ZHCSID7 June 2018 ADS8688AT
PRODUCTION DATA.
The ADS8688AT has eight analog input channels, such that the positive inputs AIN_nP (n = 0 to 7) are the single-ended analog inputs and the negative inputs AIN_nGND are tied to GND. Figure 65 shows the simplified circuit schematic for each analog input channel, including the input overvoltage protection circuit, PGA, low-pass filter (LPF), high-speed ADC driver, and analog multiplexer.
NOTE:
n = 0 to 7.The device can support multiple unipolar or bipolar, single-ended input voltage ranges based on the configuration of the program registers. As explained in the range select registers, the input voltage range for each analog channel can be configured to bipolar ±2.5 × VREF, ±1.25 × VREF, ±0.625 × VREF, ±0.3125 × VREF, and ±0.15625 × VREF or unipolar 0 to 2.5 × VREF, 0 to 1.25 × VREF, 0 to 0.625 × VREF, and 0 to 0.3125 × VREF. With the internal or external reference voltage set to 4.096 V, the input ranges of the device can be configured to bipolar ranges of ±10.24 V, ±5.12 V, ±2.56 V, ±1.28 V, and ±0.64 V or unipolar ranges of 0 V to 10.24 V, 0 V to 5.12 V, 0 V to 2.56 V, and 0 V to 1.28 V. Any of these input ranges can be assigned to any analog input channel of the device. For instance, the ±2.5 × VREF range can be assigned to AIN_1P, the ±1.25 × VREF range can be assigned to AIN_2P, the 0 to 2.5 × VREF range can be assigned to AIN_3P, and so forth.
The device samples the voltage difference (AIN_nP – AIN_nGND) between the selected analog input channel and the AIN_nGND pin. The device allows a ±0.1-V range on the AIN_nGND pin for all analog input channels. This feature is useful in modular systems where the sensor or signal-conditioning block is further away from the ADC on the board and when a difference in the ground potential of the sensor or signal conditioner from the ADC ground is possible. In such cases, running separate wires from the AIN_nGND pin of the device to the sensor or signal-conditioning ground is recommended.
If the analog input pins (AIN_nP) to the device are left floating, the output of the ADC corresponds to an internal biasing voltage. The output from the ADC must be considered as invalid if the device is operated with floating input pins. This condition does not cause any damage to the device, which is fully functional when a valid input voltage is applied to the pins.