24-Bit 2Msps SAR ADC Takes Dynamic Range To A New Level

A medical application, like an encephalograph, may require the gathering of signals in the presence of high levels of noise; the electrical activity in a cell when stimulated, known as the action potential, can range from 10uV to 100mV at frequencies from 100Hz to 2kHz. If the signals are buried in the noise you need to average the samples to resolve the signal, requiring an ADC with high dynamic range.

Seismology and seismic exploration are other demanding applications with common requirements. Seismometer and accelerometer signals can have a dynamic range of 140dB and frequencies up to 100Hz. The SNR of seismic signals received by sensors is very low due to absorption and attenuation by subsurface and deep layers during signal propagation. This creates a real challenge to measure these signals.

A gas sensor must be able to detect very low concentrations of gas, alarming at detection levels as low as 0.5ppm. High accuracy and wide dynamic range is vital for this application to ensure toxic chemicals are detected swiftly, but also ensure alarms are not activated unnecessarily.

Broader trends are also raising the data conversion bar. The move towards portable devices is resulting in increasingly complex data conversion tasks migrating to battery-powered devices. Designers must develop solutions using less space while simultaneously minimizing power consumption.

For data conversion tasks, each of the common ADC architectures brings with it a list of advantages and drawbacks.

Data Converter Architectures

Analog-to-digital converter design mostly involves a series of compromises. For converters, a lot depends on the primary goal: high resolution, high speed, or low power consumption. Note: you can't necessarily pick all three! 

To cover the full spectrum of application requirements, multiple ADC architectures have appeared over the years, but there are three primary architectures in use today.

The successive approximation register (SAR) architecture traditionally has been the workhouse, "go-to" architecture for mainstream analog-to-digital converter applications with low frequency signals. It provides the transition between high resolution, low speed delta-sigma architectures, and the high speed, lower performance, pipeline architecture. They are usually lower cost compared to pipelined ADCs, and consume a modest amount of power. The SAR converter shows no latency between successive conversions, so it is ideal for sampling multiplexed or non-periodic signals.

Pipeline converters use a multi-stage sequential pipeline architecture to increase sampling speed. They rule the market at very high sample rates for acquiring wide signal bandwidths or signals at higher input frequencies, and on a per sample basis consume less power when compared to fast SAR ADCs. They're unsuited to handle multiplexed or non-periodic inputs because they have to "flush the pipe" every time the source changes, which adds considerable latency. The main rival to the SAR ADC for higher-resolution applications has been the delta-sigma converter; this relies on a delta-sigma modulator and a digital decimation filter. This architecture is slow compared to the SAR and is not as accurate. Most importantly, the noise spectrum of a delta-sigma ADC includes vibrating noise tones whereas the SAR ADCs noise floor has a uniform power spectral density. This makes SAR ADCs better for detecting tones or vibrations at incredibly low levels.

Introducing the LTC2380-24

Despite its disadvantages, the relatively slow delta-sigma architecture has been the only option for high-resolution applications because SAR converters have traditionally not been available at resolutions above 18 bits. Recently, Analog Devices' Power by Linear introduced the LTC2380-24, a SAR converter that combines high resolution (24-bits) with high sample rate (up to 2Msps). It's the flagship member of the LTC2380 family, which includes the 20-bit 1Msps LTC2378-20, the 18-bit 1.6Msps LTC2379-18 and the 16-bit 2Msps LTC2380-16 among others. All these parts come in the MSOP-16  and 4mm by 3mm DFN packages, and are pin-compatible.

The 24-bit precision, fast 2Msps sample rate and unparalleled ±0.5ppm (typ) linearity enables the LTC2380-24 to resolve very low-level input signals in high dynamic range applications such as ECG/EEG.

The LTC2380-24 includes additional features that help simplify common design problems, such as a built-in digital filter and digital gain compression for single supply operation.

Detailed technical specifications appear on the LTC2380-24 product page; this article will discuss some of the special features of the part and how they benefit target applications, as well as touch on a couple of application details.

Digital Filtering For Averaging

Many applications, such as seismic exploration, require the accurate measurement of a weak low-frequency signal in the presence of broadband noise. Oversampling the signal at a rate much higher than Nyquist, then averaging the result of multiple conversions, reduces the effect of this uncorrelated noise. Oversampling increases the effective dynamic range of the ADC by spreading out the noise across a wider bandwidth, thus reducing the noise spectral density in the bandwidth of interest. This also reduces the complexity of the front-end anti-aliasing filter, which results in less power being consumed and less noise and distortion being introduced.

The LTC2380-24 features an integrated digital averaging filter that can provide this function without any additional hardware, simplifying the design and providing a number of unique advantages.  The high sample rate of the LTC2380-24 makes this an option for many applications. The benefit to the user is that this frees up valuable resources in the processor to perform other tasks, while the averaged data can be transferred at much slower data rates (as low as 2Msps).

Figure 1: Dynamic range improvement with digital averaging filter

Particular input frequencies may be rejected by selecting N based on the sampling rate and the desired frequency to be rejected. This is particularly useful for rejecting 50Hz or 60Hz line frequencies, which are a problem for many sensitive data-acquisition applications. 

For example, selecting N = 20,000 and a sample rate of 1Msps will reject 50Hz frequencies;  refer to the LTC2380-24 datasheet for more details.

