1. Digital to analog converter [DAC]
Digital to analog converter translates a digital signal into an analog signal to represent a physical quantity.
A n-bit DAC has n input lines to have 2n input combination and an analog output line.

2. Digital to Analog function

3. Resolution
Ability of data acquisition system to distinguish between adjacent analog input levels.
n- bit system should be capable to distinguishing between input levels that differ by 1/2n of full scale range

4. LSB (Resolution) of DAC
LSB (all bits are 0’s except the least significant bit is 1) is the smallest unit and is equivalent to ½n of the full-scale analog voltage.
This is also known as resolution of DAC.

5. MSB of DAC
The MSB (all bits are 0’s except the most significant bit is 1) represents the ½ of the full-scale value.

6. Full-scale output
It is the output signal corresponding to the maximum input signal (all bits are 1’s).

7. Full-scale Analog voltage
The full-scale analog voltage is equal to the value of the sum of the full-scale output and 1LSB input signal.

8. Bit-weight
It is the ratio between the minimum output voltage for which the digital inputs are all 1’s and the maximum count.

9. Components of a DAC Summing network of input resisters created out of
binary weighted input resistors, or
R2R ladder network
Operational amplifier.

10. Binary weighted input resistors network
Input resistors - each has double the value of the previous resistor.
The total output current is
Io = It = I1+ I2+ I3 + …
= Vin/R1 + Vin/R2 + Vin/R3 + …
= (1/2+1/4+1/8…) Vin/R
In general the output current is
Io =(A1/2+ A2/4+ A3/8…+ An/2n) Vref/R
The voltage output Vo = -RfIt .
Drawbacks:
Requirement of precision resistors

11. R2R ladder network It uses only two resistor values, R and 2R
The resistors are connected in such a way that for any number of inputs, the total current It is in binary proportion.

12. Components of a typical Digital to Analog Converter IC
It consists of an octal latch, R-2R ladder network, CMOS switches and op-amp as current to voltage converter.

13. DAC AD558: Pinout diagram
AD558 is a bus compatible commercial 8 bit DAC.

14. Analog to digital converter [ADC]
Analog to digital converter represents an analog signal by equivalent binary state.

15. Full scale output
This is the input value of analog input for which the digital equivalent saturates to all 1’s.

16. Conversion time
The total time required to convert an analog signal into its digital output . It is determined by conversion technique and propagation delay in various circuits.

17. Graph of the analog input voltage and digital output signal

18. Types of ADC Successive approximation Integrating type, Flash type,
Counter.

19. Successive Approximation A/D Components
D/A converter,
Successive approximation register,
Comparator.

20. Block Diagram

21. Working principle
The output of DAC Vo is compared with the analog input signal,
The Successive approximation register generates the digital input to the DAC,
The DAC output is the equivalent digital signal when the DAC output matches the analog signal.

22. ADC IC AD0801 Steps for data conversion
To start conversion the CS/ and WR/ signals are made low
When conversion is completed the INTR/ becomes low and
Data is placed on the output line
The INTR/ line can be used to interrupt the processor
The processor reads the data by asserting RD/,
The INTR/ is reset.
Pinout diagram

23. Instruction codes OUT Port8 ;Start ADC ( a dummy write
;operation on ADC)
NOPs ;allow for conversion time
IN Port ;read the digitized value
HLT

24. Timing diagram for reading data from ADC