The ABCs of interleaved ADCs
Across many segments of the market today, interleaving ADCs offers several advantages in many applications. In communications infrastructure there is constantly a push for higher sample rate ADCs to allow for multi-band, multi-carrier radios in addition to wider bandwidth requirements for linearization techniques like DPD (digital predistortion).
In military and aerospace, higher sample rate ADCs allow for multi-purpose systems that can be used for communications, electronic surveillance, and radar just to name a few. In yet another segment, industrial instrumentation, the need is always increasing for higher sample rate ADCs so that higher speed signals can be measured adequately and accurately.It is first important to understand exactly what interleaving ADCs is about. To understand interleaving, it is good to look at what is actually happening and how it is being implemented. With a basic understanding, the benefits of interleaving can then be discussed. Of course, as many know, there is no such thing as a free lunch, so the challenges of interleaving need to be evaluated and assessed.
When ADCs are interleaved, two or more ADCs with a defined clocking relationship are used to simultaneously sample an input signal and produce a combined output signal that results in a sampling bandwidth at some multiple of the individual ADCs. Utilizing m number of ADCs allows for the effective sample rate to be increased by a factor of m.
For the sake of simplicity and ease of understanding, we’ll focus on the case of two ADCs. In this case, if two ADCs with each having a sample rate of fS are interleaved, the resultant sample rate is simply 2fS. These two ADCs must have a clock phase relationship for the interleaving to work properly. The clock phase relationship is governed by equation 1, where n is the specific ADC and m is the total number of ADCs.
As an example, two ADCs each with a sample rate of 100 MSPS are interleaved to achieve a sample rate of 200 MSPS. In this case, equation 1 can be used to derive the clock phase relationship of the two ADCs and is given by equations 2 and 3.
Now that the clock phase relationship is known, the construction of samples can be examined. Figure 1 gives a visual representation of the clock phase relationship and the sample construction of two 100 MSPS interleaved ADCs. Notice the 180o clock phase relationship and how the samples are interleaved. The input waveform is alternatively sampled by the two ADCs. In this case, the interleaving is implemented by using a 200 MHz clock input that is divided by a factor of two and the required phases of the clock to each ADC.
Figure 1. Two Interleaved 100 MSPS ADCs – Basic Diagram
Another representation of this concept is illustrated in Figure 2. By interleaving these two 100 MSPS ADCs, the sample rate is increased to 200 MSPS. This extends each Nyquist zone from 50 MHz to 100 MHz, doubling the available bandwidth in which to operate. The increased operational bandwidth brings many advantages to applications across many market segments. Radio systems can increase the number of supported bands; radar systems can improve spatial resolution, and measurement equipment can achieve greater analog input bandwidth.
Figure 2. Two Interleaved 100 MSPS ADCs – Clocking and Samples
Benefits of Interleaving
The benefits of interleaving span across multiple segments of the market. The most desired benefit of interleaving is the increased bandwidth made possible by the wider Nyquist zone of the interleaved ADCs. Once again, taking the example of two 100 MSPS ADCs interleaved to create a sample rate of 200MSPS, Figure 3 gives a representation of the much wider bandwidth allowed by interleaving the two ADCs. This creates advantages for many different applications.
As cellular standards increase channel bandwidth and the number of operating bands, there are increased demands on the available bandwidth in the ADC. In addition, in military applications, the requirements for better spatial recognition as well as increased channel bandwidths in backend communications require higher bandwidths from the ADC. Due to the increased demands for bandwidth in these areas, there is a need created to measure these signals accurately.
Therefore, measurement equipment has increased needs for higher bandwidths in order to properly acquire and measure these signals that have higher bandwidth. The system requirements in many designs inherently stay ahead of commercial ADC technology. Interleaving allows for some of this gap to be closed.
Figure 3. Two Interleaved ADCs – Nyquist Zone
The increased sample rate provides more bandwidth for these applications but also allows for easier frequency planning and reduction in the complexity and cost of the anti-aliasing filter that is typically used at the ADC inputs. With all these great benefits, one has to wonder what the price is to pay. As with most things, there is no such thing as a free lunch. Interleaved ADCs offer increased bandwidth and other nice benefits, but there are some challenges that arise when dealing with interleaved ADCs.