Friday, 21 October 2011

Digital Voltmeter - Part II

Ramp Type DVM

The block diagram of the Ramp type ADC is given below. The principle and working of the Ramp type ADC is explained in a simplified manner in this post.
The merits and demerits of the Ramp type ADC is also listed at the end of the post.

Block Diagram:


Input voltage is converted into digital equivalent by counting the time taken for the ramp wave to decrease from the magnitude of input voltage to 0V.


The block diagram of the Ramp-type ADC can be divided into two sections as follows:
      1. Voltage to time conversion section
      2. Time measurement section

In the voltage to time conversion section, the analog input voltage is fed to the attenuation circuit. The attenuated signal is compared with the the ramp signal generated by the ramp generator given in the block diagram by the input comparator 'C1'. Similarly, The ramp signal generated is compared with 0V via a zero-crossing detector 'C2'. A sample rate multivibrator is connected to the ramp generator whose purpose is to provide an initiating pulse for the ramp generator to start the next ramp voltage for the next measurement. It is also used to reset the counter before generating the next ramp voltage.

In the time measurement section, there is counter which is triggered by a gating pulse. The inputs of the gating pulse are (i) Output of 'C1' (ii) Output of 'C2' (iii) Clock pulse from the oscillator. The counter is reset after each successful completion of time measurement by a control signal from the sample rate multivibrator. The count produced is displayed by connecting suitable display device.

Detailed Operation:

Initially, the attenuated signal is compared with a negative going ramp signal generated by the ramp generator. When the ramp voltage coincides with the input signal, the output of 'C1' becomes low. This point is called coincidence point. This initiates the counting process ( start of count ). The counter continues to count until the ramp voltage reduces and crosses zero (0V). This is detected by zero crossing detector 'C2'. The output of 'C2' becomes high which ends the counting process (end of count).

The count displayed is the count of number of clock pulses produced by the oscillator during the time in which the ramp signal is less than the input signal and greater than 0V (ie)  |input signal| > ramp > 0V. This count gives the digital equivalent of input analog voltage.

Equations Involved:

(del)t = t2 - t1= Vin/m = nT ;
 Hence, Vin = nmT;

 t1-> start of count
 t2-> end of count
 Vin-> input analog voltage
 m-> slope of the ramp curve
 n-> number of clock pulses to counter
 T-> clock period

* low cost
* simple, easy to design
* long distance transmission of output pulse is possible

* accuracy of output greatly depends on linearity of the ramp.
   (since only one ramp is used)
* input filter are needed for filtering noise from input signal.


Coming up next:

* Integrating Type DVM
* Successive Approximation type DVM


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