18:11 -
Digital system
Write the disadvantage of single slope integrator ADC and how does Dual slope integrator ADC overcome this problem.
Disadvantage of single slope
integrator ADC:
In single-slope integrating ADC on op-amp based circuit, called an
integrator to generate a saw tooth wave form is used, instead of the use
of a DAC with a ramped output.
The disadvantage of a single slope integrator ADC is the calibration
trift dilemma and the solution to this problem is found in a design variation
called the dual-slope converter. In the dual-slope converter, an integrator
circuit is driven positive and negative in alternating cycles to ramp down and
then up, rather than being reset to 0 volts at the end of every cycle.
In one direction of ramping, the integrator is driven by the positive
analog input signal (producing a negative, variable rate of output volta9e
change, or output slope) for a fixed amount of time, as measured by a counter
with a precision frequency clock. Then, in the other direction, with a fixed
reference voltage (producing a fixed rate of output voltage change), with time
measured by the same counter. The counter stops counting when the integrators
output reaches the same voltage as it was when it started the fixed-time
portion of the cycle. The amount of time it takes for the integrator’s
capacitor to discharge back to its original output voltage, as measured by the
magnitude accrued by the counter, becomes the digital output of the ADC
circuit.
Another important advantage of the dual slope integrator method is that
the input signal becomes averaged as it drives the integrator during the fixed-
time portion of the cycle. Any changes in the analog signal during that period
of time have a cumulative effect on the digital output at the end of that
cycle. If the analog signal is “noisy’ (contains significant levels of spurious
voltage spikes/dips), a dual-slope ADC, averages together all the spikes and
dips within the integration period, thus providing an output with greater noise
immunity. Dual-slope ADCs are used in applications demanding high accuracy.