SAR ADC application example.
Everything in nature is analog. Sensors allow us to acquire this data but how do we use it? Whether you are measuring the speed of a car, the position of an assembly on a production line, or the temperature in an enclosure, you will have to convert those analog quantities into digital signals to control the system.
Since we cannot perfectly reproduce these analog signals, We have to determine an appropriate step size and sampling rate to ensure that the signals will not distort.
TI’s (Texas Instruments’) line of SAR ADCs (Successive Approximation Register Analog to Digital Converters) provides a wide range of choices and Verified Reference Designs to ensure an accurate conversion.
Signal Conditioning before Analog-Digital Conversion
One of the challenges in getting data from a sensor to the ADC and ultimately the controller is signal conditioning. For instance, the signal might be too large or too small for the ADC input. This signal must therefore be adjusted to match the input of the ADC. At this point noise can become a problem, especially when amplifying the signal. Conditioning the signal with pre-amplifier filtering ensures a stable signal to the ADC.
Watch as Dwight Byrd, TI’s Product Marketing Engineer and Luke Lapointe, TI’s applications Engineer, show how to calculate, design and simulate the correct RC filter and operational amplifier to drive a SAR ADC.
The TI team also shows how to get datasheet values and achieve the maximum performance out of your high precision SAR ADC. From a few SAR ADC specs, engineers can determine the minimum Unity Gain Bandwidth and the correct RC values required to drive a fully differential SAR ADC and achieve datasheet spec performance.
SAR ADC Input considerations
Not sure what type of input or resolution will work best for your design? TI has offered a series of Blog posts to help users with their SAR ADC designs. In this blog, Amit Kumbasi discusses the Input considerations for SAR ADCs.
In Kumbasi’s post you will learn about:
- Single-Ended conversion, which digitizes the input with respect to the SAR ADC ground
- Pseudo-differential input conversion which digitizes the difference between two inputs, though one is held constant
- Fully differential conversion which digitizes the difference between two inputs which are complementary to each other (out of phase)
Fully Differential Conversion & Optical Encoding for Motor Control
Optical Encoding
To achieve the highest dynamic range, fully differential signaling is used for optical encoding in motor control applications. Two infrared emitters shine IR light through holes of varying sizes on a rotating disc which is received by two photo diodes on the other side of the encoder . As the encoder turns, the amount of light transmitted through the disc varies according to the position of the encoder. The result produced by the diode receivers are a current output that varies with the intensity of the light received and looks like two waves, a sine wave and a cosine wave. The precise motor position can be determined by calculating the arctangent of the sine and cosine waves. Precise motor positioning is a major consideration in robotics as any variance in position between the calculated actual values results in errors which is undesirable and in some cases can even result in safety issues.
TI’s 12 Bit 1 MSPS Single Supply Dual Channel Data Acquisition System for Optical Encoders in Motor Control Applications will help the design, simulation, and evaluation of this system. TI has also released a BOM (Bill of Materials) and a PCB (Printed Circuit Board) layout for reference.
Fully Differential Conversion & ECG Measurements
TI TINA ECG simulation output
Fully differential conversion is also used in acquiring an ECG (Electrocardiography) signal. Designing a system to acquire a heart rate signal is not a trivial task. The sensors are designed to detect the electric activity caused by movement in the muscles of the heart. This requires the amplification of a very small pulse before sampling the signal. Too much gain and the signal is distorted, too little gain and the accuracy is not good after sampling.
Thankfully, TI has a verified design for ECG measurement. All the theory and design considerations are covered in the TIPD116 Verified Design Reference Guide. A precise analog front end gain stage is included.
The ‘heart’ of the design is the SAR ADC - ADS8881. It is an 18-bit converter that provides very small resolution for accurate sampling. It will help your design achieve the highest performance while sampling up to 1 Msps (Million samples per second). Included with the documentation are simulation circuit files that can be used with TI TINA to simulate the circuit design.
All of this is backed up by extremely low power consumption, <1mW. This allows for flexibility in the design and lends itself to watches and other small wearable devices.
Fully Differential Conversion & Industrial Applications
Block Diagram Highlighting Primary Design Criteria for this Multiplexed DAQ Block
Conversion of fully differential signals are also found in industrial data acquisition applications. This specific example shows four separate input channels that are Time Division Multiplexed (TDM) together. Typically these signals are higher voltages than digital circuits; they can range up to ±20v.
This involves a whole new set of design considerations. One is properly managing the output of the multiplexer for fastest response time when there is an instantaneous change from a high voltage on one channel to a low voltage on another. The characteristics of the input signal such as bandwidth must be carefully considered such that the proper signal conditioning circuitry and ADC is chosen to achieve the required response time to properly acquire the signal. If achieving highest AC performance is required, careful tradeoffs need to be made to the multiplexer and amplifier signal conditioning circuitry with respect to the ADC to keep the noise sufficiently low.
TI’s 4-channel Multiplexed Data Acquisition System for High Voltage Inputs with Lowest Distortion employs the 16 bit ADS8864 400kSPS SAR ADC. The Design Reference Guide walks you through the theory and calculations required to achieve lowest distortion. Then you can simulate and verify your design using TI TINA or start with a copy of TI’s design and make your adjustments.
Other SAR ADC Designs
Achieving the lowest distortion and lowest noise might be necessary in some sensitive ADC designs. TI’s 18-bit, 1MSPS Data Acquisition Reference Design Optimized for Lowest Distortion and Noise uses the 18 bit ADS8881. It can achieve 1 Msps and does so with low power consumption.
The ADS881 is very versatile and can be used for both fast-throughput and low-throughput, low-power applications. For engineers looking to achieve lowest power and acquire signals up to 1kHz, TI’s 18-Bit, 1 MSPS Data Acquisition at 1kHz AC Reference Design optimized for lowest power is a good option. It uses the high precision, 18 bit, ADS8881 and can help you achieve a power consumption goal of under 1mW. For highest throughput requirements, with power consumption as the next most important design priority, the Data Acquisition for 10 kHz AC, 18-bit, 1Msps Reference Design may meet your needs. It achieves 1Msps throughput for up to 10kHz input bandwidth signals while consuming a measured 28.61mW of power.
Community Help
There is always a great deal of information available from the TI Community. The Blog post “Accurate data acquisition? It’s all relative,” was one of my favorites. It compares Einstein’s relativity to the use of power supplies in ADC. But TI also offers many white papers, design specs, BOM, PCB layouts, and product listings to help narrow down your search.
The right ADC can make a huge difference in your project. TI’s certified designs and resources can help take the headache out of choosing and setting up the correct ADC.
Texas Instruments has sponsored promotion of their SAR ADC Certified Designs on ENGINEERING.com. They have no editorial input to this post - all opinions are mine. Christine Halsey