# AD713: Output considerations

Document created by analog-archivist on Feb 23, 2016
Version 1Show Document

### Q

I am currently working on a design requirement that needs to create an output
of +10V and -10V at up to 20mA continuous (implies a 500ohm load). The device I
have been considering for this application is the AD713. TPC3 on the datasheet
implies that the AD713  should meet this requirement (500ohm load).

However, TPC3 also states that the curve applies for +/-15V supplies, where my
application will be working from +/-12V supplies (could go as low as +/-11.6V).
How would TPC3 be modified to reflect this?

### A

Operating from +/-11.6V minimum supply, you are going to run into output head
room problems even with the output unloaded.

Referring to page 2, the output swing from a +/-15V supply with a 2k load is
+13V to –12.5V at 25degC and +/-12V over temperature. This implies output head
room of between 2 and 3 volts to either supply. Output headroom is fairly
constant as the supply voltage is reduced as you can see from TPC 2. Assuming
+/-11.6V supply and a 2k load, the output will swing close to +10V but falls
short of –10V. If you then factor in a 500ohm load and variations over
temperature, you can be fairly confident that the output will not swing +/-10V.

Is there any possibility of increasing the supply voltages?

I presume you have selected the AD713 for availability in military grade and
it’s biFET input (low voltage offset and input bias current). If you really
need the output to swing +/-10V from +/-11.6V supply, I would suggest using the
AD713 as the input stage and using a second opamp with rail to rail output
stage to provide the required output swing. IF you close the feedback loop from
the output of the rail to rail back to the input of the AD713, you get a
compound amplifier with the input characteristics of the AD713 and a rail to
rail output ( by rail to rail output, I simply mean output head room of less
than 100mV unloaded).