2204A effective AWG resolution

Post any questions you may have about our current range of oscilloscopes
Post Reply
picothar
Newbie
Posts: 0
Joined: Mon Oct 22, 2018 6:35 pm

2204A effective AWG resolution

Post by picothar » Mon Oct 22, 2018 7:13 pm

Hello,

I am using the PicoScope 2204A AWG to generate pulses with exponentially rising and falling edges and I am curious about relatively large steps in the output near the max value of the waveform. See the attached for an example of data captured by connecting channel A directly to the AWG with waveform defined by the exponential option in the waveform editor. The screenshot also shows the average over several traces (grey). The black trace was obtained by connecting a capacitor across the AWG output to form a low-pass filter which better distinguishes the AWG steps from the 8 bit scope resolution when averaging.

According to the specification the AWG has 12 bits resolution which lead me to expect finer steps than ~10 mV for a ~1V which is the case here.

I suspect this is related to the internal DDS, but I cannot wrap my head around the details. Can someone clarify?
Attachments
Capture.PNG

picothar
Newbie
Posts: 0
Joined: Mon Oct 22, 2018 6:35 pm

Re: 2204A effective AWG resolution

Post by picothar » Tue Oct 23, 2018 7:42 am

After some more testing I found that the PicoScope software approximates waveforms in 255 discrete steps corresponding to 8 bit resolution. I checked this by selecting an exponential waveform in the editor and then saving as a CSV. After pressing 'Apply' and going back to the editor the waveform is obviously less smooth as seen from the graph. By saving again as CSV I compared the 'applied' waveform to the 'original' (see figure).

I checked the SDK and in the definition of the ps2000_set_sig_gen_arbitrary library function the waveform values are indeed stored as 8 bit values (uint8_t *arbitraryWaveform).

Why is this the case when the spec says 12 bit AWG resolution?

I also noted that in the ps2000a API used by 2206B et al, also 12 bit AWG, the waveform is defined as 16 bit signed ints that are presumably interpolated internally to give a 12 bit result.
Attachments
awg_waveforms.PNG

Gerry
PICO STAFF
PICO STAFF
Posts: 451
Joined: Mon Aug 11, 2014 11:14 am

Re: 2204A effective AWG resolution

Post by Gerry » Wed Oct 24, 2018 11:01 am

Hi picothar,

I have verified what you are seeing.

Originally our 2204 and 2205 PicoScopes only had 8-bit resolution in the Signal Generator, but our newer 2204A and 2205A PicoScopes have updated hardware capable of 12-bit resolution in the Sig Gen. It appears that our software wasn't updated to reflect the changes in the hardware, so I will raise a request to fix this bug.

Thanks for bringing this to our attention.

Regards,

Gerry
Gerry
Technical Specialist

picothar
Newbie
Posts: 0
Joined: Mon Oct 22, 2018 6:35 pm

Re: 2204A effective AWG resolution

Post by picothar » Thu Oct 25, 2018 6:27 am

Gerry,

Thank you for looking into this.

The PicoScope is convenient for my application because I can generate a sensor excitation and record the response using a single unit. However the limited resolution is a bit of a showstopper at this point because it introduces noise that I cannot easily filter out without sacrificing bandwidth.

Upgrading to 2206B would immediately solve the problem for me, with the extra benefit of having the AWG triggering capability, but it would still be helpful to evaluate the 12 bit AWG hardware in my current scope first to verify that I can get by without a dedicated signal generator. Hoping there will be a fix released soon.

picothar
Newbie
Posts: 0
Joined: Mon Oct 22, 2018 6:35 pm

Re: 2204A effective AWG resolution

Post by picothar » Sun Jul 14, 2019 11:29 am

Surprised to see that this issue has not been fixed after almost one year. Surprised because my experience with Picoscopes otherwise is well functioning hardware that is up to spec, excellent software and a very useful SDK for implementing custom applications.

The official spec still says 12 bit AWG resolution for the 2204A.

Waveforms defined in the Picoscope 6 software are still limited to 8 bit resolution with a 2204A connected.

The ps2000 SDK is still limited to 8 bits (uint8_t *arbitraryWaveform in ps2000.h).

In other words, users are not able to utilize the 12 bit AWG resolution even though that might be the spec of the DAC hardware. The current spec for these scopes are therefore misleading.

Puzzling that the issue has not been fixed yet or the spec updated to reflect the capability customers can expect from these scopes.

Post Reply