Hi,

I am developing integrating ADC as part of my thesis and i am completely new to analog design as all my course work and experience is in digital design. I am facing problems with the opamp acting as an integrator. The first one was working as integrator when the power supply is 2.5 and -2.5 V for the opamp. But the requirement has mandated to use a single supply opamp i.e. 5V and 0 and the opamp is not functioning as an integrator though it is acting as diff amp , inv amp etc fine.

I am now trying to use current mirror for producing a constant current to charge a capacitor and thus produce a linearly varying ramp voltage. I am not able to charge the capacitor though the output of the current mirror is providing me a constant current and the voltage is not a ramp voltage. Please let me know how to debug the problem.

Any help regarding the design of ramp generator using current mirror , any literature is greatly appreciated.

Thanks,
RC.

A constant current charging a capacitor results in a ramp. In the simulator we need to initialize the voltage on the capacitor to zero and then start the simulation to really see the ramp on the capacitor.

.nodeset or .force option in the SPICE simulator would help.

If we don't initialize, then the voltage on the
capacitor is some random DC value. Theoretically it is infinite but the simulator shows some random value.

Hope this helps

Thank you very much. I am now resetting the cap to zero and then able to see the ramp..