Sunday 17 March 2013
RAMP TYPE DVM
Do you like this story?
OPERATION
This DVM measures unknown voltage VM by measuring
the time taken by a linearly falling ramp voltage to fall from unknown voltage
level to zero volts
Ramp generator produces a decreasing OR increasing ramp
Input comparator generates starting pulse to open gate
circuit
Output generator produces stop pulse to close gate circuit
Clock generator produces pulses
Decade counter calculates number of pulses that pass through
it
AT INPUT COMPARATOR
An unknown voltage VM is given to the input of
non inverting terminal. When the input at inverting terminal of op-amp is less
than VM out at the input comparator is positive otherwise it is negative.
When VR decreases below VM it produces a positive going pulse,
this pulse makes the gating circuit to start and the pulse produced by the
clock generator passes through the gating circuit. These pulses are counted by
decade counter
AT GROUND COMPARATOR
Ground comparator or zero level detector .so when the VR crosses zero and moves to negative values, the input at inverting terminal is greater than non inverting terminal. So output of this comparator produces a negative going pulse which makes the gating circuit to stop. Therefore the reading displayed on the display is the unknown voltage.
Ground comparator or zero level detector .so when the VR crosses zero and moves to negative values, the input at inverting terminal is greater than non inverting terminal. So output of this comparator produces a negative going pulse which makes the gating circuit to stop. Therefore the reading displayed on the display is the unknown voltage.
VOLTAGE TO TIME CONVERSION
VR ------------TR
VM ----------TM
VM = ( VR / TR )*TM
Time period measurement
Let TCLK be time
period of clock generator. Let ‘n ‘ be the number of clock pulses between T1
and T2
T2 – T1 = TM = n* TCLK
Substitute the value of TM in VM we get
VM = ( VR / TR )* n *TCLK
FEATURES OF RAMP TYPE DVM
1)CONVERSION TIME
TCONV = TM = nTCLK
Where n is proportional to | VM | because the number of pulses depends upon VM
Where n is proportional to | VM | because the number of pulses depends upon VM
2) NOISE REJECTION
If either AC signal or noise is superimposed on unknown DC
voltage then error is introduced in voltage measurement. So noise affect is more.
Therefore noise rejection is poor. Since stability of DVM depends on noise condition.
Its stability is poor.
3) ACCURACY
Value Internal components of ramp generator changes due to
aging then linearity of ramp changes then VM also changes as shown
in the figure and accuracy of measurement is affected.
This post was written by: Author Name
Author description goes here. Author description goes here. Follow him on Twitter
Subscribe to:
Post Comments (Atom)
0 Responses to “RAMP TYPE DVM”
Post a Comment