Hi,
im trying to match impedance on my circuit and wanted to know how much impedance there is on output Y (SN74LVC1G08DBV)...didn´t find something in the Datasheet...so is it near 0 ohm?
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Hi,
im trying to match impedance on my circuit and wanted to know how much impedance there is on output Y (SN74LVC1G08DBV)...didn´t find something in the Datasheet...so is it near 0 ohm?
You can derive the worst-case output impedance from V_{OL} and I_{OL}.
The typical output impedance is roughly half that.
Thank you for the answer. So to fully understand it: In my case V_OL is 0.55V and I_OL = 32 mA. Than I get a impedance of roughly 17 ohm --> half of that is 8.5 ohm. Is that what you meant?
Why is the output impedance half?
Hi Viktor,
It just happens to work out that way. The datasheet value of 0.55V provided is a maximum limit, which means that you should _never_ see this device actually output 0.55V, including variations from the manufacturing process and across the full temperature range. We usually guard band these specs a bit on top of them being measured across all possible variations of the device to avoid any possible liability for any outlier devices.
Most devices you will find have a typical output resistance about half of what is shown in the datasheet for the 'worst case' value. If you're designing a system, it's best to design to the worst case value, but if you want to know "about how a device performs" then the typical value is more useful.
Hi Emrys,
and what about the case when the output is high....so in the datasheet V_OH = min. 3.8V (32mA)...than I have roughtly 118 ohm. Is it here also half of the calculated value (59 ohm)? In conclusion, I will get a impedance of 8.5 ohm at low level and 59 ohm impedance on high level?
Thank you for your time!
Hi Viktor,
In the high state, the voltage across the output transistor is VCC - VOH, which in this case is 0.7V, and the resistance (max) is 21.875 ohms. That's about 11 ohms typical.
To help clarify, here's a schematic for a typical CMOS output structure: