JAJSFM5 June 2018 ADS8688AT
PRODUCTION DATA.
The application circuit and system diagram for this design is illustrated in Figure 122.
The module has eight analog input channels, and each channel can be configured as a current or voltage input with software configuration. This design can be implemented using the ADS8688AT, 16-bit, 8-channel, single-supply SAR ADC with an on-chip PGA and reference. The on-chip PGA provides a high-input impedance (typically 1 MΩ) and filters noise interference. The on-chip, 4.096-V, ultra-low drift voltage reference is used as the reference for the ADC core.
Digital isolation is achieved using an ISO7141CC and ISO1541. The host microcontroller communicates with a TCA6408A (an 8-bit, I2C, I/O expander over an I2C bus). The ISO1541D is a bidirectional, I2C isolator that isolates the I2C lines for the TCA6408A. The TCA6408A controls the low RON opto-switch that is used to switch between voltage-to-current input modes. The input channel configuration is done in microcontroller firmware.
A low-cost, constant, on-time, synchronous buck regulator in fly-buck configuration with an external transformer (LM5017) generates the isolated power supply. The LM5017 has a wide input supply range, making this device ideal for accepting a 24-V industrial supply. This transformer can accept up to 100 V, thereby making reliable transient protection of the input supply more easily achievable. The fly-buck power supply isolates and steps the input voltage down to 6 V. The supply then provides that voltage to the TPS70950 (the low dropout regulator) to generate 5 V to power the ADS8688AT and other circuitry. The LM5017 also features a number of other safety and reliability functions, such as undervoltage lockout (UVLO), thermal shutdown, and peak current limit protection.
Input analog signals are protected against high-voltage, fast-transient events often expected in an industrial environment. The protection circuitry makes use of the transient voltage suppressor (TVS) and ESD diodes. The RC low-pass mode filters are used on each analog input before the input reaches the ADS8688AT, thus eliminating any high-frequency noise pickups and minimizing aliasing.