Analog Devices Inc. AD1985JST

Overview

AD1985JST from Analog Devices is an AC '97 2.3-compliant stereo audio codec IC designed for PC motherboard and embedded audio subsystems. It integrates six 20-bit Σ-Δ DACs (supporting 5.1 surround), dual 16-bit Σ-Δ ADCs, S/PDIF output, integrated stereo headphone amplifiers, and a programmable parametric equalizer. It delivers >90 dB dynamic range and supports variable sample rates from 7 kHz to 96 kHz.

For engineers reviewing the AD1985JST datasheet, AD1985JST pinout, AD1985JST application, or AD1985JST equivalent, this page provides verified technical context, validated pin functions, confirmed AC '97 interface timing, real-world audio subsystem integration guidance, and direct alternative part comparisons with documented functional differences.

Technical Context

The AD1985JST implements a full AC '97 v2.3 digital interface with SYNC, BIT_CLK, SDATA_IN, and SDATA_OUT signals operating at 12.288 MHz. Its analog core includes independent 16-bit ADCs for stereo input and six 20-bit DACs-four for front/surround/center/LFE, plus two for headphone/mono outputs-with dedicated analog mixing, programmable gain amplifiers (0 dB to +22.5 dB), and A-weighted 90 dB DAC dynamic range.

It features an on-chip PLL for system clock generation from 24.576 MHz, 14.31818 MHz, or 48 MHz inputs; software-configurable jack sensing (JS0–JS3) with auto topology switching; EAPD control; and flexible power-down states enabling <0.12 mA standby current. All DACs support independent sample rate programming (7–48 kHz in 1 Hz steps; 96 kHz double-rate mode).

Key Specifications

Pinout & Package

AD1985JST is housed in a 48-lead LQFP package (7 mm × 7 mm, 0.5 mm pitch) with exposed thermal pad. Pin assignments follow AC '97 v2.3 slot logic and include dual analog/digital ground separation, dedicated voltage reference (VREF, VREFOUT), and four jack sense inputs (JS0–JS3) for automatic peripheral detection.

Key Features

Applications

Equivalent & Alternatives

The following parts are listed as comparable options for similar audio codec applications.

Compared with ALC655 and CX20468, AD1985JST uniquely combines 20-bit DAC resolution, independent 7–96 kHz sample rate control per channel, integrated S/PDIF, and software-configurable VREFOUT-making it optimal for high-fidelity, multi-topology embedded audio where driver flexibility and analog integrity are critical.

Availability

AD1985JST is available at Aetrix Electronics and suitable for desktop motherboard designs, industrial thin clients, laptop audio subsystems, and gaming PC audio enhancement modules requiring stable component supply and long-term manufacturability.

Supply support for AD1985JST 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

Analog Devices is a global leader in high-performance analog, mixed-signal, and digital signal processing semiconductors, headquartered in Norwood, MA.

The AD1985JST belongs to Analog Devices' SoundMAX® AC '97 codec family, engineered specifically for PC and embedded audio systems demanding high SNR, flexible topology management, and seamless AC Link interoperability with Intel and VIA chipsets.

FAQ

What AC '97 revision does AD1985JST comply with, and why does it matter?

AD1985JST complies with AC '97 Revision 2.3, which mandates support for six-channel DACs, enhanced jack sensing, and extended power management states. This ensures full compatibility with Intel ICH4–ICH10 southbridges and guarantees correct slot allocation, reset timing, and EAPD behavior in production BIOS implementations-critical for avoiding audio initialization failures in mass-produced motherboards.

Does AD1985JST support true 5.1 surround output, and how is it implemented?

Yes, AD1985JST supports true 5.1 surround via six independent 20-bit DACs: two for front left/right, two for surround left/right, one for center, and one for LFE. These are mapped to dedicated analog outputs (Pins 35/36, 39/41, 31, 32) with individual volume control registers (0x36, 0x38) and independent sample rate programming-enabling simultaneous playback of discrete 5.1 content without down-mixing.

How does the AD1985JST handle microphone input sharing with headphone jacks?

AD1985JST uses VREFOUT (Pin 28) and jack sense inputs (JS0–JS3) to manage shared jacks. When stereo mic is detected, software sets VREFOUT to 0 V (via Register 0x76 D3–D2 = 11), reducing Center/LFE crosstalk to <−60 dB at 100 Hz. Combined with auto-topology switching, this enables single-jack headset support without external analog switches or layout compromises.

Can AD1985JST operate without an external crystal, and what clock options exist?

Yes, AD1985JST supports three clock modes: crystal oscillator (24.576 MHz on XTL_IN/XTL_OUT), reference clock (14.31818 MHz), or USB clock (48.000 MHz). The internal PLL locks to any of these, eliminating need for crystal in cost-sensitive or space-constrained designs-though crystal mode delivers lowest jitter (<750 ps) for highest THD performance.

What is the purpose of the LOSEL and HPSEL bits in Register 0x76?

LOSEL (Register 0x76 D5) and HPSEL (Register 0x76 D10) dynamically reassign analog output pins: LOSEL swaps Line Out (Pins 35/36) with Surround outputs; HPSEL routes headphone drive to Pins 39/41 instead of surround. This allows single PCB layout to support both desktop (line-out) and laptop (headphone-out) configurations without hardware changes-reducing NRE and inventory complexity.