Interfacing the MAX1169 ADC to a PIC Microcontroller

This application note describes how to interface the MAX1169 analog-to-digital converter (ADC) to a PIC microcontroller. Example circuits and software for the PIC18F442 are provided. The software contains function calls to interface the ADC to the PIC microcontroller at 400kHz using the internal MSSP I2C* port.
The MAX1169 is a 16-bit, low-power ADC with an I2C-compatible 2-wire serial interface. The MAX1169 interface supports both fast mode (400kHz) and high-speed mode (up to 1.7MHz).

This application note includes an example application circuit and software for the PIC18F442. The software provides function calls to interface the ADC to the PIC microcontroller at 400kHz using the internal MSSP I2C port. Because other microcontrollers have similar on-chip peripherals, the I2C communication routines provided in the example are intentionally split into separate I2C function calls to facilitate quick porting to other microcontrollers. Table 1 shows the separate I2C interface function calls used in the assembly program example.

The application circuit discussed here uses the MAX1169EVKIT, which includes the MAX1169, input buffer (MAX4430), and a proven PCB. The PIC and RS-232 transceiver IC (MAX3232) are not installed on the MAX1169EVKIT board. However, they have been added to the system, and a complete application circuit is shown in Figure 1. The SCL and SDA pads on the MAX1169EVKIT facilitate the connection of an I2C-compatible serial interface. For 400kHz I2C communication, 1.5k pull-up resistors are connected to R3 and R4.

The pinout of the MAX1169 facilitates the isolation of the analog and digital sections. The analog section is grouped on pins 8-13, distributed on the right side of the IC; the digital section is grouped on pins 1-7, distributed on the left side of the IC. Pin 14 is also part of the digital section, but can be conveniently organized with the left side of the IC. For best performance, it is recommended to use separate analog and digital power supplies, as shown in the schematic in Figure 1.

Figure 1. MAX1169 application schematic.

Separating the analog ground from the digital ground (as shown in Figure 2) can achieve better practical results. Use ferrite beads, such as TDK MMZ1608B601C, to connect the two ground planes. This layout can prevent the microcontroller system clock and its harmonic components from feeding into the analog ground. The system clock of the PIC18F442 is known to be 40MHz. Considering the special impedance and frequency characteristics of the MMZ1608B601C, we chose this ferrite bead. Figure 3 shows the impedance change curve of the MMZ1608B601C with frequency.

Figure 2. Separate analog and digital grounds.

Figure 3. TDK MMZ1608B601C ferrite bead impedance vs. frequency

The assembly program in the example reads the continuous conversion results of the 16-bit ADC MAX1169 at a frequency of 400kHz through the PIC's 2-wire interface. Once the PIC receives the data (digitized analog voltage), it will immediately send it out through the software UART at a rate of 115kbps. The RS-232 transceiver sends the data to a personal computer with a standard serial interface.

Table 1. Single I2C interface function calls

Figure 2. Separate analog and digital grounds.

Figure 3. TDK MMZ1608B601C ferrite bead impedance vs. frequency

The assembly program in the example reads the continuous conversion results of the 16-bit ADC MAX1169 at a frequency of 400kHz through the PIC's 2-wire interface. Once the PIC receives the data (digitized analog voltage), it will immediately send it out through the software UART at a rate of 115kbps. The RS-232 transceiver sends the data to a personal computer with a standard serial interface.

Table 1. Single I2C interface function calls