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AC + DC signals using summing amp -> loss of low freq

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ericmar

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Hi,

I am trying to superimpose an AC signal on a DC signal using a summing amplifier but found that the output waveform is not shown correctly. Apprently, there is something lost in the output and I think this voltage droop caused by AC coupling due to the loss of low frequencies.

May I know how can I solve this problem? How can I do to show the complete signal of combination of AC and DC signals?

Thanks in advance.

Regards,
Eric
 

post your circuit excerpt
 

"I think this voltage droop caused by AC coupling due to the loss of low frequencies. "

there is no AC coupling on the schematic.

Are you working with square waves ? If so for high freq it could be problem for oamps . You should choose high slew rate and high bandwidth oamps. Do your followers pass signal as on their input ? what is the signal frequency and form ? Do you really need so many opamps to add signals?
 

artem said:
"I think this voltage droop caused by AC coupling due to the loss of low frequencies. "

there is no AC coupling on the schematic.

Are you working with square waves ? If so for high freq it could be problem for oamps . You should choose high slew rate and high bandwidth oamps. Do your followers pass signal as on their input ? what is the signal frequency and form ? Do you really need so many opamps to add signals?
Click to expand...
I am not working with just square waves but also sinusoidal waveforms.
Yes. The followers pass signal as on their input.
The signal frequency is about 1KHz in AC form.
Those 2 followers are used to buffer the signal input whereas the last op-omp is the only one to sum up both signals.

Could it be the summing amplifier I chose not suitable for this application?

Thanks,
Eric
 

yes, it could be case . Try to sum your signals on simple resistive network (dont use high impedance resistors as capacitance of next stage will influence summed signal) and you will see the difference .

Also sum is usually done if opamp used in inverted connection with one resistor per each signal source to be summed. See the opamp for everyone or similar textbook .
 

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