Digilent Pmods: Analog-to-Digital Converters

Welcome back to the Digilent Blog!

We explored and explained a number of Pmods already, but hold on to your seats because we have more to go. Today, we’re going to check out a few of Digilent’s input Pmods: analog-to-digital converters.

ADCs are used whenever you need to accurately receive voltage signals (such as an audio signal) in order to have the signal be correctly interpreted without any excess noise. Digilent offers three of these devices– the PmodAD1, the PmodAD2, and the PmodAD5.

Digilent's PmodAD1
Digilent’s PmodAD1

The PmodAD1 has two 12-bit ADCs, which both have a two pole Sallen-Key anti-aliasing filter. In plain English, this means that the converters have a second order filter (which is better than a first order filter) to help filter out unwanted frequency noise. When this “noise” of very short, random electrical pulse occurs, the filter is able to prevent that pulse from continuing along the data line to the system board which is attempting to correctly interpret the incoming data. The anti-aliasing feature means that it helps prevent parts of a incoming repeated signal from getting skipped and ignored. In terms of communication, this Pmod uses a variant of SPI. It still has the chip select and serial clock pins, but rather than having MOSI and MISO pins, it has two input/ouput pins. The reason for this so that data is able to be read from the two ADCs simultaneously rather than having to get the data from both of them the Master-In-Slave-Out pin on two separate occasions.

Digilent's PmodAD2
Digilent’s PmodAD2.

The PmodAD2 is able to have up to four 12-bit analog-to-digital converters that use successive approximation ADCs through the use of a capacitative DAC. This style incorporates a feedback loop where the input analog voltage is compared to a guessed approximation voltage as produced by the capacitative DAC. A capacitative DAC is a series of capacitors that are appropriately charged so that their overall voltage is equal to the input voltage. When the comparator sees that the input voltage and the capacitors voltage are equal, the successive approximation register allows this data to be available to the system board, specifically through I2C.

Digilent's PmodAD5
Digilent’s PmodAD5.

The PmodAD5 is the most versatile ADC with up to eight 24-bit differential comparisons. These comparisons can be between pairs of the eight input pins, such as pin 1 and pin 2 or pin 3 and pin 4. All eight inputs can be compared to an external analog voltage input for pseudo-differential signaling. The actual conversion of an analog to digital signal is done through a type of filtering technique called sigma-delta modulation. This conversion is able to use either an internal or external reference voltage . When the internal reference voltage reference is used there are up two four general purpose output pins available for the users use. The PmodAD5 also offers extended functionality by providing two SMA analog inputs, making it an ideal analog-to-digital converter for any situation.

How would you expand your projects with these powerful Pmods?

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About James Colvin

The biggest thing that I enjoy is learning new things. Especially things involving some type of technology; computer components, fun gadgets, games, coding techniques, etc. I love spending time with my wife and our two sons and hanging out with our friends. During my normal work day, I manage the Digilent Forum and the North American Support team.

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