### Q

How to calculate the LSB value in microvolts for an AD 1877JR?There is some confusion with the 1.333 factor given in the data sheet

AD1877.pdf_page2_rev0. What is the applicable value to 2^15 Full Scale?

Vref or Vref x 1.333 ?

### A

The answer to your question is to be found on page 8 of the datasheet in thesection APPLICATIONS ISSUES:Analog Input voltage swing.

Nominally the analog input span is 3.1V, that is, Vref+/- 1.55V. Therefore

there is a little confusion when Vinpp is given as Vref x 1.333 which is 3.0V

(taking Vref at it's nominal value of 2.25V)

However, on page 8 it is explained that the analog input span actually scales

with the reference voltage.

Since the reference voltage is fairly loosely specified in terms of absolute

accuracy (+300mV / - 200mV) there is also a fairly large variation in the input

span. The formula on page 8 shows that the analog input span is in fact

proportional to the reference voltage. If you rearange the formula you get

Yvolts = Xvolts x 1.378

where:

Yvolts is the maximum swing without clipping

Xvolts is the measured reference and 1.378 is the ration between nominal

voltage swing and nominal reference voltage

So, the 1.333 factor given on page 2 is simply being cautious, by stating a

limit for the input signal which will ensure no clipping takes place. In fact,

the "real" number is 1.378, so to calculate the lsb size

1lsb = Vref x 1.378 /2^16 =47.3uV for a nominal reference of 2.25V. The lsb

size will change depending on the actual value of the reference for the

particular device in question.