Part Number:LM98640QML-SP
Hi,
I am considering the LM98640QML-SP for use in S/H mode with DC coupling of the inputs. I have a question regarding the symmetry of the input ports in S/H mode.
The datasheet seems to assume that the incoming video signal is always referenced to the top of its voltage range. For instance, in section 7.3.1.1, it states that "The Reference level must be connected to the OSx+ pin, and the Video Level signal connected to the OSx- pin".
In our system, the video signal spans 1-3V, but the reference level signal is at 1V, not 3V. The statement from section 7.3.1.1 seems to require a 3V reference level signal. However, if I'm understanding the internal architecture correctly, that statement is not actually a requirement. I believe the inputs are symmetric (so long as VCLP is not used), and either input can be used as the reference.
Figure 17 shows the input structure. In S/H mode, the static switch should be closed. If I don't use VCLP, then the grey, optional switch would be open. Finally, in S/H mode, I would anticipate that the SAMPLE and CLAMP switches would close and open simultaneously. Section 7.3.1.1.1 seems to imply that the SAMPLE and CLAMP signals are identical in S/H mode, but it does not explicitly state that.
If indeed the SAMPLE and CLAMP signals are identical in S/H mode, then I don't see any reason why I couldn't apply the 1V reference level signal to OSx- and the video signal to OSx+. Is that correct? Am I misunderstanding anything?
Thank you for your help,
Bradley