AD8616 Supply Voltage

Cheng,

  You are not alone in your confusion.

When op amps came out in the 1960's and 1970's, they ran on +/-15V and usually had an NPN diffierential pair for the input stage

and an emitter follower output stage.  See figures 2, 6, and 7 in MT-035:

http://www.analog.com/static/imported-files/tutorials/MT-035.pdf

Because the input differential pair will not work unless there is sufficient voltage across the base-emitter junction and the current

source, you usually had to stay away from the bottom power supply pin by one or two volts, sometimes more.  Most of our

datasheets have a parameter in the spec table called "Input Voltage Range" (IVR).   If both inputs are within this range, it

will act as an op amp.

For the output stage, you have the voltage required across the base driving circuit, plus the base-emitter voltage, so the output

cannot get to the top or bottom rail, usually one volt or more away.  Again, our spec table will have output swing (Voh and Vol)

into a 2k load or a 10k load or xxx.

See MT-041:

http://www.analog.com/static/imported-files/tutorials/MT-041.pdf

As the digital guys tried to go faster, they turned to CMOS where all of the feature sizes were being reduced.  This lead to some

processes that had breakdown voltages of 5.5V or 6V, so they clamored for 5V only op amps.  So for op amps to run on ground

and 5V, we went to a PNP input stage, so the inputs could go to ground, but you couldn't take the inputs to V+.  But by adding

a PNP diff pair and an NPN pair, the input stage would use one pair when it was close to one rail, and the other pair when the inputs

where close to the other rail.

For the output stage, we flipped the output transistors and instead of emitter followers, used common emitter or common source.

This still does not allow you to go exactly to the rails with the outputs, but certainly closer, say 100-200 mV.

So the official definition of a "single suppy op amp" is one where the input voltage range includes the V- pin (ground), and the

output gets close to the V- pin. (ground)

Let's say you have an op amp with a +/-15V spec table, and the IVR is +/-11V and the output swing is stated to be +/-12V into a 2k load.

See the 1981 op amp:  OP-27:

http://www.analog.com/static/imported-files/data_sheets/OP27.pdf

There is no ground pin on an op amp, so the little electrons inside don't know where your ground is; they only know what is on the pins.

You could connect the OP27 on ground and plus 30V, and if you kept the inputs between +4V and +26V, the output would be

between +3V and +27V if you had a 2k resistor connected from the output to +15V.  This would be single supply operation,

but the OP-27 is NOT a single supply op amp according to the official definition.

Some signal generators put out sine waves centered around ground, and spectrum analyzer inputs like to see sine waves

centered around ground, so we do test parts with +/- supplies.  This is valid.

For figure 44 on the d.s., the photodiode is operated with reverse bias to reduce the capacitance so the system is faster.

So they ran it on +/-2.5V.  The capacitance vs. reverse voltage characteristics are left as a homework assignment. (g)

Good luck on your studies.....

For a good reference on photodiodes and transimpedance amplifiers, see:

http://www.jensign.com/transimpedanceamp

Harry