ADS1114, ADS1118 - Internal Voltage Reference

Takushi-san,


The internal reference does not go to any pin. It is constructed from sampled portions of a reference in the modulator.

This specification is best explained with a dc specification as the offset power power supply rejection and the the gain power supply rejection. These are both listed in the Electrical Characteristics table on page 3 of both the ADS1114 and ADS1118 datasheets.


Joseph Wu

Takushi-san,


As an example. let's take the ADS1118 gain-power supply rejection specification of 10ppm/V if the change in power supply is 20mV. This becomes 0.02*10ppm or 0.2ppm change in the gain. Similarly the offset power-supply rejection is 0.2 LSB/V or a change of 0.004 LSB for 20mV. For both these changes are very small and likely not noticeable in the data.

However, the data was taken with DC measurements and not AC measurements. Note that the ADS1115 and ADS1118 have very slow data rates and are generally considered DC parts, where making AC measurements are not as much a concern as for our wide bandwidth delta sigma ADCs. I don't have any data for any AC measurements except for the frequency response of the ADC's digital filter, which is mathematically constructed from the filter coefficients.

Note that for switching noise, there are a lot of variables about how this noise might be seen by the ADC. The switching rate, the shape and rate of the switching pulse, etc. Also, there may be some noise that may show up on the analog inputs.

I don't think that the 20mV switching noise will show up in the data. However, with switching supplies we have generally recommended using an LDO to clean up any switching noise from the supply. This way the ADC is isolated from the supply noise.


Joseph Wu

Takushi-san,


Let me describe it this way: since the bandgap reference consists of multiple parts, each part can be sampled separately into the ADC. The important thing to take away is that there is no reference that can be measured from outside the device.

This should make no difference to the customer. Again, the reference is not accessible from any pin.


Joseph Wu

Takushi-san,


The offset power power supply rejection is very similar to an DC PSR measurement. The gain power supply rejection shows how much the gain can change if the power supply changes. This would be similar to a PSR for the reference used by the ADC.

Gain and offset error can be affected by several things. Temperature can affect both and are shown in the specifications as a drift. Also, changes in common-mode input can also change the offset.


Joseph Wu