ADS1120: The block diagram of ADS1120

Hi Miles,

See my responses below.

Best regards,

Bob B

Miles Li1 said:

Part Number: ADS1120

Team,

I am still confused about the application of ADS1120， so I would like to list more questions:

1. What is the function AVDD and AVSS? Where do they connect to? [Bob] AVDD and AVSS are the analog supply inputs.  AVDD  is for the most positive potential and AVSS as the most negative potential in relation to analog ground (AGND).  The analog supply can be unipolar where AVSS is connected to AGND or it can be bipolar where as an example, AVDD = +2.5V and AVSS = -2.5V.  For the ADS1120 there can also be an internal connection to allow the analog supply to be measured through a mux selection.  The analog supply can also be selected and internally connected to be used as the conversion reference voltage through the reference mux. 

2. What is the function of VREF if choose internal VREF? [Bob] The conversion output code is proportional to the reference voltage used.  There are four possible reference input selections selectable by register configuration.  The choices are in bits 7:6 of configuration register 2.  The full-scale range (FSR) of the ADC is +/-VREF/GAIN.  The default reference is the internal reference and is fixed at 2.048V which allows for a full-scale range of +/-2.048V at a gain of 1.  If an input is expected to be greater than 2.048V, then a larger reference voltage can be connected to either of the external reference inputs (REF0 or REF1).  As stated earlier, the analog supply can also be used as the reference voltage.  The input is measured differentially which means if AINP is greater than AINN the output code is positive. If AINN is greater than AINP the output code is negative. For example, if the internal reference is used, the FSR is +/-2.048V.  If AINP = +2.048V and AINN = AGND the result code will be full-scale positive (0x7FFF).  If AINP = AGND and AINN = +2.048V the result will be negative full-scale in binary 2's complement (0x8000).

3. If AVDD is tied to 5V, AVSS is tied to GND, VREF is choosing internal Vref, and bi-directional current need to be measured, which means that the shunt voltage is from -5V to 5V, could ADS1120 sample this voltage directly using the differential input? If it can, what is the value that could be read after ADC? [Bob] As stated above, the measured input is not measured relative to AGND but instead AINP relative to AINN.  If AVDD = +5V and AVSS = AGND and the internal reference is used, the FSR is +/- 2.048V.  Once the input is 2.048V there are no further codes available to represent voltages greater than 2.048V, so another reference source is required to measure +/-5V FSR.  One option is to use the internal reference mux connection to the analog supply.  This will allow a FSR of +/-5V.  However, this means with a unipolar supply that +FS is AINP = +5V and AINN = AGND.  -FS is AINP = AGND and AINN = +5V.  I emphasize that you will see output codes representing +/-5V but this does not mean relative to AGND but instead AINP relative to AINN.  The absolute input range of the ADS1120 is defined by the analog supply voltages at AVDD and AVSS.  Single-ended inputs referred to AGND can be measured using a unipolar analog supply in the + FS region only to +5V.  You cannot apply a negative voltage of -5V below AGND as this will damage the ADS1120. 

Wish to get your reply, thanks!