Digital to Analog Convertor

[Pardeep-singh]

How can i shortlist a DAC, What parameters need to consider defyning resolution

 

[sapna.dudani@gmail.com]

The resolution of the converter indicates the number of discrete values it can produce over the range of analog values. The values are usually stored electronically in binary form, so the resolution is usually expressed in bits. In consequence, the number of discrete values available, or "levels", is a power of two. For example, an ADC with a resolution of 8 bits can encode an analog input to one in 256 different levels, since 28 = 256. The values can represent the ranges from 0 to 255 (i.e. unsigned integer) or from −128 to 127 (i.e. signed integer), depending on the application.
Resolution can also be defined electrically, and expressed in volts. The minimum change in voltage required to guarantee a change in the output code level is called the least significant bit (LSB) voltage. The resolution Q of the ADC is equal to the LSB voltage. The voltage resolution of an ADC is equal to its overall voltage measurement range divided by the number of discrete voltage intervals:

where N is the number of voltage intervals and EFSR is the full scale voltage range. EFSR is given by

where VRefHi and VRefLow are the upper and lower extremes, respectively, of the voltages that can be coded.
Normally, the number of voltage intervals is given by

where M is the ADC's resolution in bits.
That is, one voltage interval is assigned per code level.
Example:
Coding scheme as in figure 1
Full scale measurement range = 0 to 10 volts
ADC resolution is 12 bits: 212 = 4096 quantization levels (codes)
ADC voltage resolution, Q = (10 V − 0 V) / 4096 = 10 V / 4096 ≈ 0.00244 V ≈ 2.44 mV.
In practice, the useful resolution of a converter is limited by the best signal-to-noise ratio (SNR) that can be achieved for a digitized signal. An ADC can resolve a signal to only a certain number of bits of resolution, called the effective number of bits (ENOB). One effective bit of resolution changes the signal-to-noise ratio of the digitized signal by 6 dB, if the resolution is limited by the ADC. If a preamplifier has been used prior to A/D conversion, the noise introduced by the amplifier can be an important contributing factor towards the overall SNR.

---------- Post added at 07:40 ---------- Previous post was at 07:39 ----------

similarly we can calculate for a DAC

 

[Pardeep-singh]

Thanks for your views, but say i want to use Dac fpr controlling motor whose specification is 0 - 10000 rpm, 1% step. so there hw to choose DAC

 

[tejashs]

if you are refering to a dc motor, a dac is not of much use, you need to make a H bridge and control its speed using PWM

 

[FvM]

Assuming you have a motor speed control which gets an analog setpoint, a 7-Bit DAC would be sufficient to achieve 1 % resolution. Available DACs will have 8-Bit or better, so any one will fit. As mentioned, there are other possible methods to control a motor, e.g. PWM. Supplying a voltage to the terminals of a DC motor doesn't exactly set the speed, there will be a certain speed drop depending on the load torque. So what's your concept to control the motor speed? Which circuit is supplying the motor? Do you have a speed sensor?