SBOU024C august 2004 – july 2023 PGA309
Two 16-bit DACs are incorporated into the PGA309 for fine adjustment of the Zero DAC and Gain DAC. These DACs are based on a Resistor String (R-String) architecture with very low integral and differential nonlinearities.
The Zero DAC incorporates a buffer amplifier in a gain of 2V/V. The DAC resistor string is connected between the REFIN/REFOUT (VREF voltage) pins and GNDA. The input digital value adjusts the point on the resistor string where the noninverting amplifier input is connected between 0 × VREF to 0.5 × VREF, thus adjusting the Zero DAC output voltage from 0V to VREF. Due to the device output saturation of the buffer amplifier, the linearity of the Zero DAC is specified from 2% to 98% of the digital scale with VREF = VSA. However, for cases when VREF < VSA (for example, when using the PGA309 internal reference), the Zero DAC is linear to 100% of full-scale.
The Gain DAC uses a similar R-String architecture. However, the Output Amplifier is performing the function of the buffer amplifier. The R-String of the DAC is connected between the output of the Front-End PGA, VFRONT, and GNDA (see Figure 2-3). The input digital value adjusts the value of the noninverting amplifier input between 1/3 × VFRONT to 1 × VFRONT, thus setting the attenuation factor of the Gain DAC from 0.333V/V to 1V/V with 16-bit precision.
The output of both the Zero and Gain DACs are calculated and adjusted on every Temp ADC measurement according to the Lookup Table stored in EEPROM (see Section 3.2, EEPROM Content and Lookup Table Calculation). This leads to DAC code adjustments on small temperature changes. Unlike some string DACs, the proprietary switch architecture of the PGA309 Zero and Gain DACs allows switching with very low glitch energy and essentially no dependency on the code being changed. The glitch energy is normally lower than the voltage noise level at the output of the PGA309.