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ADS7863A:REFINとREFOUTの関係について

MINORU YOSHIDA
Prodigy 30 points
Part Number: ADS7863A
Other Parts Discussed in Thread: THS4531A, ADS7863

ADS7863Aを使用して、ADCを使う場合、ADC入力信号が小さい場合に前段で差動AMPを使用します。
ADS7863Aのアナログ側は5V、デジタル側を3.3Vで使用します。
THS4531AのAMPの電源は5Vで、中点は2.5Vです。

質問1 THS4531Aの差動AMPを使用しますが、そのTHS4531AのCOM端子が入力の基準になるため、
THS4531Aの差動AMPのCOM端子をADS7863のREFIN端子に一般的には接続すべきでしょうか?

質問2
また、THS4531Aの差動AMPのCOM端子をADS7863のREFIN端子へ未接続でも問題なく使用できますか?


質問3
THS4531Aのリファレンス電圧が下がった場合、未接続であれば、ADCのDACのレジスタ設定を変更することで
入力側の基準電圧と、ADC側のREFIN電圧と乖離がでるが、それはレジスタ設定にて、THS4531Aのリファレンス電圧基準電圧に近づけることができるので
パフォーマンス的に問題はないという解釈でよいでしょうか?

質問4
ADCのREFOUTとREFIN端子を接続したまま、THS4531Aの差動AMPのCOM端子も接続するのが、良いという解釈でよいでしょうか?

質問5
THS4531Aの差動AMPのCOM端子をADS7863のREFINへ接続しない場合において(未接続)も
DACのレジスタ設定で、VREFを変更することで、キャンセルされますでしょうか?

  • 0
    TI__Genius 17857 points

    Minoru,

    Please repost this question in English or work with local TI office.  Sorry for the difficulty.

    Art 

  • 0 in reply to Art Kay
    Prodigy 30 points

    QA.txt 
    When using an ADC with the ADS7863A, the differential AMP is used in the previous stage when the ADC input signal is small.
The analog side of the ADS7863A is 5 V and the digital side is 3.3 V.
The AMP of the THS4531A has a power supply of 5V and a midpoint of 2.5V.

Question 1 
 The differential AMP of the THS4531A is used, but the COM terminal of the THS4531A is the reference for the input.
 Should the COM pin of the THS4531A differential AMP be generally connected to the REFIN pin of the ADS7863?

Question 2
Also, can the COM pin of the differential AMP of the THS4531A be used without problems
 even if it is not connected to the REFIN pin of the ADS7863?

Question 3
If the reference voltage of the THS4531A drops, if it is not connected, 
you can change the register settings of the DAC of the ADC.
There is a discrepancy between the reference voltage on the input side 
and the REFIN voltage on the ADC side, but it can be set close to the reference 
voltage of THS4531A by register setting.
Is it correct to interpret that there is no performance problem?

Question 4
Is it correct to assume that it is a good idea to connect the COM pin of 
the THS4531A differential AMP while connecting the REFOUT and REFIN pins of the ADC?

Question 5
Even when the COM pin of the differential AMP of the THS4531A
is not connected to the REFIN of the ADS7863 (not connected)
Will it be canceled by changing VREF in the register setting of the DAC?

  • 0 in reply to MINORU YOSHIDA
    TI__Genius 17857 points

    Minoru,

    Let's start by looking closer at the THS4531A configuration for driving the ADS7863.  I will address other questions in a separate post.  

    Answer to question 1:

    First you need to understand the input range for ADS7863.  The differential input range is -Vref to +Vref, or -2.5V to +2.5V in your case.  The absolute input range is -0.1V to AVDD+.1, or -0.1V to 5.1V in your case.  That means that either input can range from -0.1V to 5.1V, and the differential input must be between -2.5V and +2.5V.  

    You can simulate the FDA in TINA spice.  Below are simulation results and the TINA source file.  The simulation uses Vcm = 2.5V and the differential output swing is -2.5V to +2.5V.  The absolute input to the ADC from each input is 1.25V to 3.75V.  Thus, connecting the 2.5V reference to the THS input works well for the ADC.  I show the simulations as I think you should double check your configuration against the simulation and adjust the simulation as needed.  From the ADC perspective, you just need to make sure that the absolute input range and differential range are not violated.

    THS4531A.TSC