Question
we are manufacturer of high-end relative humidity transmitters and relative
humidity calibrators. We want to
redesign an existing application and are looking at designing in the AD8551.
The power supply of the AD8551 application is only 2.5Vdc (using a voltage
reference). The datasheet sets the minimum power supply voltage at 2.7Vdc. What
are the consequences for using the Ad8551 at 2.5V in terms of power
dissipation, THD, offset, output swing, noise, etc?
Answer
This kind of question arises frequently, where customers want to use the part
just a little bit outside the specification. The answer is always the same:
since the part is only being used a little outside the specifications it is not
likely that the performance will change a lot for most devices. However, the
parts have not been production tested at this supply voltage and the
specifications are not guaranteed so the customer is on their own if problems
arise. You could reasonably expect some devices to work with no problems while
other devices show a degredation in performance. We will never be able to tell
the customer that it is "perfectly okay" to use a 2.5V power supply; if it was
okay at 2.5V then we would have written that in the datasheet. However, may
possibly be able to say which parameters will be degraded and whether it is
possible that some devices won't work at all.
What does rail to rail mean?
For the input signals to the amplifier, the common mode can be anywhere between
0V and Vs. This is guaranteed.
For output voltages, the amplifier is guaranteed to reach within a few mV of
the supply rail, subject to the loading and temperature conditions which are
given on page 3 of the datasheet.
The exact amount of headroom which you need to leave depends on the structure
of the input and output stages of the amplifier. It is not correct to say that
one should not exceed Vrail - 1V. This limitation completely depends on the
kind of amplifier. Some amplifiers require up to 3V headroom from both rails
while others allow you to apply a voltage several 100mV beyond the rails. In
this case the input can go all the way to the rails while the output can come
within a few mV of the rails as long as you don't draw too much current.