Understanding Analog-to-Digital Converter (ADC) Drivers
The need for analog signal conditioning, including Analog-to-Digital Converters (ADCs), continues to grow as sensors become more and more abundant within a variety of end markets. The overall sensor market is anticipated to grow at an annual compounded rate of over 9%. End markets include expansion within various Internet of Things (IoT) applications, factory automation and control, public health and safety, healthcare, and automotive. For ADCs, the market trend is toward higher resolution, higher speed devices as the cost of such solutions becomes more affordable.
As the name implies, ADC drivers are specialty amplifiers that are designed specifically to work alongside ADCs and include successive approximation, pipelined, and delta-sigma-based architectures. These specialty amplifiers are critical circuit components to enable the ADC to function at full performance and have become more vital with the expansion of higher-speed, higher-resolution converters.
Understanding ADC Inputs
Before discussing the technical functions required from an ADC driver, a brief overview of the input architecture of today’s ADC is helpful. A differential signal can be defined as two nodes that have equal but opposite signals around a fixed point, called the common mode level. The two signal nodes are typically referred to as positive and negative—or non-inverting and inverting—as shown in Figure 1.
Figure 1: Differential sine wave.
In the previous example, the full-scale input voltage is 5V peak-to-peak differentially, with each leg swinging 2.5V peak-to-peak. The common mode level in this example is 2.5V. A majority of today’s higher-performance ADCs implement a differential input architecture, as it provides superior performance relative to single-ended inputs. These performance benefits include the ability to reject common mode noise and common interference signals and a 6 dB—or a factor of 2—increase in dynamic range.
ADCs can pose an especially difficult challenge to system designers, offering a variety of different input sampling architectures that must be considered on the system level. For the purposes of this discussion, the focus will be on ADCs that use a switched-capacitor structure to accomplish input sampling. In its most basic form, this input structure is composed of a relatively small capacitor and an analog switch, as shown in Figure 2.
Figure 2: A simple switched-capacitor input structure is employed for input sampling.
When the switch is configured in position 1, the sampling capacitor is charged to the voltage of the sampling node, VS in this case. The switch is then flipped to position 2, where the accumulated charge on the sampling capacitor is then transferred to the rest of the sampling circuitry. The process then begins all over again.
An unbuffered switched capacitor input, like the one previously described, can cause significant system-level issues. The current required to charge the sampling capacitor to the appropriate voltage must be supplied from the external circuitry connected to the ADC input. When the capacitor is switched to the sampling node (switch position 1 in Figure 2), a large amount of current will be required to begin charging the capacitor. The magnitude of this instantaneous current is a function of the size of the sampling capacitor, the frequency at which the capacitor is switched, and the voltage present on the sampling node. This switching current can be described by the following equation:
Figure 3: Switching current equation.
Where C is the capacitance of the sampling capacitor, V is the voltage present on the sampling node (in this example denoted as VS), and f is the frequency at which the sampling switch is turned on and off. This switching current results in high current spikes on the sampling node, as illustrated in Figure 2.
The implications of this switching current must be considered when designing the analog circuitry in front of the ADC. As the input current passes through any resistance, a voltage drop will occur, resulting in a voltage error at the sampling node of the ADC. Distortion can also occur if the input node is not fully settled prior to the next sampling cycle.
Solution: ADC Drivers
Maintaining the required sensor signal integrity to take full advantage of these higher resolution, higher speed ADCs becomes very challenging. As the resolution and speed of the ADC increase, the effects of noise and distortion from the sensor signal become more noticeable. At higher ADC sampling speeds, care must be taken to ensure that the input signal has settled before the sampling event and that higher bandwidth signals do not alias back into the signal bandwidth of interest.
To overcome these signal conditioning challenges, many ADC applications require an ADC driver that provides sufficient settling and anti-aliasing. As described above, most modern ADCs implement a differential input architecture. One of the main functions of the ADC driver is to provide single-ended to differential conversion of the incoming signal, although they can just as easily handle a differential input signal too.
Another function of the ADC driver is to buffer the input signal, hence isolating the rest of the circuitry from the charge injection on the input node of the ADC. The ADC driver provides instantaneous charge to ensure that the sampling node is settled within the track time, thus minimizing any distortion related to settling. Care must be taken with the board-level layout of the ADC driver and the converter to ensure minimal trace resistance from the output of the driver to the input of the ADC.
Most ADC driver amplifiers also provide a hardware pin that enables the user to level shift the common mode voltage. This feature is ideal for ensuring that the resulting differential signal is centered within the input voltage range of the ADC, hence maximizing the dynamic range. As operating voltages continue to trend lower, dynamic range becomes even more critical to ensure full resolution of the input signal.
Finally, similar to most amplifier components, ADC drivers can provide amplification of the input signal as well as active filtering. It should be noted that most ADC drivers are specified with relatively low gain, typically gains of only 1 or 2 V/V. By keeping the amplifier’s closed-loop gain low, the loop gain is maximized, resulting in the lowest distortion. For example, if an amplifier has an open-loop gain of 100 dB and is configured for a closed-loop gain of 200, or 46 dB, this leaves only 54 dB of open-loop gain margin to ensure linearity or about one part in 500. Therefore, it is common to have a separate gain stage that is located close to the signal source to maximize the signal-to-noise ratio.
Get the Best of ADC
The proliferation of sensors across a wide variety of end markets has created an additional focus on signal conditioning circuitry. With the cost of high-resolution and high-speed ADCs continuing to decrease, realizing this performance improvement becomes more challenging.
To get the most out of your data converter, the ADC driver is critical to optimizing performance while adding negligible distortion, noise, and settling time errors to the source signal. Specialty devices such as the MCP6D11 differential driver are specifically designed to maximize the performance of high-speed, high-resolution ADCs.
