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# LTC2625 voltage head room with respect to GND

In the datasheet, voltage headromm is described as follows.

When drawing a load current from either rail, the output
voltage headroom with respect to that rail is limited by
the 30Ω typical channel resistance of the output devices;
e.g., when sinking 1mA, the minimum output voltage =
30Ω • 1mA = 30mV. See the graph Headroom at Rails vs
Output Current in the Typical Performance Characteristics
section.

what is the maximum value of the output headroom to GND, when the output sink the current?

• Each of the eight rail-to-rail amplifiers contained in these parts has guaranteed load regulation when sourcing or sinking up to 15mA at 5V (7.5mA at 3V).

So 30ohm * 15mA = 450mV at 5V or 30*7.5mA = 225mV at 3V.

• Hi Clarence

My question is that 30 ohm is typical value and how much it's maximum value looks like.

• The 30ohms number is a typical value for the output resistance, we don't have a min/max for that.  We do specify the max for the load regulation which is a better spec to follow:

If you know the LSBs and current draw it is easy to back out the numbers for what you are interested in.

• Hi Clarence

the load regulation is defined when Vout is Mid-scale. so, it does NOT tell us the maximum headroom.

And the maximum headroom limits vout range.

• Yes, it is measured at mid scale.  But if you design with the mid-scale numbers you will give yourself enough margin for the typical numbers you would expect from the part, even at full scale.

• This question has been assumed as answered either offline via email or with a multi-part answer. This question has now been closed out. If you have an inquiry related to this topic please post a new question in the applicable product forum.

Thank you,