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Original question:

PGA Gain

ADS1299: ADS1299 giving 2.5x signal amplitude!

Swapniil Kanwar
Prodigy 20 points
Part Number: ADS1299

I have made a circuit as given in datasheet of ADS1299, Figure 73. Rp=47k, Rf=1Meg Ohm. 

In registers the gain of all channel is 12, I have applied the conversion factor to volts as 1 LSB = (2 × VREF / Gain) / 224 = +FS / 223 (Datasheet Section 9.5.1) 

I have enabled the BIAS LOOP. 

The problem is that whatever voltage I am giving on signal generator, the ADS1299 is giving me 2.5x bigger amplitude. I have tried voltages from 20mV to 1V peak-to-peak, All the conditions gave same result of 2.5x gain. Why is this so? 

  • 0
    TI__Mastermind 19815 points

    Hi,

    I will get back to you around 3/9

  • 0 in reply to ChienChun Yang
    TI__Mastermind 19815 points

    Hi,

    Do you have an ADS1299 Evaluation kit/board(EVM)?

    -----------------------------

    Before you use external signal, have you tried the internal test signal? It's suggested to try the internal test signal first to verify/validate your data parsing&conversion.

    It's suggested to use ADS1299 evm with an ECG or EEG simulator to do some basic experiment&testing&evaluation to get some understanding.

    -------------------------------------------------------------------

    Can you generate signal from 1 to 5mV and do you connect the signal to input using sequential or referential montage?

    Did you divide/place that gain into the equation you mentioned per datasheet? 

    --------------------

    Thanks

  • 0 in reply to ChienChun Yang
    Prodigy 20 points

    Hi, 

    I have made my circuit according to schematic and EVM board. 

    As per internal testing, I tried using internal impedance testing by generating internal signals and they are coming fine. The problem arises when I connect external signal ranging 20mV to 1Vpp. Yes whatever value I am getting from ADS1299 I am multiplying it with the scale factor as mentioned in datasheet/mentioned above. 

  • 0 in reply to Swapniil Kanwar
    TI__Mastermind 19815 points

    Hi,

    Please confirm that internal tests are fine?

    -----------------------------------------------------

    If the internal tests are fine, then you will need to

    compare your peripheral circuits with EVM

    -----------------------------------------------------------------

    and

    your external signal source?

    e.g. 

    Can you generate signal from 1 to 5mV and do you connect the signal to input using sequential or referential montage?

    Can you try DC 1mV first?

    --------------------------------

    It's highly recommended to use an ECG or EEG simulator to test this.

    Thanks