As we looked for a 12-bit system to handle data loggers, handheld meters, automotive systems, or monitoring systems, we used 12-bit analog-to-digital converters (ADCs), and even a 16-bit ADC, to solve our problem. Again, the system we are designing today must produce 4,096 possible digital outputs to a microcontroller or processor. What if we just jump away from the ADC-SAR converter and really use the unorthodox approach that was utilized in ADC Basics, Part 10B: 16-Bit Converter Gives a Gainable 12-Bit System? A 24-bit Δ-Σ ADC is an excellent place to start.
In ADC Basics, Part 8: A 4-System Matrix With PGA + 12-bit SAR, we used a programmable gain amplifier followed by a 12-bit SAR ADC. In our second system, ADC Basics, Part 9: PGA Embedded in an 8-Channel, 12-Bit SAR, an eight-channel, 12-bit SAR-ADC allowed the inclusion of an analog function between the multiplexer and ADC. With our third system, ADC Basics, Part 10A: 16-Bit Converter Gives a Gainable 12-Bit System, we jumped into using a 16-bit SAR ADC, where this converter handled most of the signal gain through process gain. This third system provided a dramatic improvement in system noise, at the expense of a reduced throughput time.
Leap to a 24-bit Δ-Σ converter
So here we go — we are figuratively going to throw away the bath water. I'm referring to the external programmable gain amplifier (PGA) and operational amplifier (op-amp), hoping to keep just the baby. To achieve excellent 12-bit specifications, consider a 16-channel (multiplexed), 24-bit Δ-Σ ADC (Figure 1). This device provides zero-cycle latency with an auto-scan rate of 23.7ksps.
The input multiplexer of this device accepts combinations of eight differential or 16 single-ended inputs. The full-scale differential range is 5 V, or a true bipolar range is ±2.5 V, while operating with a 5V reference. In this device, a fifth-order sinc ([(sin x) / x]) digital filter follows the fourth-order Δ-Σ input modulator.
Digital or process gain?
The previous systems that we designed had analog/digital gains increasing by a factor of two from 1 to 128 V/V. In this system, we are going to use the process gain technique exclusively. This system process gain maxes out at 128. Figure 2 shows this how function works.
With the 24-bit Δ-Σ ADC, you receive a 24-bit word at the converter's serial digital output pin. You can increase the process gain by a factor of two by shifting the most significant bit (MSB) position to the right in your digital controller or processor. The number of process gains that have 12-bit words available at the output of this 24-bit Δ-Σ ADC is eight.
With this device, the noise is dramatically less than half the least significant bit (LSB) of our required 12-bit system. The device in Figure 1 has a noise level of 7.2 bits (p-p), or 4.5 bits rms (noisy bits). Because of this noise performance level, bits 24 (MSB) through 6 (MSB − 18) are completely usable in this application. As you use all of these 12-bit configurations by implementing your process gain, the referred-to-input (RTI) noise across all process gains (1 to 128) is lower than our 12-bit system requirements. In fact, the process gain of 128 is the only gain configuration that shows any noise of 12μV-rms.
An analog low-pass filter helps you achieve this level of performance. You install this filter by using the MUX OUT and ADC IN pins (Figure 3).
In the second part of this two-part blog, we will calculate filter component values, take a closer look at Δ-Σ performance, and summarize performance of these system variations.
- ADC Basics, Part 1: Does Your ADC Work in the Real World?
- ADC Basics, Part 2: SAR & Delta-Sigma ADC Signal Path
- ADC Basics, Part 3: Using Successive-Approximation Register ADC in Designs
- ADC Basics, Part 4: Using Delta-Sigma ADCs in Your Design
- ADC Basics, Part 5: Key ADC Specifications for System Analysis
- ADC Basics, Part 6: Key Op-Amp DC & AC Specifications
- ADC Basics, Part 7: Key Op Amp Frequency & Timing Specifications
- ADC Basics, Part 8: A 4-System Matrix With PGA + 12-bit SAR
- ADC Basics, Part 9: PGA Embedded in an 8-Channel, 12-Bit SAR
- ADC Basics, Part 10A: 16-Bit Converter Gives a Gainable 12-Bit System
- ADC Basics, Part 10B: 16-Bit Converter Gives a Gainable 12-Bit System