Have your PCB design isolated between AGND and DGND?

    

    Generally AGND, DGND stands for Analog and Digital Ground respectively. Recently most of the microcontrollers and AFE (Analog Frond End) chips are having separate ground pins (consists of AGND and DGND separately).

     

    Because of an Analog signal can be easily affected by a noise signal, which has been produced by a digital signal related components (ie... relay switching, solid state devices, motor driver and etc.).So we have to beware about PCB layout designing side.

    The figure (b)  is showing, how the ADC signal affected by the noise signal. Actually when we are consider about the digital signal, the MCU is having min and max threshold limit value by their manufacture for input signal processing. So we don’t care about digital signal consideration. But the ADC signal continuously has been taken when the ADC sampling processing time (figure(b)).So your noise signal also may be sampled at that time. It leads to take you observe wrong ADC values.

For Digital signals consideration (figure (a)),

0 to 1.5 -0 voltage that means ‘1.5 Volts’ –min threshold voltage level.

3.5 to 5 -5 voltage that means ‘3.5 Volts’ -max threshold voltage levels.

Note: min and max threshold voltage level may be varied from one MCU to another MCU. Here I have taken that values are approximately.

Here we can get only 0V or 5V .So the noise signal has never affect the digital signal.

For Analog signals consideration (figure (b)),

0 to 5 –it have collective of range (steps resolution).So we have to think about noise signal. It may affect our original ADC signal.

                                                                figure(a)

                                          figure(b)

We can reduce our noise level by the following ways,

1. Separate your AGND and DGND plane area in CPU and all add-on boards(ie.. even ADC or motor driver board also) as a figure (c).                                                                                             
2. Moreover trying to use individual power supply for all add-on boards (should be isolated ground) and finally connected together with all the ground pins (if needed) using star ground technique.                                                                                          
3. Always kept away your filter circuit and analog related components from your Motor driver or any solid state related components.                                                                                              
4. Place your sensor, Amplifier and AFE (Analog Front End)ADC chips very closely .If the length of wire or track on the PCB may increase ,you may lose few micro volts .(For 18 bits resolution ,each and every bit can be changed by 15.625uV at 2.5V Ref. So you have to create the best PCB trace for ADC higher bits resolutions).                                                                                                
5. The isolator IC which can prevent your components as well as reduce the noise level.

Here I gave few isolator ICs details with suitable application,

GPIO –ISO7340

SPI    –ISO7341

I2C    – ISO1540

PWM – ISO7821

The isolator IC can be used by the following three ways,

a) Level swifter (5 to 3.3V or vice versa)

b) Protect the pins from reverse current or any other fault condition

c) Isolate the ground signal from the noise signal

Note: Here I have just consider about TI chips only. But most of the companies, they are designing the same type of isolator chip with different type of the parameters. So let’s consider about it.

 If your design satisfies the above conditions, you can able to get the best accuracy results and protect your ICs from unwanted signals.

figure(C)

  

                                                         figure(d)