Single-supply opamp with dual supplies?!

Hello,

i want to design a PIN diode switch control driver board with Opamps.

The prerequisites are:

- V+ 3V; V- -4V
- I+- 10mA
- cap load drive ~1500pF but behind a 30Ohm output series resistor

As my opamps will only swing from one rail to the other (setup as non-inverting amps; according to TTL input 0 or 3.3V they should either source 10mA from positive rail or sink -10 mA from neg. rail), a precise rail-to-tail amp that can reach the rails at these currents should be chosen.

But rail-to-rail amps are mostly single-supply. Can i have asymmetric V+ and V- or does V- have to be GND with a single supply amp?

Thanks

Chris

I usually pay little attention to dual vs. single supply in the data sheets unless there is an enable or balance input required. I believe the single supply designation is really just a buzz word for opamps who's inputs can go near'ish both rails.

If the opamp has no other inputs except the inverting and non-inverting inputs it has absolutely no way of knowing if it's running off a single or dual supplies, symmetrical or asymmetrical.

Very basic output equation is

Output = ( ( Non-Inverting Input - Inverting Input + Input Offset ) * Gain ) + Midpoint of the Supply Rails

For symmetric supplies the "Midpoint of the Supply Rails" becomes ground and drops out of the equation. But in all supply cases the Input Offset * Gain will dominate the output voltage. This usually causes the output voltage to be pegged against one rail or the other if it was not for the external negative feedback circuit.

Just pay attention to the common mode input voltage range and output voltage swing.

One last point, if you use the rails to limit the output swing you may have a response time problem with the output coming out of saturation.

Ray