Data Converters

Data ConvertersMuhammad Zaigham Abbas ShahMany sensors and actuators are analog!

Strain GaugeThermocouple Solenoid Valve

ThermistorsIn order to use these with digital devices, we need a way to make digital data in to analog and the other way roundDigital to Analog Converters(DACs)A device used to convert a Digital Number(binary 0s and 1s) an Analog Voltage or Current.

It scales the analog output to zero when all inputs are zero and vice versa, mathematically

where Vout is the Analog output voltage, Vref is the reference voltage which sets the maximum output voltage for the DAC

The output equation can also be written as

The term is also called the Resolution which is the analog voltage per bit increment, mathematically

The smaller the reference voltage and greater the number of bits of the DAC, better is the resolution

Output voltage can be negative as well as positive

Uses offset binary for the output

Bipolar DACsHigh SpeedBinary Weighted DACSegmented DAC (wont be discussed)

High ResolutionR-2R DACPWM DAC (wont be discussed)Integrating DAC (wont be discussed)Weighted Capacitor DAC (wont be discussed)Delta Sigma DAC (wont be discussed)

DAC structuresDACBinary Weighted (R/2nR) DAC A variation of the opamp summer amplifier

The resistors are weightedand represent bits of the input digital number

R/2R DACThe Binary Weighted DAC required resistors with many unique values(one per bit) thus manufacturing is a problem

The R/2R DAC eliminates this spread in resistor values by using a R/2R ladder network

Manufacturing becomes a lot easier

A DAC ICA DAC IC consists of the following pins:

Digital I/p Lines: To give the digital no to the DACVref: To provide the Reference voltageVout: Analog output voltage pinV+,Gnd,V-: Power pinsOffset: To make offset zero

Data Latch: To latch the digital input so as to have a steady output

Conversion Time: The time the DAC takes toConvert a signal form Digital to Analog

ComparatorUsed to perform comparison of two analog voltages

Utilizes the very high open loop voltage gain of the opamp. Due to the high gain of the opamp, a slightdifference between the input terminals results in the outputsaturating to either the positive supply(vcc) or the negative supply(vee)

One of the inputs is given the input voltage and the other is given a reference voltage

9Where does the simple comparator fail? The problem with the simple analog comparator is that in a noisy environment, when the input signal crosses the threshold level, the output might have multiple transitions

Use positive feedback to add hysteresis i-e have multiplethreshold values, a lower threshold and an upper threshold

What to do?

Images: Art of Electronics, HorrowitzSchmitts Trigger/ Hysteresis ComparatorUses positive feedback to define two threshold values

Introduces noise immunity to comparators

When the input voltage comes to avalue with in the hysteresis window, the output maintains its previous stateunless it goes below the lower threshold value(making the output go to the ve supply rail) or goes above the upper threshold level(making the output go to the +ve supply rail)

When the output is at ground(i-e the input voltage is greater than the reference voltage[Vref]) , Vref is given by the equation

If the output is at Vcc(i-e the input voltage is less than Vref), Vref is given by the equation

Thus, the additional voltage, increases Vref required to bring change thus ensuring that there are no transitions due to noise

Hint: Use Superposition theorem toget this equation Reading: Application Note: Hysteresis in ComparatorsDigital Signal ConditioningConvert the signal in to a form compatible with the system

Most sensors have an analog output

Convert an Analog voltage in to a Digital number (sampling+quantisation+coding)

It finds a binary number which is the closest approximation to the fraction formed by the input voltage and the reference

Analog to Digital Converters (ADCs)

ADC StructuresSome typical structures

High ResolutionParallel-Feedback(Successive Approximation) ADCRamp ADCHigh SpeedFlash ADC (wont be discussed)Pipeline ADC (wont be discussed)Sub-range ADC (wont be discussed)Folding ADC (wont be discussed)Delta Sigma ADC (wont be discussed)

Parallel-Feedback (Successive Approximation) ADC:

Consists of a comparator, a DAC and a Logic counting network (Successive Approximation Register)

The comparator compares the input voltage and drives the Logic counter network, the output of which is connected to DAC.

The output of the DAC is connected to the comparatorand serves as a reference

The comparator indicateswhen the two signals are same or with in a resolution of the input voltage thusstopping the Logic counting network

Images: Process Control Instrumentation Technology, Curtis D. JohnsonRamp(Integrating) ADC

Eliminates the DAC in ADCs

Consists of a an Opamp integrator which produces a sawtooth waveform. This is compared against an analog input by a comparator. The time it takes for the sawtooth wave to exceed the input voltage is measured by means of a counter timed from a crystal oscillator. The output of the counter is the digital representation of the input voltage

The single slope ramp ADC suffers from calibration drift , a better version is the Dual slope ramp ADC discussed nextExplain that the charge time of the capacitor in the integrator can be related to the input voltage17In the Dual slope RampADC, the integrator is first switched to a negative input voltage(producing an increasing ramp) for time T1, giving the outputor,since Vx is constant, After T1, the input switches to the Vref which is positive hence producing a negative ramp. The voltage at the output decreases as follows

or

A counter is started at the end of T1 and it counts till the comparator indicates V2 to be zero. At this time, Vx is given by

Thus the time for which the counter is counts(tx) is proportional to the input voltage (Vx).

As observed the output expression is free from R and C

TasksIt is required to operate a solenoid valve using a microcontroller based system. The solenoid valve has two opening positions A and B, the valve opens at position A at a voltage of 0.4125V (scaled) and at position B at a voltage of 2.0625V (scaled). Design a binary weighted DAC that can be used to perform this task and also simulate on Proteus. (Hint: Choose a resolution of 3-bits)

Read up on the following terminologies for Data Converters:ResolutionQuantization errorSamplingSpurious Free Dynamic Range

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