AD8275 rail-to-rail output ?

The AD8275 is a level translator with rail-to-rail output.

But if this is true why is there an output swing ?

OUTPUT CHARACTERISTICS

Output Swing

(VREF2 = 4.096 V, REF1 and RL connected to GND, RL = 2 kΩ)

−VS + 0.048                     +VS − 0.1

I want to use this level translator with +Vs = 5V, -Vs = GND, Ref1=Ref2 = 2,048 V, RL = 8kOhm.

The calculated output for -10V input is exactly 0,048 V

The design includes a LM7705 to generate the negative supply voltage (-0,232) for the ADC.

I never found a application with -Vs of the AD8275 connected to a negative supply voltage.

Is it allowed to connect the -0,232V fron the LM7705 to the -Vs Pin of the AD8275 ?

If the answer is yes, then I can avoid to scratch with the negative output swing the not really rail-to-rail output limitation.

  • 0
    •  Analog Employees 
    on Aug 4, 2015 8:35 PM

    Hello Klaus,

    Rail-to-rail is an output stage topology, not a description. Let me explain some of the history.

    Originally, IC op amps had to make do with NPN pull-ups and resistive or NPN current source pull-downs on the output stage. But when complimentary bipolar processes became available, op amps adopted a push-pull output stage, consisting of an NPN pull-up transistor and a PNP pull-down transistor, which provided several advantages. The output was taken from the combined emitter, which follows the base voltage. Therefore, they could get no closer than a Vbe from either rail, and might need output headroom upwards of 2V from each rail depending on complexity and biasing. This was fine (and still is) for large dual supplies, but it was a problem for single supplies.

    Essentially, the solution to this was to swap the output transistors so the PNP is a pull-up and the NPN is a pull-down, ensuring the right feedback for stable operation. In this case, the output transistors are connected as common-emitter amplifiers, taking some gain, and the output voltage can get within a Vcesat of the rail, often 0.1V to 0.2V, depending largely on load and biasing. They called this topology a "rail-to-rail" output stage, because it got very close to the rails, especially compared with traditional output stages. But really this is a marketing term, the output transistors are still only steering currents and therefore the output voltage can never completely get to the rail without a charge pump or something like that.

    There is some good info on this in the Op Amp Applications handbook. The discussion on output stage types starts on page 1.44.

    Back to the AD8275, you can connect a small negative supply like the one you mentioned. Just be sure that the ripple is okay given the PSRR of the AD8275 (you may want some extra filtering to improve performance). And make sure the output current of your supply is enough, given that any load current that the AD8275 sinks is also directed to the negative supply. Feel free to use the AD8275 SPICE model to see how much current to expect into the negative supply in your circuit.

    Best regards,

    Scott