PGA970: LASB Value for DEMOD 1 and DEMOD 2.

Thevindra Anand

Part Number: PGA970

Dear TI,

Can you please provide the LSB value for the DEMOD1_DATA and DEMOD2_DATA?

Based on the data sheet, there seems to be error in the 24bit-Noise-Free Decimator Output whereby for 1.25V, the Hex value is 0x28BE60 which in decimal value is 2667360. While on the data sheet shows 4194303. If calculation for LSB value is done for each of the following are;

LSB = 1.25 / 2667360 = 4.68628157e-7

LSB = 1.25 / 4194303 = 2.98023295e-7

Meanwhile if we calculate for output 2.5V whereby the Hex value is 0x517CC1 which is 5340354 we get,

LSB = 2.5 / 5340353 = 4.68133848e-7

For all three cases, the LSB value is different. Can you assist in providing the accurate LSB value to accurately obtain voltage value from the DEMOD1_DATA and DEMOD2_DATA?

over 2 years ago

0 Thevindra Anand over 2 years ago

Prodigy 40 points

Apologies, the Hex value for 0x28BE60 is 2670176 and the LSB value is as follows;

LSB = 1.25 / 2670176 = 4.68133936e-7

However, still a slight different than other LSB values.

0 Bob Benjamin over 2 years ago in reply to Thevindra Anand

TI__Guru** 113765 points

Hi Thevindra Anand,

There is definitely a typo on the 24-bit section of the datasheet. The data should appear similar to the 16-bit data with the addition of the least significant byte, so the 1.25 data is certainly incorrect for the 24-bit decimal value. The hex value appears to be correct.

Achieving an exact value at the 24-bit level is difficult due to noise. And the precision is also lost in rounding error. The table is not meant to be an exact value but more the area of expected values.

Best regards,

Bob B

0 Thevindra Anand over 1 year ago in reply to Bob Benjamin

Prodigy 40 points

Hi Bob,

Thank you for the explanation. In that case, what is the LSB value used to convert the hex value to decimal in the source code?

+1 Bob Benjamin over 1 year ago in reply to Thevindra Anand

TI__Guru** 113765 points

Hi Thevindra Anand,

In the example program the DEMOD data is truncated to 16-bits, so the 16-bit LSB value would be used for that use case. Consider that 24-bit data will always exhibit some noise which will directly impact the level of precision.

Best regards,

Bob B

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PGA970: LASB Value for DEMOD 1 and DEMOD 2.