Multidecade BCD DAC uses resistors of only six values
DAC uses fewer resistors than that of a previous Design Idea.
Marián Stofka, Slovak University of Technology, Bratislava, Slovakia; Edited by Martin Rowe and Fran Granville -- EDN, February 4, 2010
A previous Design Idea uses a three-decade BCD (binary-coded-decimal) DAC to precisely set the output current of a current source (Reference 1). The circuit acts as a code-to-conductivity converter. The values of resistors of this DAC are staggered by powers of two within any of the decades, and the values of resistors at corresponding bits of the decades are staggered by powers of 10. Thus, the circuit needs 12 values of resistors, ranging from 125Ω to 100 kΩ.
In comparison, the circuit in this Design Idea enables you to construct a BCD DAC using only six resistor values, regardless of the number of decades. Moreover, these six resistor values vary within a relatively narrow 1:8 range. The voltage-output DAC operates ratiometrically. That is, if the temperature coefficients of the resistors are approximately the same—and you can assume they will be within this narrow range of values—then the variation of resistance with temperature has almost no detrimental effect on accuracy. This situation is not true, however, for code-to-conductivity DACs, in which the temperature coefficient of the resistors directly influences the temperature coefficient of the DAC.
| Read More Design Ideas |
Figure 1 shows the voltage-output BCD DAC. The values of resistors are staggered by powers of two within each decade. The values of resistors at corresponding bits of the decades are of equal values. The switched ends of the resistors connect to either ground at logic zero or to the reference voltage at logic one. The voltage-output DAC thus has an advantage in that all the resistors’ ends are at a defined potential. In a code-to-conductivity converter, on the other hand, one end of the resistor remains open at logic zero, and these open ends might act as capacitive sensors or even antennas, which could introduce additional errors. The common ends of four resistors in four bits of the MSD (most-significant decade) form the output. The common outputs of resistor quads in the less-significant decades successively connect to the main output through the series resistors, RS, which all have the same value. Thus, RS=(108/25)R=4.32R, where R is the value of the resistor at the MSB (most-significant bit) of any of the decades.
The common ends of bit resistors in the LSD (least-significant decade) connect to ground through terminating resistor RT. This resistor represents the theoretically infinite number of decades having weight lower than the actual LSD, whereas these hypothetical decades are all set to zero. Thus, they contribute no voltage at the output. They do, however, influence the properties of the resistive network. RT sets this influence and is equal to (24/5)R, or 4.8R. The full-scale output of the voltage-output BCD DAC is 3/5×(1–10–N)VREF, where N is the number of decades—in this case, three.
To exploit a voltage-output BCD DAC in a single-supply programmable-current source, connect the output of the voltage-output BCD DAC to the noninverting input of an op amp, which accepts input voltages as low as 0V. The inverting input of the op amp connects to ground through resistor RB, which has the value of (3/5)×(VREF/10–²A).
| Reference |
|
-
Thank you for further improvement of our circuit.
The authors of the reference.
Gyula Diószegi - 2010-11-2 02:17:00 PST
