AD7175-8 : settling time of ADC core in alternate synchronization mode

Hi,

Please do not confused with the settling time and delay. The extra delay time 128MCLKs in single conversion mode vs continuous conversion mode is the requirement for ADC to come out of standby to power up and settle.

In terms of the settling time, the settling time is the time required to generate a very first conversion. If single channel is used, the settling time is required for the first conversion and all subsequent conversions occur at selected output data rate. If multiple channels are enabled, the first conversion in the sequence requires a settling time mentioned above. The next conversions in the sequence requires a slightly shorter time than the settling time. The settling time still depends on the filter choice and selected ODR.

The extra delay time at the very first conversion is the setup time for the signal chain before the first conversion is available when the ADC is power up, coming up from standby mode or when using SYNC pin. It is always 68MCLKs for AD717x but it varies from other parts. This delay is present on either continuous or single conversion mode.

For normal SYNC mode, if sequencing is enabled then it will only be the very first conversion that will have this delay incorporated. However if there is a manual switch of channel or a change in configuration of the part then the delay will always be seen. Any conversion triggered using SYNC will add this time. For Alt SYNC it should have the same delay every time the SYNC is used. 

May I know if you have encountered any issue regarding the timing? If you can share more details about your setup and measurement, just let me know so we can clear this up.

Thanks,

Jellenie

Hi,

Please do not confused with the settling time and delay. The extra delay time 128MCLKs in single conversion mode vs continuous conversion mode is the requirement for ADC to come out of standby to power up and settle.

In terms of the settling time, the settling time is the time required to generate a very first conversion. If single channel is used, the settling time is required for the first conversion and all subsequent conversions occur at selected output data rate. If multiple channels are enabled, the first conversion in the sequence requires a settling time mentioned above. The next conversions in the sequence requires a slightly shorter time than the settling time. The settling time still depends on the filter choice and selected ODR.

The extra delay time at the very first conversion is the setup time for the signal chain before the first conversion is available when the ADC is power up, coming up from standby mode or when using SYNC pin. It is always 68MCLKs for AD717x but it varies from other parts. This delay is present on either continuous or single conversion mode.

For normal SYNC mode, if sequencing is enabled then it will only be the very first conversion that will have this delay incorporated. However if there is a manual switch of channel or a change in configuration of the part then the delay will always be seen. Any conversion triggered using SYNC will add this time. For Alt SYNC it should have the same delay every time the SYNC is used. 

May I know if you have encountered any issue regarding the timing? If you can share more details about your setup and measurement, just let me know so we can clear this up.

Thanks,

Jellenie

Hello Jellenie,

Thank you for your support.
I'm sorry, "settling time" was not a right expression.
I'd like to know that the Alternate Synchronization Mode involve the extra delay or not.

The customer is expecting a timing of follows.
[Condition]
ODR : 125kHz
CH0 - CH7 all enable
Sequencing enable
Alternate Synchronization mode
SYNC signal : 25us interval continuous pules

> For Alt SYNC it should have the same delay every time the SYNC is used.

Does this means that the extra delay(68MCLK) is occurred ?
In normal sync mode and alternate sync mode , AD7175-8 enter stand by after conversion completed, isn't it ?

Best regards,

y-suzuki

Hi,

If you are encountering some issues with the timing, I would recommend to see this timing diagram on our Virtual Eval | Analog Devices Tool or Eval+ Software. 

If you open a Virtual Eval Tool, you will observed that when converting on the AD717x, the very first conversion (Tsettle, first conversion in VE) is longer than subsequent conversions. If you enable multiple channels, check the time for Tsettle, first conversion and Tsettle, subsequent conversions, you will see that the first conversion is again longer. When the sequencer is being used with the ADC controlling everything (SYNC not being used), the ADC performs some parallel processing, processing the conversion on one channel while setting up the next channel. For the very first conversion, it cannot perform this parallel processing, hence the longer time.

When SYNC is being used, again, the part cannot perform any parallel processing. So, the time for every conversion after a SYNC pulse with be Tsettle, first conversion.

The SYNC input allows the user to reset the modulator and digital filter without affecting the setup conditions. This also allows the user to control the start of conversion from a known point, the rising edge of the SYNC input.

In normal SYNC mode, SYNC input can be use as a start conversion for a single channel. Thus, if you have multiple channels enabled and you want to control the start conversion of each channel an alternate synchronization mode can be used. However, like I mentioned above there will be a delay of 68MCLKs cycles after each release of SYNC pulse. Since in Alternate SYNC mode we used the SYNC pin for each channel your conversion time for each channel will be Tsettle, first conversion.

AD7175-8 will only enter standby mode after a single conversion mode and performing calibration modes. It will not go to standby mode if you set it up with continuous conversion mode.

Thanks,

Jellenie