MCP3208 Analog input model

6.3 Buffering/Filtering the Analog Inputs [From MCP3208 Datasheet]

If the signal source for the A/D converter is not a low impedance source, it will have to be buffered or inaccurate conversion results may occur (see Figure 4-2).

It is also recommended that a filter be used to eliminate any signals that may be aliased back into the conversion results, as is illustrated in Figure 6-3, where an op amp is used to drive the analog input of the MCP3204/3208.

This amplifier provides a low impedance source for the converter input, and a low pass filter, which eliminates unwanted high frequency noise.

Low-pass (anti-aliasing) filters can be designed using Microchip’s free interactive FilterLab® software. Filter-Lab will calculate capacitor and resistor values, as well as determine the number of poles that are required for the application. For more information on filtering signals, see AN699, “Anti-Aliasing Analog Filters for Data Acquisition Systems”.


6.5 Utilizing the Digital and Analog Ground Pins

The MCP3204/3208 devices provide both digital and analog ground connections to provide another means
of noise reduction. As shown in Figure 6-5, the analog and digital circuitry is separated internal to the device.

This reduces noise from the digital portion of the device being coupled into the analog portion of the device.

The two grounds are connected internally through the substrate, which has a resistance of 5 -10Ω.

If no ground plane is utilized, then both grounds must be connected to VSS on the board. If a ground plane is available, both digital and analog ground pins should be connected to the analog ground plane. If both an
analog and a digital ground plane are available, both the digital and the analog ground pins should be
connected to the analog ground plane.

Following these steps will reduce the amount of digital noise from the rest of the board being coupled into the A/D converter.

.END