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TLC3578 A7 Channel Offset

Chuan Shen Tong
Prodigy 10 points
Other Parts Discussed in Thread: ADS8678

Hi,

I shorting the A7 channel to ground to do the offset correction for A0 to A6 channels. There seems to be a "residue error" of 20mV after subtracting the A0 to A6 reading with A7 offset.

It looks like A7 channel is seeing a different offset level, which is lower than other channels. There is a low pass filter to 4.096V Ref Volt input, tried shorting the series resistor still having the same observation. This has ruled out the possibility of load regulation issue to the Ref Volt.

By introducing an offset of ~14mV to A7 channel, I am able to null the offset of the A0 to A7 channel correctly.  

Can someone shed some light on this observation?

  • 0
    TI__Guru 52646 points

    Hello Chuan,

    I have a couple of questions/suggestions:

    - In this legacy device, the bipolar zero error between the different channels is not matched, therefore it is not possible to null the offset of one channel using a different channel.  Also, the offset of the buffer amplifiers may also contribute a couple of milliVolts to the offset error.

    -The schematic shows REFP at 4.096V.  This device requires VREFP=4.0V nominal; where VREF has to be in the range of 3.96V < VREFP < 4.04V. In this circuit, the REFP is outside the recommended operating range.

    - The bipolar zero error of the device is a typical of 0.08%; up to max of ±0.36% of Full-Scale (±10V).  The error of 20mV between the different channels is within the guaranteed offset.

    You could consider using a newer ADC device for your design that offers better precision/accuracy such as the ADS8678.  The ADS8678 is a 14-bit SAR ADC, 8 channels, 500 kSPS, and bipolar inputs off +5V supply.  This device offers much better precision, (i.e. ±1mV offset max), programmable input ranges, incorporates the analog front end, and internal reference.

    Let me know if you have questions,

    Thanks and Regards,

    Luis