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ADS1248/1148 IDAC Question

Robert Learney
Intellectual 280 points
Other Parts Discussed in Thread: ADS1248

Hello,

I'm trying to do some basic calculations regarding the common mode and differential inputs for the ADS1x48 device.

May I ask:

If my Pt100 sensor recommends a max current of 1mA and I use it in 3-wire mode, does this mean the maximum I can put through each limb is 500uA (total through Rbias = 1mA), or 1mA (total through Rbias = 2mA)?

What voltage am I aiming to get across Rbias? AVDD/2 or 2.048V? I'm using a 3.3V supply, so AVDD/2 = 1.65V

Can you also help me work through an example:

If I have PGA=128, this will give me a max differential input of 12.9mV. If I have Rcomp = 100ohms and I want to measure a Pt100 RTD at 37C = 114.4ohms, I'm really trying to measure 14.4ohms.

So for example with 2x500uA IDACs (refers to my question regarding how much current is passing through the RTD), my max diff input will be on the order of 2.2mV (0.5mA*14.4ohms), giving me plenty of headroom for this PGA setting. My modulator voltage will then be 128*2.2mV=280mV.

How do I then relate these numbers to choosing an Rbias value to get to the middle of the common mode voltage for my inputs (relating to my questions regarding the desired Vref)? In a 3-wire setting, do I use Vref_desired/(2*IDAC)?

Many thanks!

-Rob.

  • 0
    TI__Guru** 112765 points

    Hi Rob,

    Usually the RTD max current refers to maximum current through the RTD element, but this question is better asked of the RTD manufacturer.  You do not need to use the maximum current.  There is self heating that will occur due to the current flowing through the RTD resistance.

    The better approach is to set the bias point near mid-supply for the 3.3V AVDD.  This overcomes the common mode range issue.  This means the Rbias resistor for the reference should be sized appropriately for the desired current so that you do get about 1.65V dropping across it.  All current from both IDACs need to go through the Rbias resistor.  So if you use 2x500uA you have 1mA going through the Rbias resistor and it needs to be a stable, precision resistor of about 1.65k ohms.

    It is not clear how you did your math (maybe it is a typo), but if you use the Rcomp resistor (which must also be a stable, precision resistor) then you add the voltage drops in the loop.  If the RTD is 114.4 ohms with 500uA you will see a voltage drop of 57.2mV.  Current in the opposite direction will flow through Rcomp (100 * 500uA = 50mV).  The result of the voltage drops as seen by the analog inputs is 57.2mV - 50mV which equals 7.2mV.  The full-scale range at 1.65V reference with gain of 128 is +/-1.65V/128 = +/-12.89mV.  Using a gain of 128 will work in this case, but not very far beyond that temperature range.

    You may want to take a look at this design:

    http://www.ti.com/tool/TIPD120

    Also, you may want to take a look at the WebBench example.  Go to the landing page for the ADS1248, and select the WebBench tool.

    Best regards,
    Bob B

  • 0 in reply to Bob Benjamin
    Intellectual 280 points

    Thanks very much Bob!

    The reference design you've provided is very useful, as is your detailed answer to my second question.

    May I just ask for a little clarification on one thing - I'm not an EE by background, just a general tinkerer which is why this might not be the smartest question you've had:

    With an RTD connected to AIN0 and AIN1 in a 3-wire configuration as shown in one of TI's data sheets (attached here), and 500uA coming from each IDAC through AIN0 and AIN1, will the RTD see 1mA, or just 500uA?

    Many thanks!

    -Rob.0181.3-Wire RTD Connection.tiff

  • 0 in reply to Robert Learney
    TI__Guru** 112765 points

    Hi Rob,

    In the diagram you provided RL is the wire connected to the RTD.  Current will travel toward ground.  The current through the RTD is just the current from the IDAC coming through A0.  The current from A1 wlll sum at the junction of the RTD and the wire connecting to Rbias.  That is in shown by the red color of IDAC1 + IDAC2.  That means the current the RTD itself is seeing is 500uA by your example, but the total current is 1mA through Rbias.

    Best regards,
    Bob B

  • 0 in reply to Bob Benjamin
    Intellectual 280 points

    Excellent! Thanks Bob - I'm learning something every day.

    All the best,

    -Rob.