TMS320F28379D: 3.3v and GND are getting shorted , ADC pins are getting damaged in Launchpad F28379D ????

Hi Kailas,

Can you please describe how the 5.5K potentiometer is used with the output of the pressure sensor?  Are you sampling the output of the pressure sensor with a voltage divider (the 5.5k pot) through the analog channels of the TMS320F28379D Launchpad?  The 3.3V to GND impedance on the Launchpad should be in the order of kilo ohm.  There is an on-board LDO that is powered from USB and produces the 3.3V and the 1.2V rails that the F28379D needs in order to function properly.  The 3.3V rail is really not intended to be used as an external supply (or reference) to other external circuits.

What is output level of the pressure sensor?

As for the SMA connector,  these are intentionally not shipped wit the launchpads.  Users can purchase them separately.  They are used as input connections for the differ  that is present on the Launchpad.  The output of the differential buffer directly goes to ADCIND4 and ADCIND5.

Regards,

Joseph

Hi Joseph

Thanks for your reply.

I am using two ADC pins and one I2C module .

1. Connection is like 3.3v from launchpad  + series 470ohm resistor + series 5.5Kohm potentiometer  and output of POT is given to ADCINB2 / ANALOGIN pin.

2. Second device is pressure sensor (4mA-20mA Output), connection is like: output of of pressure sensor + Series 100ohm resistor +LaunchPad gnd and                output  accross 100 ohm resistor is directly given to ADCINA3 / ANALOGIN .(I/p to pressure sensor is 12v which is externally generated )

3. MPL3115A2 I2C Altitude pressure sensor connected on J5 jumper  which works on launchpad's 3.3V. (which needs 2mA Max Current).

These are connections on Launchpad.

Hi Kailas,

Please see my comment below:

1. Connection is like 3.3v from launchpad  + series 470ohm resistor + series 5.5Kohm potentiometer  and output of POT is given to ADCINB2 / ANALOGIN pin.

- No concerns here.  The voltage divider from the 470ohm + 5.5K pot should not cause any damage to the 3.3v rail and ADCINB2 pin.

2. Second device is pressure sensor (4mA-20mA Output), connection is like: output of of pressure sensor + Series 100ohm resistor +LaunchPad gnd and                output  accross 100 ohm resistor is directly given to ADCINA3 / ANALOGIN .(I/p to pressure sensor is 12v which is externally generated )

- I'm concerned about the implementation.  I do not fully understand how these are all integrated and if you could provide a diagram on how the 100ohm resistor and the pressure sensor is is connected to ADCINA3, that would help.  Also what is the output spec of the pressure sensor (specifically, what is the maximum output voltage)?  You mentioned that 12V is externally supplied to the pressure sensor.  Did you get a chance to measure the output statically with a voltmeter?  My concern here is that if the pressure sensor output is greater than the 3.3V rail, conversion results are not guaranteed and it can cause damage to the analog inputs.

3. MPL3115A2 I2C Altitude pressure sensor connected on J5 jumper  which works on launchpad's 3.3V. (which needs 2mA Max Current).

- No concerns here.  The 2mA supply current requirement for the alt pressure sensor should not cause the on-board LDO to overload or short.

Thanks and regards,

Joseph

Hi Joseph

here is the circuit diagram (sorry its hand made but i hope okay for understanding ) of following section:

2. Second device is pressure sensor (4mA-20mA Output), connection is like: output of of pressure sensor + Series 100ohm resistor +LaunchPad gnd and          output  accross 100 ohm resistor is directly given to ADCINA3 / ANALOGIN .(I/p to pressure sensor is 12v which is externally generated )

This sensor gives output in range of current : 4mA - 20mA.

I kept 100 ohm resistor in series so maximum output voltage is  20mA* 100ohm= 2V

Thanks for giving your comment on other two connection. (POT and Altitude pressure sensor).

Hi Kailas,

The way the 100ohm resistor is connected, it would not limit the voltage going into the ADC input.  Connection from the pressure sensor output to the ADC input of the Launchpad and the 100ohm is in parallel so the 100ohm resistor just acts as an additional load so whatever voltage that gets out of the pressure sensor output still gets directly fed to the ADC input.  This is how I interpret your schematic diagram.  Do you agree or am I missing anything?

With the current connection, there is a risk of having a voltage from the pressure sensor output that is way above the 3V3 supply and this will damage the ADC input pin of the F28379D.  What I suggest is to connect a voltage divider instead on the output of the pressure sensor.  Maybe start first with measuring the voltage of the pressure sensor output with a DC voltmeter while not connected to the Launchpad and apply different pressures that you expect in your application so you can determine the minimum and maximum voltage from the pressure sensor.  Once you have the min and max voltage values, you can use these to derive the 2 resistor values for the voltage divider and limit the voltage going in to the ADC input of the Launchpad to be less than 3.3V.

Regards,

Joseph 

Hello Joseph,

This Sensor gives 4-20 mA output and unless and until we connect resistor at output of this sensor we will get 12V as output.

This acts like current source so we have to connect resistor.

if i connect 100ohm at its output it will give maximum 2V.

I personally checked output of pressure sensor connecting to launchpad as per schematic diagram.

It never gives output voltage above 2V.

Hi Kalias,

I'm not familiar with the pressure sensor specifications but it seems like the pressure sensor has a load detection circuit and it will deliver the load current if it detects a closed loop.  Maybe during the load detection process, the pressure sensor delivers low current signal first but the voltage could be closer to 12V and if it senses a load the feedback loop in the pressure sensor slowly increases the current and decrease the voltage.  This might be a transient event that happens too fast for a voltmeter to detect.  Maybe you need to scope the output of the pressure sensor to be sure.

However, you still can safeguard the ADC input of the F28379D if there are transients greater than 3.3V going to the analog pin.  In your circuit, add a clamping diode with fast transient response with the anode connected to the ADC input side and output of the pressure sensor and the cathode connected to a separate 3.3V rail (not the 3.3V rail of the launch pad - this is important - reason being is that you do not want the clamping current to all go into the Launchpad as this can cause more damage).  If you decide to do this, ensure that all the GND connections are common for the Launchpad, the pressure sensor system and the external 3.3V source where you will connect the cathode side of the clamping diode if you opt to add this.

Regards,

Joseph

Thank You Joseph .

I will try to implement this.