7 Replies Latest reply on Apr 15, 2014 7:46 AM by DaKn_263916

# Accurate logging of ~3kHz signal using Delta-sigma ADC.

Hello. First my post here, so may be I'm posting to wrong place, sorry of that.

I have ~3kHz signal with +-0.5V amplitude that i need to record and process on PSoC3. The signal have sinusoidal-like waveform. How should I configure delta-sigma ADC to get most accuracy and speed? How should i connect ADC to the signal source for correct processing of negative parts of signal? Lost in tons of parameters.

• ###### 1. Re: Accurate logging of ~3kHz signal using Delta-sigma ADC.

To handle - part you have to offset, attached some simple methods to

accomplish this.

Regards, Dana.

• ###### 2. Re: Accurate logging of ~3kHz signal using Delta-sigma ADC.

Regarding accuracy and speed. You trade off resolution, speed.

Some questions -

1) What are you using digitized signal for ? Simple harmonic content determination

or magnitude scale is important. FFT for former, wavelet transform for latter.

2) How much T do you want to store signal, eg. how many samples do you need to accumulate.

3) If you want to preserve 3 Khz harmonic content, then meeting Nyquist will take care of

that. @ 6 Khz DelSig can generate 17 bit conversion.

Regards, Dana.

• ###### 3. Re: Accurate logging of ~3kHz signal using Delta-sigma ADC.

Nyquist doesn't quite cut it.

An SAR converter has a frequency response of sin(x)/x. The Delsig converter in the PSoC3 has a frequency response of (sin(x)/x)^4, where x = PI*f/fsample. For the SAR case, the -3dB bandwidth is 0.44* fsample. For the Delsig case, the -3 dB bandwidth is 0.22*fsample. If you want accuracy = 10% at 3.0 kHz, the minimum sample rate is 24 kHz. This is a simple calculation. This is not a circuit design or implementation or software problem. It is a mathematical certainty of the topology of nth order delta sigma converters. It can be compensated with specifically designed digital filters.

---- Dennis

• ###### 4. Re: Accurate logging of ~3kHz signal using Delta-sigma ADC.

Dennis its so much easier on calculations of we ignore the H(s) response of

other components in the signal path. Its a good thing I did not contribute to the

design on navigation systems to Mars, would have wound up crashing into alpha

centauri. I am glad you caught my error.

Question, in SAR is H(s) a simple sampler or a zero order hold response ?

Which leads me to a suggestion, encorporate either in config tool, or in a table,

1/2 lsb frequency response for all basic resolutions. Just a thought, maybe even

a graphic to remind us (me especially) response properties of the converter.

Regards, Dana.

• ###### 5. Re: Accurate logging of ~3kHz signal using Delta-sigma ADC.

The 1/2 LSB frequency response is an interesting idea, an interesting way to look at the problem.

We're looking at log(base2)(1/(1-abs((sin(x)/x))^n) where x = pi*f/fsample.The guaranteed accuracy is not as good as you might hope. See attached .xls, where series# = sinx/x order(n).

This is mathematical, entirely different from the settling problem, which is handled nicely by the settling time as specified in the component datasheet.

• ###### 7. Re: Accurate logging of ~3kHz signal using Delta-sigma ADC.

@Dana

I can assure you it is quite more difficult to hit Alpha Centauri than to hit Mars, which tonight will be visible just right to and above the full-moon from your location. Alpha Centauri (proxima) is high up in the north while Mars and Moon traveling in the ecliptic. This means that any object started from Earth gets a huge V0-kick vectored in the ecliptic and it will be VERY hard (costing MUCH energy) to leave that trajectory

Bob

• ###### 8. Re: Accurate logging of ~3kHz signal using Delta-sigma ADC.

Dennis, chrome has a problem with attachments, use IE or Firefox.....

Regards, Dana.