Selecting a High Resolution or High Frequency ADC

Your next digital system will likely have to interface with the analog world, either through sensors or wirelessly. If you’re a systems designer and you don’t plan on using an SoC or MCU with an integrated ADC, you can see comparable performance with more expensive systems when you select the right high resolution or high frequency ADC for your system. Although there is generally a tradeoff between resolution and sampling rate, there are plenty of options on the market that will meet your needs.

Sampling Rate vs. Resolution

If you look at the market for ADCs, you’ll find a tradeoff between frequency and resolution. Note that resolution refers to the number of bits used to encode the voltage level of an analog signal. A higher bit depth means that you get a better representation of the behavior of an analog signal over time. If you know that you will be working with, say, a sinusoidal signal, you can generally get away with a lower resolution and you can correct any missing information using some digital signal processing techniques. For high precision measurements at low frequency, such as optical power measurements, you’ll want to opt for the highest resolution ADC you can find while worrying less about the sampling rate.

Contrast this with sampling rate, which is the number of digital signals gathered with the ADC per unit time. When selecting an ADC that can be used to convert high frequency signals into a digital number, you’ll need to use an ADC with a higher sampling rate, thanks to the Nyquist theorem. The sampling rate for your ADC should be at least double the frequency you want to measure with your ADC. If you’re working in a specific frequency band, then you should choose your ADC based on the frequency at the high end of your desired band.

RF transceiver modules and SoCs generally contain integrated ADCs for gathering analog signals at the receive side of a wireless system. Other applications, such as microcontrollers for sensor nodes, also need to gather analog measurements from other devices and process some digital data. No matter which is the case, any device designed to interface with the analog world will need at least one ADC, whether it is integrated in an SoC or as its own IC.

Choosing a High Frequency ADC

In addition to sampling rate and resolution, designers should consider some of the following aspects when choosing any ADC:

• Bandwidth. Just like other components, the bandwidth determines the range of frequencies with which the ADC can be used. If you’re working with wireless applications or chirped radar systems, your ADC should include span somewhat beyond the frequency band you intend to use in your system. This becomes particularly important when sampling complex signals that can be decomposed with multiple frequency components in order to avoid aliasing.
• Number of channels. Some ADCs include multiple channels and an internal multiplexer for converting multiple signals in a single IC, allowing you to build a custom system without defaulting to an SoC or microcontroller.
• Power consumption and temperature stability. This effectively determines the useful range for your application.
• RMS noise. This will determine the level of error in your output digital signal. This value is normally on the order of nV for high resolution ADCs. Ideally, this value should be lower than the noise floor in your system.

Texas Instruments, ADS1262IPWR

The ADS1262IPWR ADC is an 11 channel device with low RMS noise of 7 nV and up to 130 db of 50/60 Hz noise rejection. With 32-bit resolution, this ADC provides accurate measurement of multiple analog signals with a single unit. This ADC has variable sampling rate from 2.5 Sps to 38.4 kSps in a TSSOP-28 package. The power consumption is low, even at high sampling rate. This ADC is a good choice for gathering precision measurements from analog instruments. The circuit below shows an example temperature-compensated bridge measurement circuit.

Example temperature-compensated bridge measurement with the ADS1262IPWR, from the datasheet.

Texas Instruments, ADC12J4000NKET

The ADC12J4000NKET 12-bit ADC provides a high sampling rate up to 4 GSps. This is a better choice for custom systems that require reception and conversion of wireless signals or other RF signals. This ADC operates at low voltage (1.2 to 1.9 V) and consumes 2 W of power at 4 GSps. This particular ADC only operates with 1 channel, making it less useful for sensor node applications. Some example applications include RF-sampling equipment, military communications, low frequency radar and LIDAR, and RF test/measurement equipment.

Insertion loss of the ADC12J4000NKET ADC, found in the datasheet.

Analog Devices, AD9680BCPZ-1000

The AD9680BCPZ-1000 14-bit, dual channel ADC provides a better compromise between sampling rate, resolution, and channel number. This ADC runs at a top sampling rate of 1 GSps with differential input in both channels. It also has reasonable power dissipation of ~3 W throughout a broad range of temperatures and sampling rates (see below). This product can also be configured using an SPI interface. Four integrated wideband decimation filters and NCO blocks are used to support multiband receivers, making this system adaptable to a broad range of applications.

Power output from the AD9680BCPZ-1000, from the AD9680BCPZ-1000 datasheet

Analog applications are making a resurgence, and you’ll need to include at least one high resolution or high frequency ADC in your system if you want it to interface with the digital world. If you’re looking for the right ADC for your next system, try using our Part Selector guide to determine the best option for your next product.

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