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ul-gh / PiPyADC

Python classes for interfacing Texas Instruments analog-to-digital converters with the Raspberry Pi
GNU Lesser General Public License v2.1
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Full scale is not +/-5V #14

Closed Shlior closed 6 years ago

Shlior commented 6 years ago

Hi,

I'm evaluating my setup. Injecting sine wave at 1Hz +/-5 volts to channel A0.

Configuration: The gain setting (PGA) is 1. 50 SPS. AINCOM is connected to AGND. VREF 2.5V.

I'm expecting +/-5V full scale.. I get somewhat around +~4V and ~-0.15V.

What can be the cause for such behaviour? image-2

A correct behaviour example +/-0.13V sine: image

Thanks, Shlomi

ul-gh commented 6 years ago

Hi,

what i see from your graphs is clipping.

In this case, this has two reasons:

A)

The ADS1256 has typical CMOS inputs i.e. the input common-mode voltage range is GND...VDD and the internal gate electrodes are ESD-protected by silicon diode junctions.

The inputs cannot go significantly below the GND or above VDD potential.

If you want to measure AC voltages, you have to add a DC offset. The easiest method is a high-value resistor to the 2.5V REF voltage and an AC coupling capacitor in series with the AC source. You can also resistively add the DC offset using two resistors.

If you do this, of course you have to make sure that the automatic input offset-voltage calibration happens at the right moment with actually zero volts applied.

B)

If you enable the ADS1256 input buffer amplifier (this is the default configuration with the BUFEN flag), the analog input voltage range is further limited to a guaranteed "AVDD - 2.0V" according to page three of the datasheet.

You can then simply disable the integrated buffer amplifier and do without. If you have a high-impedance voltage source, you can add an external rail-to-rail operational amplifier as a buffer.

You can also just resistively divide any input voltage range to the range specified in the datasheet.

Shlior commented 6 years ago

Hi,

Thanks, I see now that the inputs are ESD protected, meaning the analog inputs cannot go below GND (minus diodes forward voltage).

Thanks, Shlomi