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AD7675 family, timing questions

Hi all,

 

I'm working with an ADC AD7675 and I'm misunderstanding few parameters about timing.

 

The application is quite simple : a microcontroller is dealing with the ADC and memory. Datas are recorded from the process under test continuously at the beginning (50ksps during about 200s) and after, when some events detected by the microcontroller occur, few points are recorded(could be 10 or 20 points in 200ms, every 30s - 50s).

 

I started tests with 8bits A/D integrated in the microcontroller. It works but more precision is required. My problem is simple. When I worked with the integrated A/D, timing was simple and correspond to action : Start acquisition, stop acquisition, start conversion, interrupt at conversion end, data in a given register.

 

With the AD7675, I understood that CNVST start conversion and stop acquisition, BUSY show the end of conversion.

If I use PD (power down) to save energy, how long will it take to warm up the converter (Tension ref always ON)? After a PD, are input pins at '0' ?

 

What about starting acquisition ? Acquisition must length at least "t8", 8.75µs. On the diagramme 11, MODE is defined as acquire or convert, no wait period. If I want to get 1point every 10ms during 200ms (total 20 points, each point acquisition time 10µs), how can I make it ?

 

The only way I see would be to make a RESET. Is 10ns (t9) is enough to make a true '0' on the input pin ? Is it possible to keep RESET active for a longer time ? Does RESET only affect input state ? (no effect on data out for example)

 

Is this timing correct ?

PD end -> warm up ->RESET ->wait 10µs->CNVST->poll BUSY LOW-> RD->RESET hold during 10ms (approx, 10ms from the previous RESET)->((repeat 20 times to get 20points))->PD start

 

The same thing to work at 50ksps, without PD and holding the RESET during a much shorter time. Is it correct ?

 

Many thanks for your help!

Kind regards,

Adrien

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  • Hi alan,

    In fact I'd like to get the same acquisition timing during the different way of sampling, and keeping it around 10µs of acquisition duration. This for 2 reasons : first I'll keep an "integration time" constant. At the begining the signal varies fastly. So I'd like to take around 50kSPS. After signal vary slowly so lower sample rate is possible. But I'd prefer keeping an acquisition time constant to get constant signal treatment. With a much longer acquisition time, I think that sensor noise will be filtering in a different way.

    And as a second point, the most important to my mind, in the  process timing I reserve a time sequence with nearly no activities (switch communtation, DAC transition, memory exchanges, ...) to try to reach low noise level. With an acquisition lasting several ms, logic transitions perturbations may to be taken into account during acquisition. With a constant 10µs acquisition time, I could get a single routine for the microcontroller with an easier control on the 'nearby' activity.

    That's why I was looking for a way to control the acquisition timing. The only way I see is to program a reset to get only 10µs of acquisition, even if I get 10ms between the end of conversion and the starting of the next acquisition. I'm not sure about the way the reset works or if it's the rigth way to do this.

    Best regards,

    Adrien

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  • Hi alan,

    In fact I'd like to get the same acquisition timing during the different way of sampling, and keeping it around 10µs of acquisition duration. This for 2 reasons : first I'll keep an "integration time" constant. At the begining the signal varies fastly. So I'd like to take around 50kSPS. After signal vary slowly so lower sample rate is possible. But I'd prefer keeping an acquisition time constant to get constant signal treatment. With a much longer acquisition time, I think that sensor noise will be filtering in a different way.

    And as a second point, the most important to my mind, in the  process timing I reserve a time sequence with nearly no activities (switch communtation, DAC transition, memory exchanges, ...) to try to reach low noise level. With an acquisition lasting several ms, logic transitions perturbations may to be taken into account during acquisition. With a constant 10µs acquisition time, I could get a single routine for the microcontroller with an easier control on the 'nearby' activity.

    That's why I was looking for a way to control the acquisition timing. The only way I see is to program a reset to get only 10µs of acquisition, even if I get 10ms between the end of conversion and the starting of the next acquisition. I'm not sure about the way the reset works or if it's the rigth way to do this.

    Best regards,

    Adrien

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