Baron 58 Cockpit Simulator

Home Up Feedback Contents

 

Up

 

Digital to Analogue converter 

The Circuit/PCB

Below the final result of the 3 x 5 Bits DAC; The 3 DAC0808 IC's are clearly visible. 

 

The first part of the DAC design that we use for our D/A converter was taken from the Typical Application of the DAC0808 8-bit D/A converter .This part of the design included two chips that are DAC0808 and LF351. The DAC 0808 is an 8-bit D/A converter and the LF351 is a JFET input operational amplifier. Both of the DAC0806 and LF351 chips are made by National Semiconductor and work as follows. An 8-bit digital input is loaded onto input lines A1 to A8 of the DAC0808. An 8-bit data word range from 00000000 to 11111111 binary that is 0 to 255 in decimal. The DAC0808 converts this bit data into analog currents Iout. The LF351 then converts these currents into analog voltages. The resulting voltage is equal to Vout. 

The diagram above is the schematic wiring diagram of the DAC

Background info: Digital to Analog

Op-Amp Summing Junction

The simplest DACs are derived from op-amps with different amplification ratios for different bits. The DAC will be used to steer the Analogue Bus Volts and Left and Right Alternator Ammeters; a high resolution/ accuracy is not require, so a simple DAC will do.

Suppose we were to set the input resistor values at multiple powers of two: R, 2R, and 4R (picture below)

Starting from V1 and going through V3, this would give each input voltage exactly half the effect on the output as the voltage before it. In other words, input voltage V1 has a 1:1 effect on the output voltage (gain of 1), while input voltage V2 has half that much effect on the output (a gain of 1/2), and V3 half of that (a gain of 1/4). These ratios are were not arbitrarily chosen: they are the same ratios corresponding to place weights in the binary numeration system. If we drive the inputs of this circuit with digital gates so that each input is either 0 volts or full supply voltage, the output voltage will be an analog representation of the binary value of these three bits. In our case V1 = V2 = V3  = 3 Volt; so 

Vout = 3 x (D1 + D2 /2 + D3 /4 ); where Di = input i (on=1; off=0)

This gives the following output table

D1 D2 D3  Vout
0 0 0 0.00
0 0 1 0.75
0 1 0 1.50
1 0 0 3.00
0 1 1 2.25
1 0 1 3.75
1 1 0 4.50
1 1 1 5.25

The range for the scale for the bus volts is 0 to 30 Volts; so with a 3 bit DAC, each step is 30/8=3,75 Volt on the output scale

If we would use a 4-bit DAC (2^4=16) each step would be approx 2 Volts on the output scale etc.

Output Errors of this DAC method
• Dynamic range can be insufficient.
• Nonmonotonic behavior
• Nonlinear behavior
• Scale error

ref: http://www.physics.niu.edu/~labelec/lect/p475_lect231.pdf 

If we wish to expand the resolution of this DAC (add more bits to the input), all we need to do is add more input resistors, holding to the same power-of-two sequence of values:

It should be noted that all logic gates must output exactly the same voltages when in the "high" state. If one gate is outputting +5.02 volts for a "high" while another is outputting only +4.86 volts, the analog output of the DAC will be adversely affected. Likewise, all "low" voltage levels should be identical between gates, ideally 0.00 volts exactly. It is recommended that CMOS output gates are used, and that input/feedback resistor values are chosen so as to minimize the amount of current each gate has to source or sink.

 

The DAC uses an R/2R ladder. This R/2R ladder network provides a simple means to convert digital information to an analogue output. Figure 1 is a diagram of the basic R/2R ladder network with N bits. The “ladder” portrayal comes from the ladder-like topology of the network. Note that the network consists of only two resistor values; R and 2R (twice the value of R) no matter how many bits make up the ladder. The 

particular value of R is not critical to the function of the R/2R ladder.

      

I made a test circuit to test the ladder, I used 100 Ohm resistors and used 6 inputs (so this is a 6-bit Digital to Analogue converter (DAC).

