Introduction to DIGITAL INSTRUMENTATION

Digital instrumentation:
 In digital instrumentatio as we have, all the transducers  considered so far have had an analog outcome, that is the outcome  is a different  kind  from  the  views but  its value  is appropriate  to the value of  the views. Why should we go to the real and costs of transferring an analog signal into a digital one in which  the  outcome  quantity  is  personalized  into  a series  of  existing  yearnings  composed  its  statistical equivalent? The objective is precise. Once a quantity has been indicated as a wide range it cannot change  its  value like  an  analog  sign  may  do  while  it  is  being  ready.  Also,  if  the  sign is indicated in binary analysis, the managing can be done numerically in a micro-processor or pc. An analog signal is become a digital one by sampling,

 

•  sampling theorem : that convert an analog signal  into digital samples and convert them back into  original  signal  provided the  number  of  samples  per  second  is  more than  twice  the  highest frequency in the signal.

A  continuous  signal  can  be  represented   by completely    and  rebuit from  a  set  of instantaneous measurements  or  samples  of  its  voltage which  are made  at  equal spaced  times and The interval  between  such  samples  must  be  less  than one-half  of  the  period  of  the highest-frequency component in the signal.

Suppose the signal contains a higher frequency than we expected, so that in fact the number of samples per second is less than twice that frequency. We then get what is known as aliasing; the samples convert back into a false lower frequency.

• Analog to digital conversion?

As we know,The main types of A/D converter:  single slope, dual slope, successive approximation and flash.

We can say that,The sample voltages from the sample-and-hold amplifier are converted into binary numbers by another integrated circuit, an analogue-to-digital converter (usually abbreviated to A/D converter).

• Digital to analog conversion?

 here,It has as many inputs as there are digits in the binary numbers which represent the voltage values.

In digital to analog conversion,after  transmission,  processing,  etc.,  the  digital  signal  may  have  to  be  converted  back  to analog form. This is done by another integrated circuit: a digital-to-analog converter (D/A).

As we know that The D/A converter circuit, known as an R-2R ladder,

Each input operates its own electronic switch (a  logic gate) which connects that particular leg of the ladder to the reference voltage, if the binary digit is a 1, or to earth if it is a 0. If all the legs but one are connected to earth, the one connected to the reference voltage produces a current which flows  towards  the  inverting  input  of the  op-amp  and  is  halved  by  the  resistance  network  at  each junction  through which  it  passes.  Thus  the  current  contribution  of  each  leg  is  weighted  to correspond to the position of that particular digit in the complete binary number; for example, in a 4-bit D/A converter the current produced by the most significant bit (MSB) will be 8 times (2(4-1)=8) the  current  produced  by  the  least  significant  bit  (LSB). an output voltage proportional to the sum of the currents it produces by The op-amp .