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ADS1299-4: External resistor value can't be measured correctly.

Kazuya Nakai59
Guru 21595 points
Part Number: ADS1299-4
Other Parts Discussed in Thread: ADS1299

Hello guys,

One of my customers is evaluating ADS1299-4 on their own board for their new products.

They measured several resistor values using the input circuit like the attached PDF.

The result is the follow.

External resistor           Difference voltage between           Converted resistor value

value (kohm)                 the resistor terminals (V)               from read out data (kohm)

10                                  0.0576                                           10.07

56                                  0.319                                             53.33

100                                0.568                                             94.49

200                                1.110                                             185.36

300                                1.625                                             273.15

400                                2.117                                              353.18

560                                2.313                                              384.87

1000                              2.386                                              396.35

At this moment, they have the following questions.

Could you please give me your reply for them?

Q1. As the external resistor value increases, the errors between actual value and converted resistor value are increased in the negative direction.

      Do you think what the cause is? Is it the internal current source (6uA) error? Does the source current decrease when INP voltage is increased?

Q2. Should the internal current source be used for Lead-Off detection only?

Q3. Do you have any recommended circuit for external resistor value measurement?

Your reply would be much appreciated.

Kazuya.

ADS1299_4_input_schematic.pdf

  • 0
    TI__Genius 11550 points

    Hi Kazuya-san,

    Thanks for the post. Please see my comment below.

    Q1. There will be some limit to where the DC leadoff current source will start to decrease and cease to operate as the INP voltage gets closer to the AVDD supply. In your test setup, it will be approximately 400 kΩ (2.5V/6μA). To measure larger external resistance, you can use the smaller DC leadoff current source (6/24nA).

    Q2. Yes, the purpose of the dc leadoff current source is for leadoff detection.

    Q3. We do not have any unique circuit for measuring the external resistor value.

    Thanks

    -TC

  • 0 in reply to TCT
    Guru 21595 points

    Hello TC,

    Thank you very much for your reply and I'm sorry to be late response.

    Your reply is very helpful for the customer and me.

    Thank you again for your help and best regards,

    Kazuya.