The AD7643BCPZ is an 18-bit, 1.25MSPS PulSAR analog-to-digital converter designed for high-resolution data acquisition. It combines a fully differential input architecture, SAR conversion, an internal reference and both serial and parallel digital interfaces in a 48-lead LFCSP package.
The device is aimed at applications where measurement resolution and dynamic performance are more important than simply maximizing conversion speed. Its 18-bit resolution and 1.25MSPS throughput make it suitable for precision instrumentation, high-speed data acquisition, spectrum analysis and automated test equipment.
The AD7643BCPZ is the LFCSP version of the AD7643, with a 7mm × 7mm package and exposed pad. This package detail is important when evaluating the device for PCB assembly or replacement.
The AD7643 provides 18-bit resolution and a maximum throughput of 1.25MSPS.
It uses a charge-redistribution SAR architecture and performs conversion without pipeline delay.
The ADC operates from a single 2.5V supply, while its digital interface can accommodate 2.5V, 3.3V or 5V logic.
The device includes an internal 2.048V reference, although an external reference can also be used.
Its analog input is fully differential, with an input range of ±VREF, where the reference voltage can be up to 2.5V.
These characteristics make the AD7643 different from simpler single-ended ADCs that are primarily intended for general-purpose embedded measurements.
The 18-bit conversion architecture provides 262,144 possible digital codes across the selected input range.
This level of resolution is useful when the application needs to distinguish relatively small changes in the analog signal.
However, the effective performance of an 18-bit ADC depends on more than its nominal resolution.
The AD7643 specifies parameters including integral nonlinearity, signal-to-noise ratio, dynamic range and total harmonic distortion. These specifications provide a better indication of its practical measurement capability than the 18-bit number alone.
The device specifies no missing codes across its 18-bit range, making it suitable for precision applications where fine voltage changes need to be represented consistently.
The fully differential architecture is one of the most important features of the AD7643.
Instead of measuring an input voltage relative to a single ground reference, the converter processes the voltage difference between its positive and negative input terminals.
This approach can be advantageous when the signal source is differential or when common-mode interference needs to be managed within the analog signal chain.
A fully differential ADC also requires more careful attention to the driver circuit than a simple single-ended input.
The external amplifier or signal source must provide appropriate common-mode voltage, settling behavior and differential drive capability for the ADC input.
The analog input range is defined as ±VREF.
With a 2.048V reference, the nominal differential input range is therefore ±2.048V.
The reference can be configured differently, with the specified maximum reference voltage extending to 2.5V.
This gives designers flexibility when matching the ADC to an existing precision analog signal chain.
The input range should be selected together with the amplifier gain. If the preceding amplifier produces a signal significantly smaller than the available ADC range, part of the converter's code range will not be effectively utilized.
Conversely, excessive gain can drive the ADC input beyond its permitted range.
The AD7643 includes an internal 2.048V reference and reference buffer.
The internal reference simplifies the basic implementation because an additional precision reference IC is not necessary for normal operation.
The reference also includes a temperature sensor output, providing additional information that can be useful in precision measurement systems where temperature-related behavior needs to be monitored.
For applications requiring a different reference architecture, the ADC can also be operated using an external reference.
Reference selection is particularly important in high-resolution ADC designs because reference noise and drift directly affect the achievable measurement performance.
The AD7643 is designed for applications that require more than high DC resolution.
At a 20kHz input with a 2.5V reference, the device specifies a typical 93.5dB SINAD and approximately 95dB dynamic range under the applicable conditions.
Its specified total harmonic distortion is approximately −113dB typical under the same representative conditions.
These specifications make the AD7643 relevant to applications involving dynamic signals rather than only slowly changing sensor voltages.
For systems such as spectrum analysis and high-speed instrumentation, dynamic performance can be as important as static linearity.
The AD7643 uses a SAR architecture and does not introduce pipeline delay.
This gives the device a predictable relationship between a conversion event and the corresponding digital result.
For control and measurement systems, this can simplify timing analysis because the designer does not need to account for multiple pipeline stages between sampling and data availability.
It is particularly useful in asynchronous-rate applications where the conversion process needs to remain closely synchronized with external sampling events.
The AD7643 supports both parallel and serial data interfaces.
The parallel interface can be configured for 18-bit, 16-bit or 8-bit buses, while the serial interface provides an alternative for systems where processor pin count or PCB routing is more constrained.
The interface supports 2.5V, 3.3V and 5V logic levels.
This flexibility allows the same ADC architecture to be integrated into different digital platforms, including microcontrollers, DSPs and FPGAs.
For high-throughput systems, the parallel interface can provide a straightforward path for transferring conversion data. For compact designs, the serial interface can significantly reduce the number of digital connections.
The AD7643 operates from a single 2.5V power supply.
