Verification care abouts for SoC internal channel characterization using an ADC

-September 14, 2013

Editor's note: Freescale authors/engineers Kushal Kamal and Siddi Jai Prakash have put together a clear and informative article outlining the challenges and good design practice in the SoC design and verification stage. I think this is a very good How-to article on this topic for designers.

A generic Nyquist Data Converter-based Analog to Digital Converter (ADC), based on a Successive Approximation Register (SAR) or Redundant Signed Digit (RSD) Algorithm, is shown in the block diagram in Figure 1. Industrial convention is to call an ADC based on a SAR Algorithm as a SAR ADC; the one based on RSD Algorithm as a Cyclic ADC. In the generic architecture of such ADCs there are 2 switches. One is called the Sampling Capacitor Switch and the other an ADC Switch. For an ADC to convert an analog signal to its corresponding digital word there are 2 phases: Sampling Phase and Conversion Phase.

Sampling Phase

In this phase the sampling capacitor switch is closed and the ADC Switch is open, as shown in Figure 1. As is evident, the ADC Driver is connected to the sampling capacitor via the sampling capacitor switch. The potential of the ADC driver is basically the voltage that the ADC would convert to a digital word after the end of sampling and conversion phases.

During the sampling phase the voltage of the ADC driver will be sampled on the sampling capacitor depending upon the source impedance (R) of the connection between the ADC driver and sampling capacitor and the capacitance value of the sampling capacitance (C). Basically the sampling time should be 5-10 times the RC values to sample the correct voltage on the sampling capacitor. Also the profile of the potential built on the sampling capacitor depends on the current drive strength of the ADC driver.

Sampling time is generally a few clock cycles. There are a couple of ways to alter the sampling time of the ADC. The number of clock cycles may increase/decrease or the frequency of clock can be changed (within ADC clock frequency range) and thus sampling time can be changed.

Figure 1: State of the ADC switches during the sampling phase

Conversion phase

Figure 2 shows the state of the ADC in the conversion phase. In the conversion phase the ADC switch closes and the sampling capacitor switch opens. Now the ADC would convert the voltage that is sampled on the sampling capacitor. Any change in the ADC driver voltage would not be seen in the ADC output. That concept is called sample and hold. Now depending on the architecture of the ADC, the conversion would be done and it would take a few clock cycles to do so. Once the conversion is done, the ADC switch would again open and the sampling capacitor switch would again close for the ADC to sample a voltage again; this way the ADC would keep converting.

Figure 2: State of ADC switches during the conversion phase