Analog Input Architecture with ADI SAR ADC Portfolio

After the chicken screamed, the sun did not fully jump out of the horizon, the air contained a little air, and the old store welcomed the first guest.

The boss greeted: "It’s really early here, what is the point?"

“Input devices for the SAR ADC analog input architecture.”

“Well, do you want a single-ended input, a pseudo differential input or a differential input?”

The guest frowned slightly: "How can I choose so many types?"

The boss smiled and said: "These input type devices are all here, so let's talk about the doorway..."

What is a SAR ADC?

Successive approximation analog-to-digital converters, also known as SAR ADCs, are general-purpose analog-to-digital converters that produce digital discrete-time representations of continuous analog waveforms. They accomplish this through a charge redistribution process; during this process, the known quantitative charge is compared to the amount of charge taken at the input of the ADC. A binary search is performed for all possible digital codes (quantization levels) during the period, and the final result converges to a certain code, causing the internally integrated comparator to return to equilibrium. The combination of 0 and 1 represents the decision sequence generated by the circuit, bringing the system back to equilibrium.

The SAR ADC is a general purpose, easy to use, fully asynchronous data converter. However, there are still some options to make when deciding which converter to use for a particular application. This article specifically discusses the types of analog input signals offered by the ADI SAR ADC portfolio. However, it should be noted that although this article focuses on SAR ADCs, the input type is common to all ADC architectures. Depending on the type of signal source or overall goal of the circuit under consideration, specific design decisions and trade-offs need to be made. The simplest solution is to match the ADC input type to the source output configuration. However, the source signal may need to change the conditioning of the signal type, or there may be cost, power or area considerations that affect the analog input type decision. Let's take a look at the different types of analog inputs available.

Single-ended input

The simplest analog input type is single-ended input. At this point, the signal only needs one line from the source to the ADC. In this case a single input pin will be used and no source directly returns or senses the path. The conversion result is generated relative to the ground pin of the ADC. Depending on the particular device, the input may be unipolar or bipolar. In the single-ended case, simplicity is its advantage. The signal needs only one trace from the source to the ADC. This reduces system complexity while reducing power consumption in the total signal chain. Of course, simplicity can also come at a price. Single-ended settings do not suppress dc offsets in the signal chain. Single-ended systems require measurements to be made relative to the current-carrying formation, and voltage differences between signal source ground and ADC ground may occur in the conversion results. Moreover, the settings are more susceptible to coupled noise. Therefore, the signal source and ADC should be close to each other to alleviate these effects.

Figure 1. Single-ended unipolar

If the SAR ADC is a unipolar single-ended configuration, the allowable signal swing is between ground and positive full scale, usually set by the ADC reference input. A visual representation of a single-ended unipolar input can be seen in Figure 1. Devices with single-ended unipolar inputs are the AD7091R and AD7091R-8 .

Figure 2. Single-ended bipolar

Pseudo differential input

If the sensed signal is required or the relative measurement is decoupled from the current carrying formation, the signal chain designer may consider migrating to the pseudo differential input structure. A pseudo-differential device is essentially a single-ended ADC with reference ground. The device performs a differential measurement, but the sensed differential voltage is a single-ended input signal measured relative to the input signal ground level. The single-ended input is driven to the positive input of the ADC (IN+) and the input ground level is driven to the negative input of the ADC (IN–). It should be noted that the signal chain designer must pay attention to the analog input range of the negative input.

Figure 3. Example of absolute input voltage

If a pseudo-differential device with a finite IN–voltage range (such as the AD7980) needs to reject interfering signals that are greater than the absolute input voltage range, the signal chain designer may need to consider the instrumentation amplifier to eliminate large common modes before the signal reaches the ADC. There are three pseudo differential configurations: unipolar, pseudo bipolar, and true bipolar. The ADI SAR ADC portfolio offers devices in each of these configurations. Devices with unipolar pseudo differential inputs are the AD7980 and AD7988-5 .

Figure 4. Unipolar pseudo-differential

In the pseudo bipolar setup, the single-ended unipolar signal is driven to the positive input of the ADC . However, the source ground is not driven to the negative input of the ADC, which reaches half of the full-scale voltage. In this example, the input range is ±VFS /2 instead of 0 to VFS. There is no increase in dynamic range, and the difference between the unipolar case and the pseudo-bipolar case is the relative voltage at which the positive input is measured. Devices that offer pseudo bipolar input options are the AD7689 .