This is an important distinction between the ADC's analog supply and its digital interface capability.
The device can communicate with digital logic operating at different voltage levels while maintaining its core analog supply at 2.5V.
Typical power dissipation is approximately 65mW at 1.25MSPS when using the internal reference.
In power-down mode, typical power consumption can fall to approximately 2µW, making the device more suitable for systems where the ADC is not required to operate continuously.
A high-resolution ADC is only as effective as the analog circuit driving its input.
The AD7643's fully differential switched-capacitor input requires the source to settle within the available acquisition period.
For this reason, the preceding amplifier should be selected according to its output-drive capability, noise, bandwidth and settling performance.
A low-noise precision amplifier may provide excellent DC accuracy but still be unsuitable if it cannot settle quickly enough for the selected sampling rate.
The ADC driver and AD7643 should therefore be treated as a single analog subsystem.
The AD7643 is a high-resolution converter, so PCB layout can have a significant effect on actual performance.
The analog input traces should be protected from noisy digital signals and high-current switching paths.
Power supply decoupling should be positioned close to the relevant supply pins, and the reference circuitry should be given appropriate attention.
The LFCSP package includes an exposed pad, which also needs to be connected according to the manufacturer's recommended PCB implementation.
For an 18-bit converter, layout errors that might be insignificant in a lower-resolution design can become measurable sources of noise or distortion.
The AD7643BCPZ uses a 48-lead LFCSP package with a 7mm × 7mm body.
The package includes an exposed pad and is designated by Analog Devices as package option CP-48-4.
The AD7643 is also available in a 48-lead LQFP configuration.
These package variants should not be considered interchangeable at the PCB footprint level.
For a new design, the LFCSP package can provide a compact solution for a relatively high-performance ADC. For an existing board, however, the exact package must be matched before selecting an alternative AD7643 variant.
The AD7643 is designed for applications where high-resolution analog conversion and relatively high throughput are both required.
Relevant applications include:
High-speed data acquisition
Medical instrumentation
Precision instrumentation
Spectrum analysis
Digital signal processing
Communications equipment
Automated test equipment
The 18-bit architecture makes the device particularly useful when the application needs substantially more resolution than conventional 12-bit or 16-bit data acquisition systems.
In a data-acquisition system, the AD7643 can provide the conversion stage between a precision analog front end and a digital processing system.
The 1.25MSPS throughput provides sufficient sampling capacity for many measurement signals while retaining 18-bit resolution.
The fully differential input can also be useful when the signal-conditioning stage already produces a differential output.
For a complete acquisition system, the amplifier, reference, ADC and digital interface should be designed together.
The ADC's nominal 18-bit resolution should not be considered independently from the noise and distortion introduced by the preceding analog circuitry.
The main reason to select the AD7643 over a lower-resolution converter is not simply the number of digital bits.
The additional resolution becomes valuable when the analog signal chain has sufficiently low noise and distortion to take advantage of it.
If the sensor or amplifier produces excessive noise, moving from 16-bit to 18-bit conversion may provide little practical improvement.
The AD7643 is therefore best suited to applications where the complete analog front end can preserve the additional resolution.
Its dynamic specifications and fully differential architecture further extend its usefulness beyond basic low-speed sensor measurement.
A replacement for AD7643BCPZ should match the complete ADC architecture rather than only the 18-bit resolution.
Important factors include sampling rate, differential input configuration, reference arrangement, input range, interface type, power supply and package.
A converter with the same 18-bit resolution but a single-ended input or a different reference architecture may require substantial changes to the analog front end.
The 48-lead LFCSP package is also essential for direct PCB replacement.
The AD7643 family includes other package variants, so the complete Part Number should be checked when sourcing or replacing the device.
The AD7643BCPZ is particularly suitable when a design requires 18-bit precision, 1.25MSPS throughput, fully differential inputs and flexible digital interfacing.
Its internal 2.048V reference simplifies the reference circuit, while the option for an external reference allows greater flexibility in precision analog designs.
The main engineering considerations are the input signal level, differential driver, required bandwidth, reference configuration, digital interface and PCB package.
For applications that can take advantage of its resolution and dynamic performance, the AD7643 provides a substantially more capable data-conversion stage than general-purpose low-resolution ADCs.
The AD7643BCPZ combines an 18-bit SAR converter, 1.25MSPS throughput, fully differential input architecture, internal reference and flexible serial or parallel interfaces in a compact 48-lead package.
Its value is particularly clear in precision data-acquisition systems where the analog signal contains useful information below the resolution normally available from 12-bit or 16-bit converters.
For new designs, the most important consideration is whether the complete analog front end can preserve the performance that the 18-bit ADC provides.
For replacement projects, the complete AD7643BCPZ Part Number, LFCSP package, input architecture and interface configuration should all be verified before selecting an alternative.
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