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ADS1243 clarification

Harini Ramanujam
Prodigy 635 points
Other Parts Discussed in Thread: ADS1243

 

Hi,

 

Can you please help with below clarification for one our customers on the ADS1243?

 

ADS1243 init and operation question; Unstable conversion results A. A few questions about ADS1243 usage: My code initialize and read data from ADS1243 with the following sequence: Init: 1. Reset ADC: send 0xFE 2. write to SETUP Reg: Send command 0x50, send 0x00 (to indicate 1 reg will be written), send data for SETUP reg. 3. write to MUX Reg: Send command 0x51, send 0x00, send data for MUX reg. 4. write to ACR reg: Send command 0x52, send 0x00, send data for ACR reg. 5. Wait for 250 instruction cycles Send SELFCAL command, 0xF0, to ADC Wait for 100ms delay Poll the /DRDY pin and wait until /DRDY pin=0 Read data: 6. write to MUX Reg: Send command 0x51, send 0x00, send data for MUX reg. 7. Send DSYNC command, 0xFC, to ADC 8. Read data but not using it -- Send RDATA, wait 25 instruction cycles, and then poll the /DRDY pin, clock out 24-bit data when /DRDY=0. 9. Poll the /DRDY pin, and wait until /DRDY goes back to high, get ready to read the 2nd conversion -- Send RDATA, wait 25 instruction cycles, and then poll the /DRDY pin, clock out 24-bit data when /DRDY=0. B. Unstable conversion results We're not getting 24bit resolution, and not even close to 19-bit resolution. Hardware: => Vdd=3.6V, Vref+=1.203V, Vref-=Gnd => Xin=1.8432MHz Software settings: => RANGE = 0 , UNIPOLAR output format, channels pair: Ain2 (+ve) and Ain3 (-ve, connect to Gnd), Gain=1, no burnout current, SPEED=0, the rest use the default settings. For example: => Vsense (V drop across RTD) = 1.165V ADC output = 2^24 [Vsense*Gain/Vref

 

Thanks,

Harini

  • 0
    TI__Guru* 97465 points

    Harini,


    The setup described looks ok and the order in which the registers are programmed and the device is setup looks ok. However, getting this part to have a low noise performance requires a lot of attention to detail.

    First I would start by looking at what type of noise performance you should expect to see based on the configuration. With the given PGA=1, you would still need to know what the DR setting is, and if the buffer is on or off, but you might expect somewhere near 20 as the effective number of bits (from rms value).

    However getting this noise performance means that you need to pay close attention to your noise sources. The first thing I would do is to short the inputs together and measure the noise when the input voltage is 0V. If this noise much lower than when the input is near full-scale, your reference noise may be contributing to the measurement and you need to create a lower noise reference for the circuit.

    If this is not the case, then the source you are measuring may have be contributing a large amount of noise and you will have to figure out how to make a lower noise measurement for the ADC to measure.

    Much of this may depend on the system that the customer has created and the type of measurement being made. If this is a measurement made from sending a current through an RTD and another resistor being used as for the reference voltage, then this may already reduce some of the noise coming from the current source.

    Another thing to consider is to take the raw data (the hex code readout coming from the ADC) and plot that over time. It is possible that there is some periodic noise coming from another source (supply noise or digital supply interference that is contributing to noise.


    Joseph Wu

  • 0 in reply to Joseph Wu
    TI__Prodigy 635 points

     

    Thanks Joseph. They are using your suggestions to dig into this. Can you help with some other clariifcations below?

     

    Question about the Vref Setting for ADS1243 Based on the datasheet: Vdd=3V => Vref typ. = 1.25V, and Vref max.=1.3V. If Vdd=3.6V, can my Vref to be set to 1.5V (max=1.56)? Since the ratio 1.25/3 = 0.41667, where 3.6V*0.41667=1.5V.

  • 0 in reply to Harini Ramanujam
    TI__Guru* 97465 points

    Harini,


    While there isn't a basic equation to rely on for this specification. You should be able to use a 1.5V reference easily with the 3.6V supply.

    The limitation of the reference comes from the operation of the ADC and the output range of the modulator.


    Joseph Wu