth is used?

The LMK04821NKDT achieves 88 fs RMS jitter by combining ultra-low-noise PLL2 design (–227 dBc/Hz normalized noise floor), dual integrated VCOs with optimized KVCO, and advanced charge-pump/loop-filter architecture. This specification is measured over the 12 kHz to 20 MHz offset bandwidth - the industry-standard integration range for evaluating jitter impact on high-speed data converters. The LMK04821NKDT's jitter performance is validated using commercial phase noise analyzers and confirmed in TI's SNAS605AS datasheet, ensuring repeatability across production lots.

Is the LMK04821NKDT pin-compatible with other members of the LMK0482x family, such as LMK04826 or LMK04828?

No, the LMK04821NKDT is not pin-compatible with LMK04826 or LMK04828. Although all share the same 64-pin QFN package and pinout layout, functional differences exist: the LMK04821NKDT includes a VCO1 divider (÷2 to ÷8) not present in LMK04826/LMK04828, and its VCO frequency ranges differ (LMK04821: 1930–2075 MHz / 2920–3080 MHz; LMK04826: 1840–1970 MHz / 2440–2505 MHz). Register maps and default power-on configurations also vary. Therefore, direct substitution requires firmware and layout validation - the LMK04821NKDT must be treated as a distinct part number in design and procurement.