SBAA275A June 2018 – March 2023 ADS1120 , ADS112C04 , ADS112U04 , ADS1147 , ADS1148 , ADS114S06 , ADS114S06B , ADS114S08 , ADS114S08B , ADS1220 , ADS122C04 , ADS122U04 , ADS1247 , ADS1248 , ADS124S06 , ADS124S08 , ADS125H02 , ADS1260 , ADS1261 , ADS1262 , ADS1263
After selecting the IDAC current, use the maximum reference resistor possible, but consider several factors in the setting the reference. The reference voltage must be within the minimum and maximum reference voltages for operation. Many ADCs will have a minimum value of the reference of 0.5 V. Some devices will have a reference maximum of AVDD – AVSS, while others may have a lower maximum of AVDD – AVSS – 1 V. Consult the ADC data sheet for more specifications on the external reference input range.
A good selection for the reference voltage is using a value close to the midpoint of AVDD – AVSS. Often, this reference voltage is used to set up the common-mode voltage for the input measurement. PGA amplification may be limited by its input range and output swing. By setting the input common-mode voltage to the midpoint in the supplies, the PGA will have the maximum range possible. Many precision ADCs have a PGA that can amplify small input signals. These PGAs often will have gains from 1 V/V to 128 V/V in factors of 2.
Also, select a reference resistance that maximizes the usable input range of the ADC. As an example, it helps to show this with several values. Start with a 2-wire RTD ratiometric measurement with a PT100 where the maximum resistance is 400 Ω. This is the setup shown for a basic ratiometric measurement in Figure 1-4.
If the IDAC current is selected to be 1 mA, then the reference resistor could be chosen to be 1620 Ω. The measurement of the 400 Ω could be set to a PGA gain of 4. This would make the input voltage 1.6 V, while the reference voltage is set to 1.62 V. This would maximize the input voltage range of the ADC to 98.8% of the positive full-scale range. A reference resistor of 1600 Ω could have been chosen to maximize the ADC, however a small gain error or resistance error may push a 400 Ω measurement out of the range of operation. For this example, the next largest 1% resistor value above 1600 was selected.
Another benefit of setting the reference voltage to 1.62 V is that it sets the RTD measurement near the midpoint of the supply voltage. A reference of 1.62 V sets the input voltage for the ADC negative input. The input voltage is highest at the maximum RTD resistance is 0.4 V using an IDAC current of 1 mA and RTD resistance of 400 Ω. This sets the input voltage to 2.02 V for the ADC positive input.
Selecting a marginally larger resistance only reduces the resolution of the measurement. If the reference resistor is selected to be 2400 Ω then the reference voltage becomes 2.4 V. With an input to the ADC of 1.6 V (from 0.4 V after PGA gain of 4) compared to a reference voltage of 2.4 V, the ADC uses only 67% of the positive full-scale range.