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ADS1220: gain issue getting fully-differential signals both with the PGA enabled (channels A0-A1 and A2-A3)

misteralex
Intellectual 760 points
Other Parts Discussed in Thread: ADS1220

Hello,

I am using ADS1220 in standard electric configuration to collect measures from sensors

with fully-differential signal ranging within [0-10] mV.

I use the multiplexer configuration A0-A1 and A2-A3 for the channels (all with PGA enabled).

I successfully get values but with different gain for the same sampled signal.

Let's consider for example a differential signal input of 2 mV. I will read following

values from both channels:

A0-A1 = 867102         and           A2-A3 = 150619

I do reset and offset operations before polling data from channels.

Here my configuration:

Register 1 = 0x04  (20 SPS, normal mode and continuous conversion mode)

Register 2 = 0x00

Register 3 = 0x00

Offset Calibration: register 0 = 0xEE (Gain = 128)

A0-A1: register 0 = 0x0E (Gain = 128)

A2-A3: register 0 = 0x5E (Gain = 128)

The electric circuit configuration is the same for both channels.

I tried also the Gain = 1 but with the same result. The channel A0-A1

provides always a value about 6 times greater than A2-A3.


Welcome any advice.

Thanks

Alex

  • 0
    TI__Guru** 113765 points

    Hi Alex,

    The AIN0-AIN1 appears to be closer to the applied voltage then the AIN2-AIN3 pair.  Have you tried to keep things more simple by not trying to reset between reads and adjusting for offset?  Have you verified your communication?  How exactly are you reading the data (just sending SCLKs and capturing data or by using the RDATA command)?

    Can you send me your schematic?  And do you have any logic analyzer/scope shots of the communication?  Also, what is your SCLK frequency?

    Thanks,

    Bob B

  • 0 in reply to Bob Benjamin
    Intellectual 760 points

    Hello,

    thanks for your fast reply.

    We do reset and adjusting for offset only one time at startup.

    Anyway it is a pairing issue as you suspected.

    We have an HSP051-4N10 to protect differential signals against

    EDS between channels. Without it the issue is fixed.

    We need a better solution to manage properly signals from each

    sensor against EDS.

    Thanks

    Alex

  • 0 in reply to misteralex
    TI__Guru** 113765 points

    Hi Alex,

    On occasion I've seen board construction issues at the CM.  Possible conditions with small packages and closely spaced connections can have issues where there was either too much paste applied, flux residue,  or inadequate reflow time creating leakage paths.

    Best regards,

    Bob B

  • 0 in reply to Bob Benjamin
    Intellectual 760 points
    Hi Bob,

    this is exactly what we are experimenting with our small board.

    The EDS component gives issue also to the I2C bus.

    Welcome advice to fix this electric design question.

    Thanks

    Alex