Friday 1 March 2013

18:11 -

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.