Best Way for +/- Rails

Hi Rodney,

what currents shall the supply voltages source and sink?

Keep in mind that 10mV is only 0.055% of 18V. So your circuit will need a very precise voltage reference and very precise resistors.

If the currents to be sinked and sourced is rather high, the LM317 could be used with the ADJ pin driven (or better said adjusted) by the output of a DC servo. The input of DC servo would compare the divided output voltage with a very precise reference voltage.

For the negative voltage you could use an identical circuit powered by a floating input voltage (two isolated windings of a mains transformer). The output voltages could be connected in such a way that a positive and negative supply voltage is generated, just like to generate +/-12V by the help of two identical LM7812.

If the currents to be sinked and source are rather small, on the other hand, the outputs of DC servos could directly be used.

Can you explain why you need such precise supply voltages?

Kai

"what currents shall the supply voltages source and sink?"

The currents would be around 10mA to an opamp and potentiometer load.

"very precise voltage reference"

Is the REF102CP precise enough? It can produce a 10V +/- 0.0025V output. 

"by the output of a DC servo"

What is a DC servo? 

Kai, could you explain the differences between a opamp solution vs a linear regulator solution? 

Hi Rodney,

the REF102 sounds good.

A simple trick to generate 15V is to stack a REF102 and REF02, or three REF02. See the datasheets.

You could add a buffer with gain to generate 18V from the 10V output voltage of REF102 or from the 15V output voltage of stacked REF102 and REF02 and an inverting buffer with gain to generate the -18V. The OPAmp used for these buffers must be precise enough and must allow the 18V or -18V output voltage swing. It also must allow to source and sink your 10mA supply currents at that output voltages.

Another way is to stack two REF102 to generate 20V from which the 18V is derived by a voltage divider and unity gain buffer.

The gain of the buffers must be programmed by state-of-the-art precision resistors referring to temperature drift and long term drift. The initial tolerance of the circuit can be adjusted by a multi-turn cermet trimmer which you put in series with one of the precision resistors but which makes only a very small fraction to total resistance. Only as an example: A 10R cermet trimmer in series to a 10k precision resistor.

When stacking the REF102 and REF02 take care that the maximum sink and source currents of the individual voltage references are not exceeded. So eventually dummy loads have to be added from some of the individual outputs to signal ground.

Kai

Thanks for the info. I will try some of this. Hopefully, this works.