Analog Devices Inc. AD7828LP

Summary of AD7828LP by Analog Devices 8-Bit, 8-Channel Flash ADC

The Analog Devices AD7828LP is an 8-bit, high-speed, 8-channel analog-to-digital converter (ADC), designed to deliver fast and efficient digitization for multi-signal measurement systems. Offered in the robust 28-lead PLCC package (11.51 × 11.51 mm), the AD7828LP employs a half-flash architecture to balance conversion speed, accuracy, and integration within compact footprints. The device supports a sampling rate of up to 50 kHz per channel and operates from a single 5 V supply, making it a prime candidate for data acquisition, multichannel instrumentation, process control, and embedded system designs where multiplexed analog monitoring is needed.

Fundamental Architecture of the AD7828LP

At the heart of the AD7828LP is a half-flash conversion technique, leveraging two cascaded 4-bit flash ADCs to obtain full 8-bit resolution. This approach significantly reduces conversion time—achieving 2.5 μs per channel—and lowers comparator count, improving power efficiency. The integrated 8-channel multiplexer allows any of the eight analog inputs to be selected and digitized in sequence or on demand, maximizing hardware resource sharing in multichannel applications.

A built-in track-and-hold circuit enables simultaneous sampling across all input channels, ensuring precise conversion even under rapidly changing signal conditions. This feature eliminates the requirement for an external sample-and-hold, simplifying signal chain design and reducing system latency.

Main Specifications and Performance Highlights of the AD7828LP

The AD7828LP distinguishes itself with a blend of speed, accuracy, and ease of integration:

8 analog input channels via onboard multiplexer for cost-effective high-density monitoring.

8-bit resolution with a maximum total unadjusted error of ½ LSB, ensuring reliable measurement fidelity in critical applications.

5 μs conversion time per channel, supporting 50 kHz per-channel sampling rates—ideal for real-time embedded and industrial systems needing concurrent multi-signal capture.

Track-and-hold function ensuring 10 kHz bandwidth per channel (max 157 mV/μs slew rate).

Wide input voltage range (0 V to 5 V) determined by external differential references, providing compatibility with industry-standard sensor and signal levels.

Low power consumption (50 mW typical), with no need for a conversion clock, enhancing suitability for power-constrained designs.

Tight channel-to-channel matching, eliminating the need for user-performed trims or calibrations at the system level.

Broad operating temperature options: Commercial (0°C to 70°C), Industrial (–40°C to +85°C), and Extended (–55°C to +125°C), enabling deployment in a range of environments.

Reference Input Versatility and Signal Conditioning Options

The AD7828LP features fully differential reference inputs, allowing engineers to tailor both reference voltage span and common-mode offset according to application requirements. By varying the voltage at the V_REF(+) and V_REF(–) pins, designers can set a reduced or shifted input range (e.g., using a 2 V reference for improved sensitivity, where 1 LSB = 7.8 mV).

Such flexibility facilitates ratiometric configurations—common in sensor interfacing—and allows the ADC's measurement ground to be offset from system ground for improved noise immunity or application-specific signal referencing. This is crucial in high-accuracy instrumentation, industrial process monitoring, or systems with disparate ground domains.

Analog Input Properties and Sample-and-Hold Operation

A unique aspect of the AD7828LP's design is the nature of its input circuitry. When a channel conversion is initiated, all switches corresponding to the selected input momentarily close, charging a network of sample capacitors (31 pF total for all comparators, plus ~14 pF stray capacitance). The result is a requirement for a low source impedance driver—preferably under 100 Ω—to effectively settle the input during the ~1 μs track interval.

Opamp drivers such as the AD544 or AD644 are recommended for their low output impedance and suitable bandwidth. This structure natively accomplishes the sample-and-hold function critical to flash ADC operation, making the device well-suited to capturing fast, multiplexed analog waveforms without the complexity or cost of an external S/H stage.

Unipolar versus Bipolar Operating Modes of the AD7828LP

While the AD7828LP’s default input mode is unipolar (0–5 V), its flexible reference design allows for straightforward configuration to bipolar operation, typical in precision measurement or audio signal capture. A front-end signal conditioning circuit, as shown in the reference documentation, can transform a ±4 V analog signal into the 0–5 V range expected by the ADC input, yielding a complementary offset binary digital output. The flexibility to digitize both unipolar and bipolar signals enhances the versatility of the AD7828LP in field and lab applications.

Digital Communication, Timing Specifications, and Microprocessor Integration

The AD7828LP is designed for direct microprocessor and microcontroller interfacing, employing standard Chip Select (CS) and Read (RD) signals for conversion control and data access. Data output is provided as an 8-bit parallel bus with three-state drivers, suitable for integration with popular processor buses.

Two modes of digital operation are supported:

Mode 0: Supports processors with WAIT state capability. Data read and conversion are simultaneous; RDY and INT pins can manage handshaking.

Mode 1: For processors without WAIT states. Reads always return data from the previous conversion cycle; additional timing control is required.

Detailed timing characteristics, voltage thresholds, and handshake conditions are documented, enabling robust integration with industry-standard microprocessors such as the Z80, MC68000, and TMS32010. Engineers must observe the minimum delay for conversion and adhere to valid address/data timings to ensure reliable sampling cycles and data integrity.

Physical Packaging and Mechanical Dimensions of the AD7828LP

The AD7828LP is supplied in multiple package variants to support both through-hole and surface-mount assembly:

28-lead PLCC (11.51 × 11.51 mm), ideal for compact PCB footprints and automated placement.

28-lead PDIP, SOIC, and SSOP options for alternative mounting and mechanical constraints.

Ceramic DIP (for extended temperature or high-reliability use cases), including JEDEC-compliant outlines for compatibility in defense or aerospace assemblies.

Comprehensive mechanical drawings and tolerancing specifications are available to support layout, production, and system-level design compliance.

Comparable or Replacement Alternatives to the AD7828LP

System designers evaluating replacements or alternatives to the AD7828LP should consider the following:

Analog Devices AD7824: A 4-channel variant in the same series, identical in architecture and performance features but with half the input capacity. This is suitable where only four channels are required.

Other comparable 8-channel flash or half-flash ADCs with parallel interface and similar resolution from established manufacturers (e.g., Texas Instruments TLC0838, Intersil ICL7109 with appropriate channel multiplexing).

Pin compatibility, timing, input range, and interface style should be cross-checked to ensure drop-in functionality in existing designs.

For applications requiring higher resolution, follow-on models or similar 10-/12-bit ADCs in the same package and pinout may also be considered, prioritizing conversion speed and track-and-hold capability to match system requirements.

Conclusion

: Application Value and Selection Advice for the AD7828LP

The AD7828LP from Analog Devices is a robust and efficient solution for engineers seeking an 8-bit, high-speed, multichannel ADC for embedded and measurement systems. Its fast half-flash architecture, integrated sample-and-hold, and flexible reference configuration remove the traditional obstacles of multi-signal data acquisition, offering high sampling rates, channel density, and interface simplicity in a low-power, widely deployable footprint.

For engineers and procurement professionals selecting ADCs for multiplexed sensor arrays, process control, or real-time measurement applications, the AD7828LP provides a compelling balance of speed, accuracy, integration, and versatility—making it a strong fit wherever space, performance, and reliability are critical system criteria.