|Supply||Attenuated Input Signal|
The signal recovery circuit is used in digital systems to retrieve distorted clock or data waveforms. These clock and data signals can be attenuated and distorted on long traces due to stray capacitance, stray inductance, or reflections on transmission lines. The comparator is used to sense the attenuated and distorted input signal and convert it to a full scale digital output signal. A dynamic reference voltage will be connected to the inverting terminal of the comparator which is extracting the common-mode voltage from the input signal.
A ramification to consider when balancing the accurate filtering of the signal versus is the start-up time. As the system starts in an uncharged state, once the system is active, there is a time period (around 5 ) until the voltage level at the inverting input is at an accurate level.
Is this comparator fast enough for this input signal?
Toggle frequency, fToggle, is the metric that measures how fast a comparator can handle input signal speeds. This metric is measured as the input-signal frequency at which the output swing is a certain percentage compared to itself at low-input signal frequencies. The percentage varies by manufacturers and even by products, so it is important to check the data sheet of the part to see how this parameter is being met.
When fToggle is not included in data sheet of a part, there may be some concern as to whether that part is suitable for use in a system. In that case, here is a general approximation to gauge fToggle of the part:
It is important to note that this approximation is conservative and may not completely match a part's fToggle inside a data sheet if specified, especially when evaluating higher speed comparators as these tend to be multi-stage comparators. Using the values included in TLV3603 data sheet:
While the data sheet states that the toggle frequency is 325MHz, this approximation indicates that this product only handles 173.9MHz and lower signals. Why is this the case? This can be due to multiple factors, but an important consideration must be made when evaluating single (or near-single) stage products versus multi-stage products.
When using a near-single stage comparator, the input signal read by the comparator needs to pass through a low number of stages until its output transitions. fToggle is dependent on the stage with the longest propagation delay in the chain (whether that chain be one or multiple stages), rather than passing all the way through to the output before the next bit is fed in.
In the previous diagram, an input signal consisting of bits 1, 2, and 3 are both fed into a single stage comparator and a multi-stage comparator. The single stage comparator only has stage A, while the multi-stage comparator consists of stages A, B, and C. When bit 1 enters both comparators, it takes a period of time to get through stage A. Once it gets past stage A, on the single stage comparator, it reaches the output while on the multi-stage comparator, it enters stage B. At that point, bit 2 can begin to enter stage A. After another period of time, bit 2 reflects on the single stage output while also entering Stage B of the multi-stage comparator. Bit 1, at this point, begins to enter stage C.
This illustrates that while the propagation time may differ between a multi-stage and single stage comparator (it may be smaller, larger, or nearly the same depending on the stages), the rate at which each comparator handles these signals is dependent on when the bit clears the stage with the greatest propagation delay so that the next bit can come through the pipeline.
Transient Simulation Results
See Analog Engineer's Circuit Cookbooks for TI's comprehensive circuit library.
See circuit spice simulation file, SNOM712.
For more information on many comparator topics including hysteresis, propagation delay and input common mode range please see, TI Precision Labs.
Design Featured Comparator
|Vss||2.4V to 5.5V|
Design Alternate Comparator