Hello,
I had a few questions about the spectrum I'm measuring at the output of my buffer circuit with the switched integrator, an ACF2101. The buffer amplifier is the OPA4131.
http://www.ti.com/product/acf2101
http://www.ti.com/product/opa4131
The datasheet shows the frequency response for a ACF2101 below. I agree, fourier spectrum of a rectangular pulse is a sinc function with null at the sampling frequency and all harmonics.
The timing signals I generate are the same as presented in Figure 9 of the ACF2101 datasheet. I use a microcontroller to generate the signals with a PWM or software timed. It's periodic with sampling frequency fs.
The circuit is the same as below, just the OPA2107 has been replaced by the OPA4131. The input network i s a photodiode with shunt resistance 500 Megaohm and shunt capacitance 10 pF. No reverse bias applied. A 10 nF poly cap is for S/H operation. No external cap for ACF2101, just the internal 100 pF.
I ran simple experiment where I set the hold time to roughly 600 usec. The select and reset were set to 30 usec, so the entire integration period is ~660 usec. I was expecting a clean output like the datasheet, instead I get the following for my spectrum analyzer response in a bandwidth of 5 kHz. The first null should be around 1/660usec or 1.515 kHz so thats pretty spot on. But there are big tones at every sinc NULL spot.
I do not understand this response so I'm requesting some help. What exactly am I seeing here? I don't believe this is a grounding error. I will post back when I pull the photodiode and try 1) grounding input and 2) putting in a current source. The instrument noise floor is clean too in this bandwidth with no spurs down to -127 or so. The instrument used for analysis is a HP 3561A dynamic signal analyzer.