# AD7176-2 has unknown noise in sample data.

Hi ADI,

I got EVAL-AD7176-2SDZ (Rev-A), and I use it to test audio signal.

I use AD7176-2 to sample a pure 12.5 kHz 0.1 Vrms sine wave, and sample rate is 125kHz.

But there is a stable low frequency noise as below picture, so it make the wave envelope is unstable.

I

My setting as below:

The attachment is my test data.

Do you have any idea about this problem?

Thanks.

Homeway

12.5kHz.csv.zip
Parents
• Hi Jellenie,

Thanks for your reply.

May I ask for your input configurations?

[Source : Audio precision AP515, single-end 12.5kHz 0.1Vrms sine-wave]

How did you connected your AC input voltage?C

[connector J8 ,  AIN0 as AIN+, AIN1 as AIN- for channel 0]

Are you operating at single or dual supply?

[Single power supply for AD7176-2 J4, AVSS = -2.5V]

Can you also try to lower down the number of samples, let say try to make it 20 or just try to zoom in data to take a look at one or two cycles only. As I've seen the voltage values it looks like it swing to +/-140mV which is equivalent to your 0.1Vrms. Can you also checked your inputs at scope and compared it with the ADC results?

[After check with scope, I find that if the sample rate is not enough, then the envelope is not fix.

For 12.5kHz signal, sample rate should be more than 200 kHz, and capture data will be stable.

I am a little confused.

According to Nyquist theorem, sample rate just need to twice as input signal frequency.

Why should I have to use sample rate that more than twice to get stable sample?]

==========================================================

And I have a new question about AD7176-2 sample rate 250 kHz.

Sample point # of 1 cycle = Sample rate / Signal frequency. =250 kHz / 12.5 kHz = 20

But you can see below picture, I sample 40 points, but it seems have 3 cycles.

The problem only happens at sample rate 250 kHz.

Could you help to check this issue?

Thanks a lot.

B.R.

Homeway

Reply
• Hi Jellenie,

Thanks for your reply.

May I ask for your input configurations?

[Source : Audio precision AP515, single-end 12.5kHz 0.1Vrms sine-wave]

How did you connected your AC input voltage?C

[connector J8 ,  AIN0 as AIN+, AIN1 as AIN- for channel 0]

Are you operating at single or dual supply?

[Single power supply for AD7176-2 J4, AVSS = -2.5V]

Can you also try to lower down the number of samples, let say try to make it 20 or just try to zoom in data to take a look at one or two cycles only. As I've seen the voltage values it looks like it swing to +/-140mV which is equivalent to your 0.1Vrms. Can you also checked your inputs at scope and compared it with the ADC results?

[After check with scope, I find that if the sample rate is not enough, then the envelope is not fix.

For 12.5kHz signal, sample rate should be more than 200 kHz, and capture data will be stable.

I am a little confused.

According to Nyquist theorem, sample rate just need to twice as input signal frequency.

Why should I have to use sample rate that more than twice to get stable sample?]

==========================================================

And I have a new question about AD7176-2 sample rate 250 kHz.

Sample point # of 1 cycle = Sample rate / Signal frequency. =250 kHz / 12.5 kHz = 20

But you can see below picture, I sample 40 points, but it seems have 3 cycles.

The problem only happens at sample rate 250 kHz.

Could you help to check this issue?

Thanks a lot.

B.R.

Homeway

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