Low Noise, Low Power, High Speed: An 18-bit, 2 MSPS Precision SAR ADC from Analog Devices

The AD4002 from Analog Devices is a low-noise, low-power, and high-speed 18-bit successive approximation register analog-to-digital converter.

Analog Devices recently released their new AD4002, which is an 18-bit pseudo differential successive approximation register (SAR) analog-to-digital converter (ADC) that is capable of taking up to 2 million samples per second (MSPS). The MSPS sampling rate is also referred to as the throughput rate (see the figure below).

The AD4002 has a maximum sampling rate, or throughput rate, of 2 MSPS. Table taken from the datasheet (PDF).

According to the datasheet (PDF), which also serves as the datasheet for the AD4006 and AD4010 devices—which offer 1 MSPS and 500 kSPS throughputs, respectively—this ADC is suitable for applications ranging from automatic test equipment to precision data acquisition systems, to battery-operated applications.

A simplified block diagram of the AD4002 SAR ADC. Image taken from the datasheet (PDF).

This IC is designed for digital communications by use of an SPI-/QSPI-/Microwire-/DSP-compatible serial interface bus and can be used with either a 3- or 4-wire interface scheme. For more information on these digital communication techniques, review the section entitled Digital Interface located on page 24 of the datasheet.

Also, if you have a need for using multiple AD4002 devices, you may want to consider connecting them in a daisy-chain fashion (see image below), which is designed to reduce component count and wiring connections, thus making your design efforts a little easier. For more information regarding the daisy-chain connection method, check out the Daisy-Chain Mode section on page 34 of the datasheet.

Connection diagram, from the datasheet (PDF), for illustrating the daisy-chain mode.

What Does "Pseudo Differential" Mean Anyway?

The term pseudo differential is in reference to the ADC's inputs, so pseudo-differential input is the more technically correct term. In this application note from Maxim Integrated, three different types of inputs are explained: single-ended, fully-differential, and pseudo-differential.

 

Single-ended ADCs

With single-ended input ADCs, as the name implies, the input signals are "single-ended," meaning that they are referenced to ground. Maxim's previously mentioned app note tells us that "single-ended inputs are generally sufficient for most applications." Of course, you'll want to have a clear and complete understanding of your design's requirements before assuming that a single-ended ADC is sufficient for your design.

 

Fully-differential ADCs

Fully-differential input ADCs measure the voltage "difference" between the positive (IN+) and negative (IN-) input terminals of the sensor. These types of analog-to-digital converters, again according to Maxim's app note, have "superior DC and AC common-mode rejection" and have an increased dynamic range. If you're not familiar with, or simply need a refresher on, some of these technical terms, then be sure to review the Terminology section on page 15 of the AD4002 datasheet. And speaking of technical terms, I have found that this video, from Texas Instruments, is a great resource for explaining various key terms associated with ADCs.

 

Pseudo-differential ADCs

Pseudo-differential input ADCs are like fully differential inputs in the sense (no pun intended) that they separate signal ground from the analog-to-digital conversion ground, thus allowing for the rejection of DC common-mode voltages. However, these pseudo differential ADCs have little effect on dynamic common-mode noise. A typical application for such pseudo-differential input ADCs includes "measuring sensors that are biased to an arbitrary DC level," as explained in the Maxim app note.

While the focus of this article is on the AD4002 device, its datasheet lists other AD40xx family member part numbers, which include some true differential (also known as fully-differential input) analog-to-digital converter IC's. Also note, per the figure below, that many of these ADCs are pin-to-pin compatible.

The datasheet (PDF) includes part numbers for both true-differential and pseudo-differential ADCs, some of which are pin-to-pin compatible.

Need Help with Layout?

If you, or your layout team, is in need of some PCB layout tips and hints, then take a look at the section entitled Layout Guidelines on page 35 of the datasheet. Besides offering an example layout of the AD4002 IC (see the figure below), Analog Devices also discusses the importance of separating the analog and digital areas on the PCB as well as using decoupling capacitors near the power supply pins.

The datasheet (PDF) offers layout guidelines, including these layout examples.

Interested? Try out the Evaluation Kit

To test drive this low-noise, low-power, and high-speed 18-bit SAR ADC prior to committing it to a new design, consider using its evaluation kit. Take note that this eval kit is designed to accommodate the AD40xx family of ICs, which range from 16-bit to 20-bit ADCs, making it a good method for determining if the lower resolution 16-bit ACD is acceptable for your design, or if the more powerful 20-bit device is necessary.

The AD4000-AD4003, and AD4020 family evaluation board. Picture courtesy of Analog Devices.

Have you had a chance to use this new 18-bit pseudo differential SAR ADC, the AD4002, from Analog Devices in any of your designs? If so, leave a comment and tell us about your experiences.