FFT simulation of D/A converter

[himadri_debnath]

I am doing FFT simulation of my 8bit D/A converter, but I have very much less idea about FFT. I am attaching the simulation result here. Please tell me if the simulation result is good or bad. By the I am confused about negative spikes. Shouldn't I supposed to get positive spike? Please somebody explain t his.

 

[FvM]

You forgot to tell what's the input signal in your simulation.

Meaningful FFT converter tests are mostly using single or two-tone sinusoidal signals. Apparently your input signal is something different.

 

[himadri_debnath]

I applied 8 pulse signal with time periods 12.5ns, 25ns, 50ns, 100ns, 200ns, 400ns, 800ns, 1600ns from bit0 to bit 7 respectively. Duty cycle is 50%. That means every steps of total 256 steps are 12.5ns apart(80 MHz). The spike I am getting at 160 MHz.

 

[Dominik Przyborowski]

So, you are generating sawtooth waveform by DAC and look on FFT. It does not provide you any useful information about dynamic performance of DAC. The proper way is to generate single tone sinusoid and ensure that update rate to sinusoid frequency ratio is not an integer number.

 

[himadri_debnath]

Yes you are right. But why negative spike? Even if I apply pulses that generate sawtooth wave there may be some frequency where it should give positive spike, right? Please correct me if I am wrong.

 

[FvM]

I presume you don't completely understand the generated spectrum of your test signal. It's well possible that it has some zeros.