2. (10 pts) An old exam problem on ADCs: The TI ADS8331 is an IC that provides a

Question

2. (10 pts) An old exam problem on ADCs: The TI ADS8331 is an IC that provides a 16-bit, low-power sigma-delta ADC. It is being used inside a data recorder to collect data at 250ksps and is powered by a single 1.8V power supply. Answer the questions below about the ADS8331 in this configuration assuming that Vcc=1.8V and that it uses an external reference voltage VREF+-Vcc and VREF--0V. What is the full-scale range of this ADS8331? What is the resolution of this ADS8331? In this configuration, what was the input voltage to the ADS8331 if the output code is 0x8021? What would the output of the ADS8331 be (in decimal) for an input voltage of 0.87V? a. b. c. d.

Explanation / Answer

Solution:

2.a) The full scale voltage of ADC is Vref+ and Vref-.

Given that Vref+ = 1.8V and Vref- = 0V

So Full scale range of ADC is 1.8V

2.b) The resolution of ADC is depending on the no of bits that ADC can give.

ADC is 16 bit So signal is quantized to 216 levels

So resolution is = FS Range/216 = 1.8/65536 = 27.46?V.

2.c) Input of ADC is positive so straight binary is used to represent the Input values.

For straight Binary, FFFF (16bit) represents 1.8V

The code is linear with respect to the code word.

So 0x8021 represents = (0x8021/0xFFFF)*1.8V = 0.9009V

Output code 0x8021 represents input value of 0.9009V

2.d) Input of ADC is positive so straight binary is used to represent the Input values.

For straight Binary, 1.8V represents FFFF (16bit).

The code is linear with respect to the code word.

So 0.87V represents = (0.8/1.8)*65535 = 31675 = 0x7BBB

Input value 0.87V represents code word of 0x7BBB.

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