Figure 5. Pseudo bipolar

The pseudo-differential true bipolar case is similar to the unipolar pseudo-differential case except that the single-ended positive ADC input can swing up and down at a low voltage. Typically, the peak-to-peak input range is twice the reference voltage or a multiple of this ratio. The device that provides the pseudo differential true bipolar input is the AD7606 .

Figure 6. Pseudo-differential true bipolar

Differential input

The pseudo differential architecture is superior to the single-ended architecture in that it can suppress specific disturbance signals within the conversion system. However, there are architectures that provide the same suppression advantages while also increasing the dynamic range of the system.

ADI offers two devices with differential inputs. The first one introduced in this article is differential inversion. In this example, the ADC converts the difference between the positive and negative inputs of the ADC, while the positive and negative inputs swing 180° out of phase with each other. Typically, differential inverting devices are unipolar. Therefore, each side of the differential device will swing between a low voltage and a positive full scale (set by the reference input). The input common mode is fixed because the differential device is 180° out of phase on each side. Similar to pseudo-differential devices, differential inverting devices limit their allowable common-mode input range. This range can be found in the specification sheet of the product data sheet. As shown in Figure 7 below. For devices with an absolute input range of ADC input from 0 volts to positive full scale, the common mode voltage is V FS/2. In most cases, for high resolution (16-bit and higher) differential inverting SAR ADCs, the common-mode voltage range is ±100 mV typical to common-mode voltage.

Figure 7. Differential Common Mode Input Range

When absolute optimum performance is required, a differential inverting device is usually selected. The differential signal will provide maximum noise rejection and tend to eliminate even-order distortion characteristics. As shown in Figure 8, the dynamic range and SNR are improved relative to single-ended and pseudo-differential configurations due to the differential device pins swinging in opposite directions.

Figure 8. Dynamic range increase due to differential signaling

If you want to maximize system performance in a single-ended signal chain, use a single-ended to differential amplifier, such as the ADA4940-1 or ADA4941-1, to properly condition the input signal and match its common-mode to the ADC. Figure 9 shows a differential inverting input range plot. Devices with differential inverting inputs are the AD7982 , AD7989-5, and AD7915 .

Figure 9. Differential inversion

Common mode range limits are necessary to achieve optimal performance and to avoid affecting the dynamic range of the converter. There are some common mistakes when using differential inverting devices that may violate the common mode range. Figure 10 shows the user error that often occurs when implementing a differential inverting device. A data sheet for devices that violate the limits of Figure 7.

Figure 10. Violation of common mode

In this case, the differential signal is not 180° out of phase. Therefore, the common mode varies drastically between the two ADC input pins. Another common differential inverting error is a 180° inverting, but improperly common, signal, or connecting the IN- pin of the ADC to a DC base voltage. Providing a DC voltage at the negative ADC input quickly violates the common-mode range specification while eliminating the dynamic range advantage of the differential signal. The second differential signal measures the difference between any two signals, regardless of the common mode.

ADI offers a comprehensive range of data acquisition solutions based on SAR ADC technology to measure fully differential signals. For signal chain designers looking for integrated data acquisition solutions with a wide allowable input common-mode range, ADI offers the ADAS3022 and ADAS3023. They are bipolar continuous and simultaneous sampling data acquisition systems with a common mode range of ±10 V. Within this range, they show the difference between any two signals.

The analog input type can affect the digital output encoding. Converters with unipolar input ranges, such as single-ended unipolar and pseudo differential devices, use direct binary encoding. Code 0 will represent the negative full-scale input voltage and code 2N – 1 (N is the number of bits) will represent the positive full-scale input. Devices with ± polarity inputs will use two's complement to provide the sign bit to the user. Devices with ± polarity include single-ended bipolar, pseudo differential bipolar, pseudo bipolar, and fully differential devices. For these ADCs, the negative full-scale input will be represented by the code –2N – 1, and the positive full-scale input will be represented by the code 2N – 1 – 1.

in conclusion

The SAR ADC is a versatile, low power, high performance option for creating analog to digital conversion signal chains. These devices are easy to implement. However, in order to achieve the required performance of the system, specific architectural choices must be made. This article specifically discusses the choice of analog input types offered by the ADI SAR ADC portfolio. Each input type offers specific advantages and specific trade-offs must be made. As mentioned above, the right choice is critical to achieving optimal performance.