PGA305: PGA Transfer function

Allen12

Intellectual 435 points

Part Number: PGA305

Hello Scott, I did not find this information in the datasheet.

What is the formula to convert ADC counts to volt for the PGA305.

thanks

over 4 years ago

0 Scott Cummins over 4 years ago

TI__Mastermind 31655 points

Hello Allen,

This depends on the PGAIN setting.

The LSB voltage of the ADC is equal to the input voltage range divided by the number of codes (5V/2^24= 29.8uV). The input voltage corresponding to a single code is equal to the LSB voltage divided by the PGAIN setting. Therefore

Input voltage = ((ADC code) * 29.8uV) / PGAIN

Regards,

0 Allen12 over 4 years ago in reply to Scott Cummins

Intellectual 435 points

Hello Scott, The excitation voltage = 2.5V, the PGA is in the 16bit mode.

ADC voltage = ((Excitation voltage * 2) /2^16) /gain = (2 * 2.5V/65536) /Gain = 76.29uV

Is this correct?

One bit is used for the sign bit, is there a loss of resolution?

thanks

0 Scott Cummins over 4 years ago in reply to Allen12

TI__Mastermind 31655 points

Hello Allen,

The LSB size is determined by the full scale input range of the ADC, not by the bridge excitation voltage. Please refer to this blog post for more information:

It’s in the math: how to convert an ADC code to a voltage (part 1)

Many newcomers to analog-to-digital converters (ADCs) wonder how to convert their ADC code to a voltage. Or perhaps their question is specific to the application, such as how to convert the ADC code back…

By Christopher Hall over 9 years ago in Archives > Precision Hub

4 comments

In your case the equation would be the same as previously, but the total number of codes would be changed to 2^16 as you mentioned.

The ADC codes are stored as binary twos complement, and the resolution is still 16 or 24 bits.

Regards,

0 Allen12 over 4 years ago in reply to Scott Cummins

Intellectual 435 points

thanks

0 Scott Cummins over 4 years ago in reply to Allen12

TI__Mastermind 31655 points

Hello,

The LSB size is related to the full scale range (FSR) of the ADC, not the VREF. FSR is related to the VREF, though. In this case the input limits of the ADC are +/-VREF, so the FSR is is 2*VREF or 5V. Please check the link I posted in my last reply for more detail.

Regards,

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PGA305: PGA Transfer function