In the unrelenting march towards high channel density, many system designers are searching for data acquisition solutions that use less board area, while still meeting strict performance criteria. ADI is meeting these challenges head-on with its first family of μModule® Data Acquisition Systems, the ADAQ7980 and ADAQ7988. The ADAQ798x family integrates common signal processing and conditioning blocks into a system-in-package (SiP) design that enables high channel density, simplifies the design process, and provides exceptional performance.
The integration of the ADC driver, critical passive components and the SAR ADC into a single package simplifies the design process, reduces component count and enables increased channel density while guaranteeing signal chain performance. The ADC driver configuration is flexible as well, and enables the ADAQ798x to directly interface with sensors and input sources with varying input voltage and frequency ranges. This flexibility makes the ADAQ798x suitable for a variety of industrial, instrumentation, communications, and healthcare applications.
The goal of this blog series will be to help system designers take full advantage of the ADAQ798x family’s flexible front-end, and show how it can be configured to fit their application. We will be examining common and useful ADC driver configurations, how to implement them using external passive components and some of the “gotchas” to look out for in each configuration.
Why Configure the ADC Driver Anyway?
The ADC driver is used to condition the input signal and acts as a low-impedance buffer between the signal source and the SAR ADC’s switched capacitive input. The ADAQ798x takes a “best of both worlds” approach with its ADC driver by providing the benefits of signal chain integration while still providing design flexibility that supports a variety of applications. The integration of the ADC driver into the ADAQ798x reduces board area and eliminates the (sometimes daunting) task of selecting an appropriate amplifier (as explained here). The ADC driver's configuration is still flexible, however, because its inputs and output are routed directly to the pins on the device, allowing for the addition of external passive components to implement gain, filtering, etc. This enables the ADAQ798x to support signal amplitudes and bandwidths present in many precision applications.
We'll be looking at several common ADC driver configuration options for the ADAQ798x in future posts. Before we get into the specifics of those configurations, though, let's establish some of the common design considerations for the ADC driver for many applications. First, we’ll start with input voltage range:
The ADAQ798x’s integrated ADC converts unipolar, single-ended signals from 0 V to VREF to a 16-bit result. VREF is the reference voltage, which is generated externally and can be set from 2.4 V to 5.1 V. The ADC driver must be configured to translate the input source’s output range to fit the integrated ADC’s input range.
The ADAQ7980/ADAQ7988 data sheet specifies performance with the ADC driver in a unity gain configuration, where the voltage input at the IN+ pin is 0 V to VREF. This configuration is the simplest design (it only requires shorting the IN- and AMP_OUT pins together!) and achieves the best noise performance and power consumption, but isn’t always practical, as many sensors and sources don’t adhere to the ADC’s input range. Industrial applications, for example, frequently involve bipolar signals with amplitudes as large as 20 VPP! Luckily, with the addition of a few passive components, we can implement gain, attenuation, bipolar-to-unipolar conversion, and active filtering, potentially eliminating the need for more amplifiers in the signal chain.
As we delve into some configuration options in future posts, we need to keep some key design considerations in the back of our minds. Examples of these include:
- Power Consumption
- System noise
- Large- and small-signal bandwidth
- Input impedance
- Settling characteristics
- Offset error
- Gain error
The requirements for each will differ for each application, but all of them are impacted by the ADC driver configuration, and the components used. For example, using large-valued resistors will typically reduce power consumption and increase input impedance, but can increase system noise, distortion, and offset and gain errors. We’ll examine each of these parameters as they pertain to specific configurations in future blog posts.
Have any questions? Feel free to ask in the comments section!