The input range is -Vref ---+Vref , If could i set the the Vref is 0V-5V / then input is,0V--+5V ? If set Vref is -5V---+5V, the input range is -5V---+5V?
what is the Analog input impedance of ADS8861?
Hello Andrew,
The ADS8861 uses differential inputs. Both the VREF input and AINP, AINM pins can only have positive voltages applied to them. VREF can range from +2.5V to +5V. AINM and AINP can range from 0V to +VREF. The difference in voltage between AINP and AINM (AINP-AINM) will be interpreted as either a positive or negative differential voltage.
For example, with VREF=+5V
AINP=+5V, AINM=0V, (AINdiff=+5V) or +VREF, the conversion result will be 0x7FFFh, or positive full scale equal to +VREF.
AINP=0V, AINM=+5V, (AINdiff=-5V) or -VREF, the conversion result will be 0x8000h, or negative full scale equal to -VREF.
The input of the ADS8861 is a switched capacitor with a total capacitance of around 59pF. During acquisition, this capacitor will chare to the input voltage. Once it is fully charged, there will be no more current flowing other than a leakage current of ~5nA.
I suggest you take a look at TI Precision Labs -ADCs that discusses these concepts in more detail. Section two specifically covers the differential input types and how to drive them.
Regards,
Keith N.
Precision ADC Applications
