
Digital to Analogue converterThe Circuit/PCBBelow 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 8bit D/A converter .This part of the design included two chips that are DAC0808 and LF351. The DAC 0808 is an 8bit 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 8bit digital input is loaded onto input lines A1 to A8 of the DAC0808. An 8bit 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 AnalogOpAmp Summing JunctionThe simplest DACs are derived from opamps 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 V_{1} and going through V_{3}, this would give each input voltage exactly half the effect on the output as the voltage before it. In other words, input voltage V_{1} has a 1:1 effect on the output voltage (gain of 1), while input voltage V_{2} has half that much effect on the output (a gain of 1/2), and V_{3} 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 V_{1} = V_{2} = V_{3} = 3 Volt; so V_{out} = 3 x (D_{1} + D_{2} /2 + D_{3} /4 ); where D_{i} = input i (on=1; off=0) This gives the following output table
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 4bit DAC (2^4=16) each step would be approx 2 Volts on the output scale etc. Output Errors of this DAC method 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 poweroftwo 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 ladderlike 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 6bit 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. The fullscale output is less than Vr for all practical R/2R ladders, and for low pin count devices the fullscale output voltage can be significantly below the value of Vr. An R/2R ladder of 4 bits would have a fullscale 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 fullscale 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
Integrated Circuit DACThe R2R ladder concept is integrated in the DAC0808 IC.
The DAC0808 (Conrad) is an 8bit monolithic digitaltoanalog converter (DAC) featuring a full scale output current settling time of 150 ns while dissipating only 33 mW with ±5V supplies. No reference current (I_{REF}) trimming is required for most applications since the full scale output current is typically ±1 LSB of 255 I_{REF}/256. Relative accuracies of better than ±0.19% assure 8bit monotonicity and linearity while zero level output current of less than 4 µA provides 8bit zero accuracy for I_{REF}>=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 pinlayout 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 8bit D/A converter .This part of the design included two chips that are DAC0808 and LF351. The DAC 0808 is an 8bit 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 8bit digital input is loaded onto input lines A1 to A8 of the DAC0808. An 8bit 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

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