About the 0Hz-100kHz range signal processing

Hello Tatsu,

The short answer is, no SigmaDSP can handle the full 0 -- 100 KHz spectrum. Some SHARC processors can also be programmed via SigmaStudio so you may wish to ask about your application in the SigmaStudio for SHARC forum -- although swimming with the SHARCs is much more involved.

Since SigmaDSPs are designed for audio processing; they find relatively few application outside that realm. Most SigmaDSPs have the MIPS to operate at a 192 KHz sample rate -- enough for up to about 90 KHz. Thus they have found their way into varied higher frequency applications. Search this forum for "ultrasonic" to see a couple. They also work at DC, with reservations. Their internal calculations work fine with non-varying numbers (hence, DC). Since they are meant for audio, those SigmaDSPs with internal converters (like the ADAU1701) are usually set up with external AC coupling caps at both input and output. Operating from a single 3.3V supply, both input and output are biased at Vcc / 2, about 1.6 V. They respond down to DC, yet you'll need external circuitry to supply and cancel the Vcc / 2 offset which varies from chip to chip and perhaps with other factors like temperature. They simply aren't specified for DC. For example, a DC error of -40 dB is plenty good in the audio world -- but that's a whopping 1%, which op-amps like the 741 bettered in the 1970's. Audio codecs with the I2S interfaces supported by SigmaDSP are also meant for audio and likely have the same limitations as the built-in converters. I don't know of any DC-capable ADCs and DACs supporting I2S.

Fortunately it's relatively easy to come up with a simple auto-zeroing arrangement at power-up to reduce chip-to-chip variations -- yet if you're looking for the sort of DC precision easily found with op-amps and such, you'll won't find it here. So it depends upon your application. Search the forum for "DC' and you'll find that engineers have asked about DC operation in non-critical, niche applications.

Best regards,

Bob

Hello Tatsu,

Everything that KJBob said is true with the slight exception of the sample rate. The ADAU1452 family of parts can actually run the core at 384kHz fs which will give you the desired range with a Nyquist frequency of 192kHz.

That said, the problem is getting the data into and out of the DSP. The serial ports are limited to 192kHz but you can play some tricks to get 384kHz data into and out of the part. The easiest is if your application is OK with 16 bits of resolution then you can "pack" two 16 bit words into one 32 bit word and send them in/out of the serial ports as 32 bit data at 192kHz fs. It takes a little more explaining but it can be done.

If you need 24 bit data then you have to use other tricks which require a GPIO and an LRCLK format of 50/50 duty cycle to interleave data that is coming in at a 192kHz rate.

So the top end can be dealt with leaving only the 0Hz issue that KJBob detailed quite well. Another issue with the DC performance of the DACs is the DC drift over time and temperature. It is not an important design factor for audio DACs and ADCs so effort goes into making it sound better etc. I know some customers disable the internal reference generators and use an external low drift DC source to drive the ADC or DAC reference voltages. We are audio guys who support these parts so this is not our area of great expertise but I know the DC drift can be improved.

So you have the details of where the difficulties will be to hopefully help you with your decision.

Dave T

Hi Bob,

Thank you.

Hi Dave,

Thank you.