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ADS1261: Dual Bridge with AC Excitation - Schematic Review

Miguel Sanches
Prodigy 150 points
Part Number: ADS1261
Other Parts Discussed in Thread: , ADS1235

Dear all,

I am trying to use an ADS1261 in AC Excitation mode to read two full Wheatstone Bridges. Thus, I am planning to use AIN0 and AIN1 as REFP0 and REFN0, and use AIN2 and AIN3 as REFP1 and REFN1. The AC Excitation is 2 wire and then use the pairs ANI6 - AIN7 and AIN8 and AIN9 as differential inputs for each bridge.

The schematic that I came across with is as follows, but I not sure of it and I also have some doubts:

1) - the AINCOM must be connected to some signal or can be left floating?

2) -  the REFOUT must be connected to 5V? or should I use the 2.5V. The bridge is being powered with AVDD = 5V and DVDD = 3.3V for the MCU

Thanks in advance,

Miguel

  • +1
    TI__Mastermind 40211 points

    Hi Miguel,

    The schematic you have is pretty straightforward, and is effectively a copy of the ADS1261EVM (which includes the UCC gate driver for AC excitation). In fact, if you wanted to verify the performance of your system in advance, you can implement it directly on this EVM.

    The only comment I have is I am not sure why you have 100ohm resistors between the REF inputs. If you want a ratiometric reference relative to the bridge excitation voltage then I presume these resistors could be removed.

    Answers to your questions:

    1. Pursuant to section 10.1.4 in the ADS1261 datasheet "to minimize input leakage of the measurement channel, tie unused inputs to mid-supply voltage (AVDD+AVSS)/ 2 or to AVDD."
    2. REFOUT is an output from the ADC, specifically the output from the ADC's internal 2.5V reference voltage. This does not need to be connected to anything if you do not need to use it. Since you are using a ratiometric reference I am not sure you even need to enable the ADC's internal reference.

    -Bryan

  • 0 in reply to Bryan Lizon86
    Prodigy 150 points

    Hi Bryan,

    The resistors should have been 100k. I based this on the ADS1235 application example. That is why I was wondering if I should have them or not. But I am looking for a ratiometric reference, so I will take them out

    Regarding my questions:

    1 - I will tie AINCOM to AVDD then

    2 - I connected a 10uF because it says in the datasheet in the pin description. But since REFOUT is an output I don't think I will need to use it, so leave it floating

  • 0 in reply to Miguel Sanches
    TI__Mastermind 40211 points

    Hi Miguel,

    Okay, the 100kohm resistors make more sense. This is actually described in section 9.3.3.4 in the ADS1261 datasheet, and these resistors are used to detect a missing reference voltage. If the bridge supply becomes disconnected, that resistor pulls the REF pins together so they approach 0 V, which can be detected by the ADS1261's internal VREF monitor alarm.

    -Bryan