EE174 – SJSU Lecture #4 Tan Nguyen

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EE174 – SJSU Lecture #4 Tan Nguyen
ADC TYPES EE174 – SJSU Lecture #4 Tan Nguyen

A n-bit flash ADC uses 2n-1 comparators and a encoder logic. Advantage: the fastest type of ADC. Disadvantages: limited resolution, expensive, large power consumption and low accuracy. Applications: Data acquisition, satellite communication, radar processing, sampling oscilloscope and high density disk drives.

Start Conversion (SC) A DAC is used to generate approximations of the input voltage. A comparator is used to compare Vin and Vappr. In each cycle, SAR finds one output bit using comparator. To start conversion, set SC = 1. When conversion ends, EOC = 1. Quite fast, expensive, high accuracy and one of the most widely used design for ADCs.

Goal: Find digital value Vin 8-bit ADC Vin = 7.65 Vfull scale = 10

Vfull scale = 10, Vin = 7.65 MSB  LSB Average high/low limits Compare to Vin Vin > Average  MSB = 1 Vin < Average  MSB = 0 Bit 7 (Vfull scale +0)/2 = 5 7.65 > 5  Bit 7 = 1 1

Vfull scale = 10, Vin = 7.65 MSB  LSB Average high/low limits Compare to Vin Vin > Average  MSB = 1 Vin < Average  MSB = 0 Bit 6 (Vfull scale +5)/2 = 7.5 7.65 > 7.5  Bit 6 = 1 1  1

Vfull scale = 10, Vin = 7.65 MSB  LSB Average high/low limits Compare to Vin Vin > Average  MSB = 1 Vin < Average  MSB = 0 Bit 5 (Vfull scale +7.5)/2 = 8.75 7.65 <  Bit 5 = 0 1  1  0

Vin = 7.65 MSB  LSB Average high/low limits Compare to Vin Vin > Average  MSB = 1 Vin < Average  MSB = 0 Bit 4 ( )/ 7.65 <  Bit 4 = 0 1  1  0

Vin = 7.65 MSB  LSB Average high/low limits Compare to Vin Vin > Average  MSB = 1 Vin < Average  MSB = 0 Bit 3 ( )/2 = 7.65 <  Bit 3 = 0 1  1  0

Vin = 7.65 MSB  LSB Average high/low limits Compare to Vin Vin > Average  MSB = 1 Vin < Average  MSB = 0 Bit 2 ( )/2 = 7.65 <  Bit 2 = 0 1  1  0

Vin = 7.65 MSB  LSB Average high/low limits Compare to Vin Vin > Average  MSB = 1 Vin < Average  MSB = 0 Bit 1 ( )/2 = 7.65 >  Bit 1 = 1 1  1  0

Vin = 7.65 MSB  LSB Average high/low limits Compare to Vin Vin > Average  MSB = 1 Vin < Average  MSB = 0 Bit 0 ( )/2 = 7.65 >  Bit 0 = 1 1  1  0

Vin = 7.65 = 8-bits, 28 = 256 Digital Output 195/256 = Analog Input 7.65/10 = 0.765 Resolution (Vmax – Vmin)/2n  10/256 = 0.039 Voltage Bit 1  1  0

Binary search for a 3-bit ADC Vref: 5V full scale value D2 D1 D0 Vin= 3.4V 4.375V 3.750V 3.125V 2.500V Vappr 1.875V 1.250V 0.625V 0.000V clock cycle 1 D2 = 1 [Vin > Vappr] clock cycle 2 D1 = 0 [Vin < Vappr] clock cycle 3 D0 = 1 [Vin > Vappr] 3.4 > (5 + 0)/2 = 2.5  Bit 2 = 1 3.4 < ( )/2 = 3.75  Bit 1 = 0 3.4 > ( )/2 =  Bit 0 = 1

Counter Ramp Converter
Counter-ramp converters comprise a D-A converter, a single comparator, a counter, a clock and control logic When a conversion is required a signal (conversion request) is sent to the converter and the counter is reset to zero. The purpose of the sample-and-hold amplifier is to freeze the analogue voltage at the instant the HOLD command is issued and make that analogue voltage available for an extended period. A clock signal increments the counter until the reference voltage generated by the D/A converter is greater than the analogue input At this point in time the output of the comparator goes to a logic 1, which notifies the control logic the conversion has finished encoder input signal digital output The value of the counter is output as the digital value. The time between the start and end of the conversion is known as the conversion time. A drawback of the counter-ramp converter is the length of time required to convert large voltages. A 10 bit a/d converter will require 1024 iterations to resolve the maximum input voltage. The worst case must be assumed when calculating conversion times

Counter Ramp Converter

Integrating ADC Speed: Low Cost: Low Accuracy: High

References: www.ti.com/lit/an/slaa587/slaa587.pdf
1. Understanding Data Converters – SLAA013 2. ADS8318 data sheet – SLAS568A Evaluating High Speed DAC Performance by Walt Kester – Analog Devices MT-013 Tutorial Home > Products and Services > White Papers > Understanding Resolution in High-Speed Digitizers/Oscilloscopes ume.gatech.edu/mechatronics_course/ADC_F10.pptx