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ADS1261EVM: simple two wire measurement

Yukio Kato
Prodigy 30 points
Part Number: ADS1261EVM
Other Parts Discussed in Thread: UCC27524, UCC27523, ADS1261

Tool/software:

I am trying to make very simple 2-wire measurement. I got 1kohm resistor between AIN0 and AINCOM, nothing extra connected.

I am doing following settings in console(apperantly GUI register map window, doesnt change these, so im running them in console)

// 1) REF
//    REFENB=0  (disable internal 2.5 V ref, use 5V)
//    RMUXP=INT reference P
//    RMUXN=INT reference N
WREG 06 00


// 2) INPMUX
//    MUXP=AIN0
//    MUXN=AINCOM
WREG 11 10

// 3) PGA_GAIN
//    PGA_GAIN=1 V/V (default)
WREG 10 00

// 4) MODE0
//    FILTER=SINC4
//    DR=100 SPS
WREG 02 3B


// 5) MODE1
//    CHOP:normal mode
//    CONVRT=Pulse (one-shot) mode

// DELAY 17.8ms
WREG 03 1D

// 6) IMAG
//    IMAG1=1000 µA
WREG 0E 06

//7 IMUX)
//    IMUX1=AIN0
WREG 0D F0


and then running RDATA command on console, gives me 0x00F5B4

That is 0.037491322 Volts.
meaning with my 1mA current it is 37.5ohms. Not 1kohm that i have connected between AIN0 and AINCOM



What i am doing wrong?

  • 0
    TI__Mastermind 45435 points

    Hi Yukio Kato，

    Did you turn on the VBIAS? I did not see it.

    BR,

    Dale

  • 0 in reply to Dale Li
    Prodigy 30 points

    Why does Vbias need to be enabled?

  • 0 in reply to Yukio Kato
    TI__Mastermind 45435 points

    Hi Yukio Kato，

    As you can see the state from the datasheet below, it can bias the input to the mid-power supply ((AVDD + AVSS) / 2) for internal PGA.

    9.3.4 Level-Shift Voltage (VBIAS)

    • The purpose of the voltage is to shift the signal level of floating sensors to within the input range of the ADC. Isolated thermocouples and piezoelectric sensors are examples of sensors that are suitable for connection to the level-shift voltage. For these sensors, connect the negative lead to the AINCOM pin and enable the level-shift voltage.

    I would suggest you to try.

    BR,

    Dale

  • 0 in reply to Dale Li
    Prodigy 30 points

    Thank you, that solved the issue.

    I am also trying to use AC exication mode. I can see the square wave with my ADC data, but i cannot see any difference in Vpp, i tried floating and connecting resistors there.

    My configuration is:
    MUXP:AIN2
    MUXN:AIN3
    IMAG2:500uA
    IMAG1:500uA
    IMUX2:AIN3
    IMUX1:AIN2
    REFENB: INT reference disabled
    RMUXP: AVDD
    RMUXN:AVSS
    CHOP:2-wire AC excitation mode
    CONVRT:Continous mode
    Delay:50uS
    DR:400SPS
    Filter: FIR


    Rest registers default.

    1kohm resistors connected between AIN2 and AIN3.
    I can see ADC outputting "square wave", but it seems not registering anything that is connected between those pins.

    DO i need to use ADS1261EVM pins EXC_out- and EXT_out+, or are those interconnected to AIN2 and AIN3 pins?

  • 0 in reply to Yukio Kato
    Prodigy 30 points

    My aim is to try out if AC mode can be used to measure impedance.
    So i want to input AC voltage through resistor and measure its peak to peak voltage.


    which mode should I use, 2-wire AC excitation mode or 4-wire AC excitation mode?
    Do I need GPIO registers, like GPIO_CON[1] and GPIO_CON[2]?

  • 0 in reply to Yukio Kato
    TI__Mastermind 45435 points

    Hi Yukio Kato，

    You need EXC+ and EXC- output FOR AC excitation on ADS1261EVM. A4 and A5 are used to control the EXC+ and EXC- through U1 UCC27524 on the EVM board by default, please see the schematic below.

    You also need to set these pins to GPIO to control UCC27524 or UCC27523 if you installed and replaced it.

    I strongly suggest you to check the details in the ADS1261EVM user guide and the application note: Reduce Bridge Measurement Offset and Drift Using the AC Excitation Mode (Rev. A)

    BR,

    Dale

  • 0 in reply to Dale Li
    Prodigy 30 points

    Hello,

    What are the actual register values that i need to turn on to get EXC+ and EXC* enabled?
    Especially which GPIO registers? GPIO_CON[X]?

    Would it be OK take this discussion to email?

  • 0 in reply to Yukio Kato
    TI__Guru 52146 points

    Hi Yukio Kato,

    Can you provide a picture of how you have your system connected? A high resolution picture would be best showing the sensor (resistor) and all of the wiring connections to the ADC.

    Regarding the register settings: the footnote in the image Dale provided tells you what to do (enable ACX1 and ACX2 GPIOs). These are GPIO2 and GPIO3 on the ADS1261, which are connected to AIN4 and AIN5 respectively

    You should connect these GPIOs to the appropriate analog input and set them as outputs in the MODE2 register

    -Bryan