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BCBaker
BCBaker
11/25/2013 12:07:19 PM
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Re: input signals specification
Etnapowers,

If you look at the ADS7886 specifications, typically there is a slight variation in gain error in temperatures below 15C. Typically, the offset error seems to be rock solid. This is true for this product, but if you choose to use another, please check the product data sheet for information.

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etnapowers
etnapowers
11/25/2013 11:58:46 AM
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Master
Re: input signals specification
Hi Bonnie, thank you again for your informations.

Does the temperature change rate play a role in the recalibration of the ADC ? Is there a threshold value?

 

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BCBaker
BCBaker
11/25/2013 11:44:38 AM
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Re: input signals specification
Etnapowers,

Your calculations are correct, but remember, you are calibrating the offset and gain error of the ADC, not the PGA. Re-calibration is not needed as you change the Mux or Gain of the PGA. You would want to recalibrate if there is a temperature change or at start-up.

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etnapowers
etnapowers
11/25/2013 10:53:27 AM
User Rank
Master
Re: input signals specification
Thank you Bonnie for explanation. I intended the calibration routine that, as per your description should take 2 x 7.4 us each time it is performed. This 14.8 us time interval could be a long time if the calibration is required many times and the data trasmission effective rate could be decreased accordingly.

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BCBaker
BCBaker
11/19/2013 12:30:56 PM
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Blogger
Re: input signals specification
Etnapowers,

The simple answer to your question is that the PGA channel changes are obvious as the user is interested in the data from another channel. As you change the channel, it is possible that a new gain is required per the channel requirements.

If you intend to use the PGA system calibration routine, you will need to configure CH0 appropriately and run the measurement algorithm described in the PGA116 data sheet. As you go through this calibration routine, the internal calibration channels need to be changed. After you select each calibration channel, the ADC must collect measurement data. Each calibration cycle (total two) requires a full 7.4 us to perform.

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etnapowers
etnapowers
11/19/2013 5:21:30 AM
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Master
Re: input signals specification
"The time limitation for this system is when you change the PGA gain and/or channel"

 

@Bonnie, could you cite some cases in which the change of the PGA gain or channel is required by the application? In case of failure or missed calibration of a channel for example?

 

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BCBaker
BCBaker
11/18/2013 2:32:55 PM
User Rank
Blogger
Re: input signals specification
DaeJ,

On the analog side, the PGA minimum full-scale frequency response (across all gains) is 100 kHz. You can tweak this to a higher frequency if you choose a PGA setting that allows this. There are some settling time issues, however in the audio band (assuming no step responses) this will have minimal impact.

 

The maximum clock rate of the PGA is 10 MHz and the maximum clock rate of the ADC is 20 MHz. The PGA clock does limit this system, however if you keep the channel/gain of the PGA constant, you can convert the PGA signals with the ADC at a rate of 0.8 us or 1.25 MHz. Taking Nyquist into account the maximum rate would be at 625 kHz.

 

This analysis suggests that this system will operate with a vocal signal greater than 100 kHz. This is good, because there are many audio-files that believe that we 'hear' or feel sounds well above the scientifically measured hearing range.

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DaeJ
DaeJ
11/18/2013 2:05:47 PM
User Rank
Master
Re: input signals specification
->If you want to measure a vocal signal (<  40 kHz), the PGA116 is fully equipped to amplify that type of signal, independent of the programmed gain

I wonder if there is a limitation for PGA 116 to measure data in the condition that vocal signal is greater than 40 kHz. I think that this would be related to sampling rate.

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BCBaker
BCBaker
11/18/2013 11:56:42 AM
User Rank
Blogger
Re: input signals specification
Etnapowers,

If you want to measure a vocal signal (<  40 kHz), the PGA116 is fully equipped to amplify that type of signal, independent of the programmed gain. The time limitation for this system is when you change the PGA gain and/or channel. This change requires 1.6 us. Additionally, when you make this type of change you need to allow 4.2 us (worst case) for the PGA signal to settle.

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etnapowers
etnapowers
11/18/2013 8:58:03 AM
User Rank
Master
Re: input signals specification
@Bonnie, thank you very much for your clarification.

As I understand this the minimum bandwidth of the PGA is the bottom limit of the system bandwidth.

Input signals having a bandwidth of 6.75 kHz are non vocal signals (that have  40 KHz bandwidth). To transmit a vocal signal only one of the inputs could be utilized for this system, is this correct?

 

 

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