Analog Devices Inc. 5962-89518012A

Overview

AD7821 from Analog Devices is a high-speed, 8-bit LC2MOS analog-to-digital converter with integrated track-and-hold, 660 ns conversion time (WR-RD mode), ±1 LSB total unadjusted error over full temperature range, and dual-supply (±5 V) or single-supply (+5 V) operation for unipolar (0 to +5 V) or bipolar (±2.5 V) input ranges. It interfaces directly to microprocessor buses using CS, RD, WR, RDY, and INT signals and supports DSP applications requiring dynamic performance up to 100 kHz input bandwidth.

For engineers reviewing the AD7821 datasheet, AD7821 pinout, AD7821 application, or AD7821 equivalent, key selection considerations include its half-flash architecture eliminating external clock dependency, built-in 100 kHz track-and-hold enabling direct sampling of full-scale sine waves without external circuitry, ratiometric reference inputs supporting precision sensor interfacing, and MIL-STD-883 Class B availability via /883B suffix for ruggedized deployments.

Technical Context

The AD7821 employs a half-flash conversion architecture using two cascaded 4-bit flash ADCs - one for MSBs and one for LSBs - with an internal DAC reconstructing the MSB output to generate the residual for LSB conversion. Its sampled-data comparators inherently provide track-and-hold functionality without external components.

Digital interface supports two modes: RD mode (conversion and read initiated by single READ cycle, RDY-driven wait-state extension) and WR-RD mode (separate WRITE to start, READ to fetch, with INT-driven or timing-bypassed data access). MODE pin selects between them, and all logic signals comply with TTL voltage thresholds (VINH ≥ 2.4 V, VINL ≤ 0.8 V).

Key Specifications

Pinout & Package

AD7821 is available in multiple packages including 20-lead DIP (N-20), PLCC (P-20A), SOIC (RW-20), CERDIP (Q-20), and LCCC (E-20A). The 20-pin configuration is consistent across variants, with Pin 1 = VIN, Pin 10 = GND, Pin 19 = VSS, and Pin 20 = VDD.

Key Features

Applications

Equivalent & Alternatives

The following parts are listed as comparable options for similar 8-bit, high-speed ADC applications.

Compared with AD7821, AD7820 lacks bipolar capability and bandwidth for modern sensor interfaces, while MAX118 trades integration and speed for serial simplicity and voltage flexibility - making AD7821 optimal for high-fidelity, microprocessor-connected, wide-temperature analog capture.

Availability

AD7821 is available at Aetrix Electronics and suitable for industrial process monitoring, avionics sensor acquisition, DSP-based voice recognition, and portable test equipment requiring stable component supply across extended temperature grades and military-grade screening options.

Supply support for AD7821 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, serving precision instrumentation, communications, and industrial markets since 1965.

The AD7821 belongs to Analog Devices' LC2MOS high-speed ADC product line, engineered for applications demanding fast, accurate digitization of analog signals in resource-constrained, microprocessor-coupled systems - especially where external clocking or track-and-hold circuitry must be minimized.

FAQ

What is the maximum analog input frequency the AD7821 can accurately digitize without an external track-and-hold?

The AD7821 can accurately digitize analog input signals up to 100 kHz without external track-and-hold circuitry. This is enabled by its inherent sampled-data comparator architecture, which provides a 100 kHz signal bandwidth and supports full-scale 5 V_PP sine wave inputs with ≤±1 LSB error. The specification is verified under conditions of VREF(+) – VREF(–) = 5 V and f_SAMPLE = 500 kHz.

Does the AD7821 require an external clock signal to operate?

No, the AD7821 does not require an external clock signal. Its half-flash conversion architecture generates internal timing for both MSB and LSB stages, eliminating the need for CLK input. Conversion is triggered solely by control signals (WR in WR-RD mode or RD in RD mode), simplifying system timing design and reducing jitter sources in the signal chain.

Can the AD7821 operate with a single +5 V supply for bipolar input signals?

No, the AD7821 cannot digitize bipolar input signals (e.g., ±2.5 V) using only a single +5 V supply. Bipolar operation requires VSS = –5 V to bias the analog input stage correctly. For unipolar operation (0 to +5 V), VSS is connected to GND and only +5 V is needed. The VSS pin (Pin 19) is essential for dual-supply functionality and must not be left floating.

What is the meaning of "total unadjusted error" for the AD7821, and how is it specified?

Total unadjusted error for the AD7821 is a composite specification encompassing offset error, full-scale error, and relative accuracy (integral nonlinearity), measured across the full operating temperature range without calibration. It is guaranteed at ±1 LSB max for all versions (K, B, T), corresponding to ±19.53 mV for both unipolar (0 to +5 V) and bipolar (±2.5 V) input spans - ensuring monotonic behavior and no missing codes.

How does the MODE pin affect the AD7821's digital interface behavior?

The MODE pin configures the AD7821's interface protocol: when low, it enables RD mode (conversion and read initiated by a single READ cycle, with RDY output driving processor WAIT state); when high, it enables WR-RD mode (separate WR to start conversion, RD to read data, with INT signaling completion). MODE is internally pulled low (50 µA), so RD mode is default unless actively driven high.