Be a bit cautious trying the Sparko discrete opamps in your Ultra DAC Larry until I try them out here first.
I do note there is some input current flow in these devices as they are bipolar inputs and in your DAC we are using a fet input device (OPA627) that does not have this issue.
I am not certain how this will interact with the D to A chip ahead of the op-amp. The device is used as a current to voltage amplifier in the Philips based chip set design. You might want to bring the DAC over and let us try it while monitoring the circuit as we bring it up on our AC variac to make sure nothing bad happens.
Regards,
Frank
Input bias current should not cause any problems. Input bias current will be supplied to the devices from the I output of the DAC. Be careful using a variac to bring up a system. The digital ICs might get under-voltaged and latch up. Slow dV/dT on VCC rails will occasionally give one grief.
I would expect to see slew induced distortion when a sparko opamp is substituted for an OPA 627 or OPA 637 in an I/V application.
Scotty
There is an equation that can be used to calculate the slew rate requirement of a device if we know the maximum frequency and the peak amplitude of the signal that the device must amplify. This equation is -
Required Slew Rate (in V/µS) = 2π F Vpk / 1,000,000
We divide by 1 million to get an answer in volts / microsecond.
Most op amps in line level circuits process signal amplitudes of a few volts peak. Typically, these op amps are supplied from something like ± 15 V, so it’s impossible for them to process signals much beyond 10 or 12 volts peak anyway due to the limitation of the power supply rails. (Headroom) The upper limit of audio frequencies is widely considered to be around 20KHz, but for the sake of discussion lets call it 160KHz, which is 3 octaves higher than is presumed to be audible by the human ear. So how much slew rate is required of an op amp to reproduce a 10V peak signal at 160KHz? Turns out, its 10V/µS. If we get more reasonable, and calculate a slew rate requirement for 5V peak at 50KHz, we only need a paltry 1.5V/ µS slew rate device to accomplish this.
Perhaps I misunderstand slew rate distortion, but under mathematical inspection it would appear to be a non issue with op amps. Power amps are somewhat of a different story as they must swing far more peak voltages.
Hope this helps.
Andrew
