LMH7322_14 Datasheet, PDF (19/33 Pages) Texas Instruments – Dual 700 ps High Speed Comparator with RSPECL Outputs

LMH7322
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SNOSAU8H – MARCH 2007 – REVISED MAY 2011
If Δt_PD is not zero, duty cycle distortion will occur. For example when applying a symmetrical waveform (e.g. a
sinewave) at the input, it is expected that the comparator will produce a symmetrical square wave at the output
with a duty cycle of 50%. When t_PDH and t_PDL are different, the duty cycle of the output signal will not remain at
50%, but will be increased or decreased. In addition to the propagation delay parameters for single ended
outputs discussed before, there are other parameters in the case of complementary outputs. These parameters
describe the delay from input to each of the outputs and the difference between both delay times (See
Figure 27.) When the differential input signal crosses the reference level from L to H, both outputs will switch to
their new state with some delay. This is defined as t_PDH for the Q output and t_PDL for the Q output, while the
difference between both signals is defined as Δt_PDLH. Similar definitions for the falling slope of the input signal
can be seen in Figure 19.
t_PDH
V_REF
time
V_O
time
't_PDLH
t_PDL
V_O
time
Figure 27. t_PD with Complementary Outputs
Both output circuits should be symmetrical. At the moment one output is switching ‘on’ the other is switching ‘off’
with ideally no skew between both outputs. The design of the LMH7322 is optimized so that this timing difference
is minimized. The propagation delay, t_PD, is defined as the average delay of both outputs at both slopes: (t_PDLH +
t_PDHL)/2.
Both overdrive and starting point should be equally divided around the V_REF (absolute values).
Dispersion
There are several circumstances that will produce a variation of the propagation delay time. This effect is called
dispersion.
Amplitude Overdrive Dispersion
One of the parameters that causes dispersion is the amplitude variation of the input signal. Figure 28 shows the
dispersion due to a variation of the input overdrive voltage. The overdrive is defined as the ‘go to’ differential
voltage applied to the inputs. Figure 28 shows the impact it has on the propagation delay time if the overdrive is
varied from 10 mV to 100 mV. This parameter is measured with a constant slew rate of the input signal.
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