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7805 is NOT isolated, how can it be when it's ground pin is common to the input and output.
An isolated supply has no electrical connection between its input and output, it is usually achieved by using a transformer to magnetically transfer the power from input side to output side. The '1212' mentioned by Klaus is a small box that takes 12V in and gives an isolated 12V out, it uses a high switching frequency so a small ferrite transformer can be used. You can use a standard 50Hz/60Hz transformer but it would be physically bigger and heavier. The '1212' part number represents 12V in and 12V out but you can get other variations such as 1205, 0505 and 0512 and you can get versions that produce two outputs, for example +5, 0 and -5.
However, you can get op-amps that work on a single +5V supply so you don't need a negative rail at all.
To explain a little more about the need for isolation:
Think of 'ground' as being a connection to the Earth beneath your feet. The two incoming wires are not 0V and 48V, they both have voltage on them with respect to ground. The reasons for that is back many years ago, a large inductance had to be wired in series with each wire and by virtue of the wires in the inductor they had a high resistance and that dropped a voltage from the teleco supply in one wire and a voltage on the return wire before it reached telco's ground. A telephone is inherently insulated from any external connection so it can only draw current from across the wires, it can't divert current out of the phone or draw it in from outside. If you leak current to ground from either wire it unbalances the line and can stop it working or in some cases kill broadband signals. On phones connected through a private switch (PBX or PABX system), it actually uses a connection to ground as a way of signaling to the operator. However, in a normal system, a small leakage is likely to introduce buzzing on the line and as the leakage increases it eventually disconnects the line and alerts the telco there is a line fault.
Brian.
An isolated supply has no electrical connection between its input and output, it is usually achieved by using a transformer to magnetically transfer the power from input side to output side. The '1212' mentioned by Klaus is a small box that takes 12V in and gives an isolated 12V out, it uses a high switching frequency so a small ferrite transformer can be used. You can use a standard 50Hz/60Hz transformer but it would be physically bigger and heavier. The '1212' part number represents 12V in and 12V out but you can get other variations such as 1205, 0505 and 0512 and you can get versions that produce two outputs, for example +5, 0 and -5.
However, you can get op-amps that work on a single +5V supply so you don't need a negative rail at all.
To explain a little more about the need for isolation:
Think of 'ground' as being a connection to the Earth beneath your feet. The two incoming wires are not 0V and 48V, they both have voltage on them with respect to ground. The reasons for that is back many years ago, a large inductance had to be wired in series with each wire and by virtue of the wires in the inductor they had a high resistance and that dropped a voltage from the teleco supply in one wire and a voltage on the return wire before it reached telco's ground. A telephone is inherently insulated from any external connection so it can only draw current from across the wires, it can't divert current out of the phone or draw it in from outside. If you leak current to ground from either wire it unbalances the line and can stop it working or in some cases kill broadband signals. On phones connected through a private switch (PBX or PABX system), it actually uses a connection to ground as a way of signaling to the operator. However, in a normal system, a small leakage is likely to introduce buzzing on the line and as the leakage increases it eventually disconnects the line and alerts the telco there is a line fault.
Brian.
