• State Resolved
  • Locked Locked
  • Replies 5 replies
  • Answers 1 answer
  • Subscribers 46 subscribers
  • Views 520 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

Original question:

ADS1241: Unclear value for Vref in the datasheet

ADS1241: ADS1241 reading negative millivolts

Akash Kasturi
Prodigy 105 points
Part Number: ADS1241

I am using ADS1241 to measure Thermocouple millivolts. How can I measure negative thermocouple millivolts with ADS1241. Can I connect negative voltage to VREF- (-0.23V) .

  • 0
    TI__Mastermind 49041 points
    Hi Akash,

    Please see VOLTAGE REFERENCE INPUT in the ELECTRICAL CHARACTERISTICS: AVDD = 5V table. You will see that applying a negative voltage is not within the device spec. Also take a look at Full-Scale Input Range - it is ±VREF /PGA.

    Looking at the inputs individually, or from a Single-ended standpoint, all signals must all be positive with respect to GND. Differentially however, it is possible to measure negative signals.

    Say that you configure the PGA to measure Ch1 - Ch2 with a gain of 1. If Ch2 is held at mid supply (2.5V for a 5V AVDD), then when Ch1 = 0.5V, the ADC will see -2V (0.5-2.5V). Alternatively, if Ch1 = 4.5V, the ADC will see positive 2V (4.5-2.5V).
  • 0 in reply to Alexander Smith
    Prodigy 105 points
    Hello Alex,

    What will be the ADC counts in unipolar configuration, because it is giving 0x000000 in case differential input is negative. I have taken 1.235 V Vref+ and Vref- is connected to gnd.

    Further How can I get stable counts of ADC?

    Thank You,
  • 0 in reply to Akash Kasturi
    TI__Mastermind 49041 points

    Hi Akash,

    Can you provide additional information regarding what exactly is being applied to the inputs? 

    What gain setting is being used? 

    Please see the Full-Scale Range (FSR) section on page 22 of the datasheet. 

    The count will never be truly stable due to noise saturating the least significant bits - there will always be a slight variation. 

  • 0 in reply to Alexander Smith
    Prodigy 105 points

    Hi Alex,

    I have applied millivolts differentially to channel AIN4 and AIN5 using a stable and accurate millivolt source. The Thermocouple Millivolts range from -8.9 mV to 70 mV. I want to cover the whole millivolt range using this ADC.

    I have set the PGA to 1.

    How can I measure the negative millivolts?

    I applied -10mV to the input and getting counts FF47D3 in hex. The reference is 1.235V and the format is bipolar. I am not able to decode these ADC counts.

  • 0 in reply to Akash Kasturi
    TI__Mastermind 49041 points
    Hi Akash,

    With an input range of -8.9 mV to 70 mV, and a reference voltage of 1.235V, you will need to increase the gain beyond 1 such that the input voltage is closer to the full-scale range of the ADC. The input signal is multiplied by the programmable gain amplifier (PGA) then read by the inputs of the ADC.

    The full-scale range of the ADC will be +/-VREF or -1.235V to 1.235V.
    Using a PGA gain of 16 would mean the range of your signal multiplies by 16, or -142.4mV to 1.12V. This is still not a great as much of the negative range of the ADC is not being utilized, but it is much more than was previously configured. If the input signal is level shifted and theinput magnitude is reduced slightly, then a gain of 32 could be used.

    Please see this thread for help with decoding ADC counts: e2e.ti.com/.../2856893