Question

what is a split dc supply?

Answer 1

I believe this normally refers to a circuit that requries both a positive and negative power supply voltage. Usually, but not always, they are equal in magnitude but opposite in sign. For example, +12 v and -12 v. This was very common in the past, but many modern circuits, including analog circuits, can be made with a single supply voltage - such as a single +5 vdc supply. Sometimes making an analog circuit with a single supply involves creating an "artificial ground" at 1/2 the power supply voltage. This is usually done simply by using a resistive divider and the "artificial ground" is commonly attached to very high impedance circuits, such as the non-inverting input of op amps.

Answer 2

why can't we just make an ordinary battery connection?i mean positive supply to positive terminal and negative supply to negative terminal of the opamp from a single supply? why do we need a ground?

Answer 3

We CAN and do power op amps from a single supply. Suppose we put +12 on the Vdd terminal and ground the Vss terminal. However, this means that any signal presented to either of its inputs or generated by its output can only be a positive voltage between ground and +12 v. In fact, older or cheaper op amps might only allow a signal to be between 1.2v and 10.8 v. The point is that you cannot process most ac voltages with such a setup because the ac voltage is centered around zero. Take an audio signal - think of a perfect sine wave at 1khz as an example. Half of the time the signal is positive and half of the time it is negative. This signal cannot be applied directly to the op amp because it cannot handle negative voltages. The output will hit one of the power supply rails and stop there. So what we do is to create an "artificial ground" at +6 volts and connect this to the non-inverting input of the op amp. The "zero signal" output voltage of the op amp will then be +6v. You could think of this as biasing the amp to the mid point of the ac signal. Then we capacitively couple the signal to the op amp. As long as the signal is less than about 12 v pk-pk, the op amp will handle it without saturation or distortion. (In real life, running the output voltage close to the power supply rails will induce some distortion in many op amps - but op amp choice is a topic for another day). This is how we handle input signals that go negative in a system with a single power supply - regardless of the input waveform shape.

Obviously, it is easier to connect the op amp Vdd terminal to +12 and Vss to -12. This is a "split supply" design. The input signal can be directly coupled and we can handle both positive and negative voltages between -12 and +12 volts. We can handle any waveform, even DC. This is more flexible but comes at the cost of an additional power supply.

The current trend toward battery powered mobile systems has pushed many designs toward the single supply architecture and many ICs have now been made to accommodate such designs (such as op amps that can swing all the way to the power supply rails). Only the more exacting systems use split power supplies. Your iPhone uses a single supply. I have not looked, but I wager your nice new Tektronix scope uses split supplies. :-)

Mike