AD7176-2

Hi ,

 

I have planned to use AD7176-2 ADC in our project to read thermocouple, thermistor, RTD , Voltage , Current and strain gauge sensors. I am finding it hard to fix what is the conversion time and settling time.

 

I will be using one channel with output datarate @ 10ksps and Sinc5+sinc1 filter settings. In "Table 18. AD7176-2 Output Data Rate (ODR)". It mentions that @ 10KSPS Settling time is 100uSec. But it doesnt mentions about the conversion rate.

Please let us know what is Settling time? Is it applicable for ever conversion as I will be using external mux to provide input to the ADC and what is the conversion time. I have 16 channels to be converted.

Also I would like to know whether to use an continues conversion mode or single conversion mode. As the device is a battery operated I want an optimal solutions.

Regards,

ChiranjeeviM

  • 0
    •  Analog Employees 
    on Jun 9, 2013 4:34 PM

    Hi Chiranjeevi M,

    We are looking into this. We'll get back to you as soon as possible.

    Thanks and Best Regards,

    Chris

  • 0
    •  Analog Employees 
    on Jun 12, 2013 4:34 PM

    Hi Chiranjeevi M,

    The settling time is the time required for the digital filter to fully settle any step at the input or any data from another channel in the case where multiple channels are used. The duration of the settling time varies on what degree of filter is used and what output data rate is selected. The settling time is required for the first conversion on a newly selected channel or when moving from standby mode to any other mode. Subsequent conversions would then follow the selected output data rate (ODR) (ODR = conversion rate).

    The Sinc5 + Sinc1 filter is targeted at fast switching multiplexed applications and achieves single cycle settling at output data rates of 10 kSPS and lower. This means that for output data rates of 10 KSPS and less the conversion time and settling time are equal which is also known as single cycle settling.

    When using continuous conversion mode, the part will perform a new conversion as soon as the previous conversion was completed even if it has not been read. If there is a need to convert continuously, then, this mode can be used. If not, single conversion mode would work better as the ADC returns so standby mode after each conversion and waits for a write to the mode register before starting another conversion. This would save significant power if only occasional measurements are to be done.

    Thanks and Best Regards,

    Chris

  • HI Chris,

    Thanks for the info.

    I have one doubt in continuation to your explanation. If I select 10kSPS data rate, then the ADC will have 100uSec settling time and 100uSec conversion time. Is my understanding correct?

     

    Also in our circuit, we have 16 Channels which are externally multiplexed (Analog Switch matrix) and fed to AD8231. Output of AD8231 is fed to AD8639 Opamp. Finally differential output of AD8639 is fed to AD7176-2.

    1. In this case pls clarify if there is any aliasing effect on the input of the ADC, since we are using external Mux.

    2. What will be the timing requirement to avoid aliasing between the channel switching?

    Regards,
    Chiranjeevi M

  • 0
    •  Analog Employees 
    on Jun 20, 2013 1:24 PM

    Hi Chiranjeevi,

    100us will be all it takes to get a conversion sample.

    So switch -- start conversion -- 100us -- DOUT/nRDY will be pulled low and you can get your conversion data.

    For your system, in case you didn't notice there is a "MUX to GPIO" feature in AD7176-2(search MUX_IO in the datasheet) which allows you to sync each switching of the external MUX with the start of conversion.

    However there are only 4 internal channels in AD7176-2.

    My suggestion is:

          Wire the GPIO pins to lower 2-bits of the 16-chn MUX decoder, control the higher 2-bits with you MCU;

         Enable MUX_IO;

         Enable all four channels, use same setting for"CH_SELx" on each channel, also same setting for "AINPOSx" and "AINNEGx".

    Now the loop:

         1.Set the higher 2-bits of MUX decoder to "00";

         2.Set ADC to Single Conversion;

         3.Watch the DOUT/nRDY line and read back conversion when it's ready;

        DRDY will happen once every 100us till all four channel is converted, then ADC puts itself to standby mode.

         4. Set higher 2-bits of MUX decoder to "01", repeat from step 2 and 3 to convert next 4-channels;

         ......

    There is also a delay that can be programmed ("DELAY" bits in MODE register) so the ADC will wait a short time before start of each conversion allowing external circuit to settle after switching. You can look into that.

    Regards

    Cheng