  

 Digital information is presented to the ladder as individual bits of a digital word switched between a reference voltage (Vr)(in my case 5 Volt) and ground. Depending on the number and location of the bits switched to Vr or ground, Vout will vary between 0 volts and Vr. 
  - If all inputs are connected to ground, 0 volts is produced at the output, 
  - if all inputs are connected to Vr, the output voltage approaches Vr, and 
  - if some inputs are connected to ground and some to Vr then an output voltage between 0 volts and Vr occurs. 
These inputs (also called bits in the digital lingo) range from the Most Significant Bit to the Least Significant Bit. As the names indicate, the MSB, (D0) when activated, causes the greatest change in the output voltage and the LSB, when activated, will cause the smallest change in the output voltage. If we label the bits (or inputs) bit 1 to bit N the output voltage caused by connecting a particular bit to Vr with all other bits grounded is: Vout = Vr/2N where N is the bit number. For bit 1, Vout =Vr/2, for bit 2, Vout = Vr/4 etc. The table shows the effect of individual bit locations to the Nth bit. Notice that since bit 1 has the greatest effect on the output voltage it is designated the Most Significant Bit. Since an R/2R ladder is a linear circuit, we can apply the principle of superposition to calculate Vout. The expected output voltage is calculated by summing the effect of all bits connected to Vr. For example, if bits 1 and 3 are connected to Vr with all other inputs grounded, the output voltage is calculated by: Vout = (Vr/2)+(Vr/8) which reduces to Vout = 5Vr/8. The R/2R ladder is a binary circuit. The effect of each successive bit approaching the LSB is 1/2 of the previous bit. If this sequence is extended to a ladder of infinite bits, the effect of the LSB on Vout approaches 0. 

The full-scale output is less than Vr for all practical R/2R ladders, and for low pin count devices the full-scale output voltage can be significantly below the value of Vr.  An R/2R ladder of 4 bits would have a full-scale output voltage of 1/2 +1/4 + 1/8 + 1/16 = 15Vr/16 or 0.9375 volts (if Vr=1 volt) while a 10 bit R/2R ladder would have a full-scale output voltage of 0.99902 (if Vr=1 volt).

Picture below shows the output Voltage with the Bits D0, D1 and D4 'on': this gives a mathematical output Voltage of

V out = Vr/2 + Vr/4 + Vr/ 32 (with Vr = 5V)

V out = 2,5   + 1,25 + 0,15 = 3,95; the voltage gauge indicates 3,93 Volt 

Back

Integrated Circuit DAC 

The R2R ladder concept is integrated in the DAC0808 IC.

                                                 

The DAC0808 (Conrad) is an 8-bit monolithic digital-to-analog converter (DAC) featuring a full scale output current settling time of 150 ns while dissipating only 33 mW with ±5V supplies. No reference current (IREF) trimming is required for most applications since the full scale output current is typically ±1 LSB of 255 IREF/256. Relative accuracies of better than ±0.19% assure 8-bit monotonicity and linearity while zero level output current of less than 4 µA provides 8-bit zero accuracy for IREF>=2 mA. The power supply currents of the DAC0808 is independent of bit codes, and exhibits essentially constant device characteristics over the entire supply voltage range.

Above: Block Diagram and pin-lay-out of the 0808

The first part of the DAC design that we use for our D/A converter was taken from the Typical Application of the DAC0808 8-bit D/A converter .This part of the design included two chips that are DAC0808 and LF351. The DAC 0808 is an 8-bit D/A converter and the LF351 is a JFET input operational amplifier. Both of the DAC0806 and LF351 chips are made by National Semiconductor and work as follows. An 8-bit digital input is loaded onto input lines A1 to A8 of the DAC0808. An 8-bit data word range from 00000000 to 11111111 binary that is 0 to 255 in decimal. The DAC0808 converts this bit data into analog currents Iout. The LF351 then converts these currents into analog voltages. The resulting voltage is equal to Vout. 

The diagram above is the schematic wiring diagram of the DAC

Back

 

 

Send mail to info@baron58 dot com  with questions or comments about this web site.