This technical article was updated on July 23, 2020.

In Part 2 of this series, I explained a serial interface in which a conversion result for sample S is transferred to the host controller after the conversion is complete but before the start of the next conversion. I also noted that with this type of interface, both the throughput and response time of the analog-to-digital converter (ADC) degrade when using lower clock speeds.

Interface type 3: Data bits are transferred when the ADC is converting for the next sample

In such ADCs, the conversion result for sample S is transferred to the host controller when the ADC is converting for sample S+1.

Figure 1 shows such a serial interface.

(**Click image to enlarge.)

Figure 1: Type 3 serial interface

Note that:

• tDTX = tCONV = n*tCLK
• tTHROUGHPUT = tCONV + tACQ (as long as n*tCLK < tCONV – for faster clock speeds)
• tTHROUGHPUT = n* tCLK + tACQ (as long as n*tCLK < tACQ – for slower clock speeds)

The equations show that a SAR ADCs employing a type 3 interface scheme will also have a restriction of minimum clock speed to achieve the desired throughput.

• tCLK < tCONV/n

As an example, let’s use a hypothetical 18bit/1MSPS ADC with a conversion time of 710ns.

• tTHROUHGPUT = 1 µs and tCONV = 710ns and n = 18
• tCLK < 710/18 and fCLK > 25.4MHz to achieve 1MSPS throughput

To achieve the same 1MSPS throughput, the type 2 interface required fCLK > 62MHz! The type 3 interface meets the same throughput requirements at less than half the clock speed.

Clearly, the type 3 interface is more user-friendly, unless we’re missing something.

What about the response time? From the diagram, it is also clear that:

• tRESP-ADC = tCONV + tACQ + n* tCLK
• tRESP-ADC > tTHROUGHPUT

Table 1 compares the throughput and response times of type 2 and type 3 interfaces:

Table 1: Throughput and Response time comparison between Type 2 and Type 3 serial interfaces

(**Click image to enlarge.)

Figure 2: Throughput and Response time comparison between Type 2 and Type 3 serial interfaces for different clock speeds

This example shows that for the same clock speed, interface type 2 will give you a better response time, while interface type 3 will give you better throughput.

In my next blog post, I’ll give some tips on which interface is more suitable for particular applications.