Digital Gain Compression

Figure 2: Digital gain compression

The LTC2380-24 -24 includes digital gain compression (DGC). This feature defines the full-scale input swing to be between 10% and 90% of the +/-VREF analog input range. The REF/DGC pin is held low to enable this feature. DGC allows the signal conditioning circuit in front of the LTC2380-24 to be powered from a single +5V supply, since each ADC input swings between 0.5V and 4.5V when using a 5V reference with the LTC2380-24, as shown in Figure 2. This eliminates the requirement for a negative rail on the ADC driver, which lowers system cost and gives additional power savings for the entire system. Battery-power and portable applications, in particular, will benefit from this feature.

Power Management

The LTC2380-24 powers down after conversion is complete, although the conversion data can still be clocked out. In power-down mode, the total power consumed is only 2.5uW (typ), making it well suited to low-power applications that only require periodic input samples. The LTC2380-24 consumes only 28mW from a single 2.5V supply when sampling at 2Msps.

Application Detail: optimization of the input driver

Figure 3: LTC2380-24 input filtering

A buffer amplifier is recommended to provide low output impedance for fast settling of the analog signal during the sample and hold phase. The distortion and noise of the buffer amplifier and signal source must be taken into account, of course, since they add to the ADC noise and distortion.

To minimize noise, input signals should be filtered before the buffer amplifier input with an appropriate filter. The simple RC low pass filter (LPF1) shown in Figure 3 is sufficient for many applications.

An additional low pass filter, LPF2, is needed between the output of the driver and the ADC input. This is important because the LTC2380-24's sampling process creates a charge transient when the sampling capacitor is switched in at the start of the sample and hold process. This briefly "shorts" the amplifier output as charge flows from the amplifier to the sampling capacitor. The driver must be able to recover from this load transient before the end of the sample period. Otherwise the signal at the ADC input pin won't be a valid representation. LPF2 decouples the sampling transient of the ADC; the capacitance provides the bulk of the charge and the resistors dampen and attenuate any charge injected by the LTC2380-24.

LPF2 provides both differential and common-mode low-pass filtering. The differential cutoff frequency is given by 1/2πR(2CD + CC), and the common-mode cutoff frequency by 1/2πRCC .

It's critical that the CC capacitors match as closely as possible. Since resistors and capacitors can add distortion, the design should use high-quality components such as metal film resistors and zero-drift ceramic (NPO) or silver mica capacitors.

Choice of Driver op amp

Two different opamps are commonly used with the LTC2380-24 for best AC performance.

To buffer a fully differential source, or convert a single-ended input to differential form, the LT6203 is recommended. The LT6203 is a unity gain stable, dual low power opamp with rail-to-rail input and output. It features 100MHz GBW product, ultra-low noise voltage of 1.9nV/√Hz, and harmonic distortion of less than –80dBc at 1MHz. The LT6203 draws only 2.5mA of supply current per channel, making it suitable for low-power applications.

The LTC6362 SAR ADC driver is recommended for single supply operation with a 5V supply. It also features rail-to-rail input and output but is fully differential and has a noise density of 3.9nV/√Hz, 180MHz GBW product and -116dB distortion at 1kHz.

Figure 4 shows the LTC6362 used in a single supply application together with the LTC6665, a low drift precision voltage reference that has peak-to-peak noise of only 0.25ppm and accuracy of 0.025% maximum. Notice that the voltage at pin 2 of the LTC6362 sets the common-mode voltage level. If left floating, an internal resistor divider develops a default voltage of 2.5V with a 5V supply.

Figure 4: single-supply application circuit using the LTC6362

A seismic oil exploration rig may use 1,000 or more sensing elements, called geophones. Each one produces low-level signals in a noisy environment at frequencies of up to 100kHz. The high sampling rate of the LTC2380-24 allows oversampling and the use of the digital averaging filter for maximum dynamic range.

Similarly, medical applications such as MRI, gas chromatography, and digital X-ray machines involve precision measurement of low-level signals and impose severe demands on the data acquisition front end dynamic range, so the LTC2380-24 is an ideal choice.

Design Support

Evaluating the LTC2380-24 -24 is made easy with the DC2289 demonstration board, shown in figure 5.   

The DC2289 demonstrates correct layout and recommended device selection to achieve the highest performance design. The board includes the LT6203  as an input buffering amplifier for a fully differential signal source and built-in lowpass filter.

To demonstrate DC performance parameters such as peak-to-peak noise and DC linearity, the DC2289 can connect to the DC590B USB Serial Controller or DC2026C Linduino One Isolated Arduino-Compatible demo board. Alternatively, the  DC890B PScope™ data collection board can be used to demonstrate AC performance metrics such as SNR, THD, SINAD and SFDR.

Figure 5: DC2289A demo board

LTC offers a variety of free data acquisition and analysis tools, including the QuikEval system for the DC590, and the PScope software for the DC890. The entire code base for the DC2026, LTSketchbook.zip, is available for download on the Analog Devices' website. The package includes demo code and libraries for all covered devices, including the LTC2380-24. 

Conclusion

The LTC2380-24 is a ground-breaking precision analog to digital converter with a unique combination of high resolution and high speed.

It has a number of features that help designers solve problems and perform tasks in a variety of precision analog fields such as data acquisition, seismic exploration, medical, industrial process control and ATE.

When coupled with Analog Devices' Power by Linear's comprehensive suite of development tools, the LTC2380-24 will help designers quickly get started on their next precision data acquisition project.