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Original question:

TM4C1294NCPDT: Baffled why ANIx sample amplitude decreases

TM4C1294NCPDT: AIN-16 unrelated switching noise

Genatco
Guru 54057 points
Part Number: TM4C1294NCPDT
Other Parts Discussed in Thread: LM94022

The gremlins seem to get more active when potentials around the ADC channels are elevated above 24v. The very last channel scanned (AIN-16) for conversion appears to spit out perhaps VREFA+ potential onto channels sample hold frame.

Is there possibility datasheet left out conditions where a VREFA+ termination resistor may be required on AIN-CH16? Why does adding a termination resistor appear stop the LM94022 output random elevated current gain when it should not have any gain just because PWM0 is active? Seems the powers of the universe are on our side pointing out certain anomaly otherwise flew under the radar.  Seem to recall long ago experimenting with Motorola ADC chip that required some value termination resistor on the last channel or a specific pin!

Given we sacrificed use of AIN-0 PE3 for conditions errata #13 silicon Rev.3, may also have similar origins we are currently having relative to AIN-CH16?

Is it not odd that the very last ADC channel is having elevated issues when PWM0 is mostly active, yet with a relative turtle slow 80us period?   

  • 0
    TI__Guru 72500 points
    Hi BP101,
    I can only think of two scenarios where more than the 2uA of leakage current comes out of ANI-16. The first is the very brief period when the internal sample capacitor (max of 10pF) discharges when the new sample voltage is lower than the previous sample. The second is if the internal ground of the ADC is higher than the voltage at the AIN-16 pin which is usually caused by induced noise.
  • 0 in reply to Bob Crosby
    Guru 54057 points

    Hi Bob,

    Well the internal ADC ground AGND is common with DGND. So AIN-16 pin starts out at +2.235v (22*C) and when PWM0 starts AIN-16 jumps to +2.978 or (-29*C) in direct opposition to AIN-8 which remains at 22-23*C. 
     
    The LM94022 datasheet shows a high impedance output (Totem Pole) directly coupled to SAR ADC inputs and only suggest to add series resistor if the output cap is above 1nf. The odder part is AIN8 LM94022 sensor has 1nf cap & remains fairly stable with PWM0 active. Recently tested AIN16 sensor with 0.2nf output made the voltage higher & temperature roll more negative than it did with 1nf. The only time the ANI16 input produced any kind of stable reading with PWM0 active and relative to ANI8 sensor occurred as the AIN16 sensor output had 2.9k short to its own ground pin. Both use sequencer 1 of ADC1 (AIN-16 step 0 & AIN-8 step 1).

    Very odd the last channel AIN16 (configured) seems to need a termination resistor to handle the high output impedance but AIN8 does not. Perhaps that is because AIN8 has channels on either side and AIN16 is located on the very end of the configured silicon gate chain.