PGA 309 use questions

Hi Joe,

The PGA309 calibration sequence measures the differential voltage of the bridge sensor (whether a real-world sensor or emulated one) at various temperature points. It does not calibrate the temperature sensor itself, per se.

When you execute the calibration procedure, you are filling out the entries in a lookup table (see section 3.2.1 for more details) where the indices correspond to readings from the Temp ADC. Once calibrated, when the Temp ADC returns a measurement, the PGA309 will use the Gain and Zero DAC values from the table that correspond with that temperature range. Thus, it is important to make sure the Temp ADC frontend is properly configured before running calibration. When using the PGA309EVM, these settings may be adjusted by selecting “Config Temp” when creating your Pre Cal File.

Note that when using the external temperature sense (as in your case), the "temperature" read by the Temp ADC is just a voltage. On the PGA309EVM, there is a temperature emulator DAC that generates a voltage that corresponds to the "temperature" you choose when setting up the cal, but the actual PGA309 itself just reads a series of voltages over the course of the calibration and then sorts them to make the table. Thus, the "temperature" readings you get will be specific to your RT resistor, bridge sensor, Temp PGA setting, Temp ADC reference setting, excitation voltage, etc.

You will need to select your desired Temp PGA gain (1x, 2x, 4x, or 8x) manually. You’ll also need to choose the appropriate Temp ADC Mux configuration for the circuit topology you are using. If you are employing a top-side resistor, then you’ll likely need Configuration 2, unless you are not using the linearization features of the device – in that case you’ll want Configuration 4. If you are using a bottom-side resistor you’ll want Configuration 1.

If you can share some details about the specific bridge sensor you are using, we can help determine the best PGA gain values and RT value for the circuit. If you can share the sensor datasheet, that would be best; otherwise, you will need to know the following information about the sensor:

• Nominal resistance (at room temperature)
• Expected drift (maximum and/or typical)
• Nonlinearity (if this information is available)

You will also need to determine:

• What range of temperatures the sensor will experience in the application
• Nominal excitation voltage of the sensor, whether using the internal reference voltage (2.5V or 4.096V) or an external reference. Often, a sensor’s datasheet will suggest an optimal range

Regarding the sensor output impedance, the impedance of the Temp sense circuit is fairly high (2.8MΩ divided by the Temp PGA gain) so there are a wide variety of sensors that may be used. If using the Vexc output as the bridge excitation, you will want to make sure the max current draw through the network is below 50mA. In terms of minimum signal levels, the resolution of the Temp ADC will dictate the signal step required in order for a change in temperature to be registered – just make sure the TEMPin signal level is no lower than 200mV below GND and no higher than 200mV above VSA.

Cheers,

Jon

Jon,

Jon,

Jon,

Hi Joe,

It appears your reply did not go through successfully, can you try to submit it again?

Cheers,

Jon

Jon,

I have been unable to respond to you. The forum was blocking me it said because I was using a personal email in my profile. This just started yesterday. I put my business email in my profile but it still is not working. Another Ti coworker (Carlo) is addressing this now. Meanwhile I figured out that I can communicate using the abbreviated version of our conversation. It doesn't allow me to see more than three sentences of the previous conversations but at least it isn't blocked. For now I also am not allowed to start an Associated question or a new question.

Now for work...

Thank you for the complete and very understandable answers to my questions. I had a conversation with my Boss and he would like very much to get your advice on the best way to implement temperature measurement. See his comments below.

Joe,

Yes, it is to our advantage to have TI advice.

You can send them the Excel file we have been using.

I have no other drift and nonlinearity values other than what can be derived from the individual gage resistance values found in the Measurements tab.

I would like to implement the bridge impedance method (TI PGA 309 User Guide, SBOU024B, revised Jan 2011, Section 2.7.2) for our application.

I would like to know how to select between the ‘top side series resistor’ Fig. 2-20 or the ‘bottom side series resistor’ Fig. 2-21 configuration and how to choose the value of the series resistor, R_T.

Thanks,

Frank

I have a file I want to send you so you can get the information you need to do your work. In this abbreviated thread area I cannot attach a file. hope fully you can send me another way that I can get it to you.  

Best,

Joe 

Jon,

I have been unable to respond to you. The forum was blocking me it said because I was using a personal email in my profile. This just started yesterday. I put my business email in my profile but it still is not working. Another Ti coworker (Carlo) is addressing this now. Meanwhile I figured out that I can communicate using the abbreviated version of our conversation. It doesn't allow me to see more than three sentences of the previous conversations but at least it isn't blocked. For now I also am not allowed to start an Associated question or a new question.

Now for work...

Thank you for the complete and very understandable answers to my questions. I had a conversation with my Boss and he would like very much to get your advice on the best way to implement temperature measurement. See his comments below.

Joe,

Yes, it is to our advantage to have TI advice.

You can send them the Excel file we have been using.

I have no other drift and nonlinearity values other than what can be derived from the individual gage resistance values found in the Measurements tab.

I would like to implement the bridge impedance method (TI PGA 309 User Guide, SBOU024B, revised Jan 2011, Section 2.7.2) for our application.

I would like to know how to select between the ‘top side series resistor’ Fig. 2-20 or the ‘bottom side series resistor’ Fig. 2-21 configuration and how to choose the value of the series resistor, R_T.

Thanks,

Frank

I have a file I want to send you so you can get the information you need to do your work. In this abbreviated thread area I cannot attach a file. hope fully you can send me another way that I can get it to you.  

Best,

Joe 

Sorry about the second message. I clicked a second time because it didn't respond

Hi Joe,

That is quite odd, hopefully Carlo will be able to figure out a solution for you. I have sent you a "friend request" via the forum. This should allow us to send messages directly. You may have better luck attaching the file under those conditions.

Cheers,

Jon

Hey Joe,

Sounds good! Feel free to start a new thread if you have any other questions. Good luck!

Cheers,

Jon