AD7675 family, timing questions

Hi Adrien,

Why bother powering the part down? The power of this device scales very well with throughput so if your not converting very often it is very low power. If you look in the datasheet it consumes 17mW @ 100ksps and 15uW @ 100sps. In power down it consumes 7uW max so not really much of a saving by powering down completely. See figure 10 for even more power dissipation numbers. It looks like if your not converting the power is in the region of <1uW. This way you don't have to worry about power up times.

Best Rgds,

Alan

Hi alan,

.

Thanks for your answer. It's true that I could not bother with power down. I use the ADC with parallel port. I need to put databus on high impedance (to read back data). As I need to deal with CS or RD, I thougth why not PD ? It's true that RD will solve power up time trouble. Do you think it's the rigth way to use RESET to start the acquisition ? Troube to keep it active for a "long time" ?

Best regards,

Adrien

Hi Adrien,

You do not need to reset the part to start acquisition. The acquisition time is basically the time that the part is not doing a conversion. So at slow throughputs you have a very long acquisition time. As you can see from Figure 10 the part can convert at very low throughput rates (10sps) and even lower if necessary. I am not aware of any issues with very slow throughput rates.

Best Rgds,

Alan

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

Hi Adrien,

A longer acquisition time will not integrate more noise, the input is sampled at the CNVST edge. The bandwidth of the AD7675 is 3.9MHz so it's time constant is fast relative to your measurement interval. If you really want to keep the acquisition time at 10us you can use reset or do a dummy conversion first, then give 10us acquisition time and do the conversion you want to capture. If you use reset it may reconfigure the device to a state you don't want.

Best Rgds,

Alan

Hi Alan,

Thanks for your answers. I've got a better a view on the problem. The next step is to get a good pcb and to put all the things working fine together!

Best regards,

Adrien