AD1938 output coding

Question

Hello 
 
 I will use AD1938 as 24bit ADC and could I ask how to calculate input voltage ->LSB ? 
 ( which the number of full-scale or VREF in the datasheet should I use ?) 
 Which equations are correct ? 
 Differential input voltage / ( VREF (1.5V) *2) = outpu code ( LSB) or differentail input voltage / (Full-scale(1.9Vrms) *root(2)) =output code ? 
 
 Please let me know your advice. 
 
 Regards, 
 Teli

DaveThib · Accepted Answer

Hello Teli, 
 I do want to ask why do you need this? Most people who use this part for audio do not need to calculate the exact code. They just need to know what the full scale peak to peak voltage will be. If you are planning to use this as a data converter keep in mind that it is designed for audio and not data so it will have a lot more drift and gain error than a data converter will have. Its DC performance will not be great. 
 This will be a little difficult to follow. Here is the formula simplified: 
 code=(4*(Ain-CM)* 2^23-1)/Vpp 
 Ain is the voltage on the pin relative to ground and assuming a nominal CM voltage. 
 CM= nominal Common Mode voltage which in this case is 1.5V. 
 Vpp is full-scale input voltage as a differential peak to peak number. 
 The result will be the 2's compliment code number. Keep in mind that this is one pin and the assumption is that the other pin is sitting at the complimentary voltage. If you just use this single ended with the other pin sitting at the CM voltage, then the result will need to be divided by 2. You need to use both pins to reach digital full scale input. 
 I got this by doing the following calculations then simplifying it afterwards. 
 Ain-CM gives the full scale positive voltage swing. 
 You then divide that by the voltage per bit which is obtained by taking the Vpp and dividing it by 4. Each pin takes half the peak to peak voltage and then each pin further divides it by two around the CM voltage. So you get Vpp/4 
 
 Then take the Vpp/4 and divide that by the number of codes and that is 2^23-1 
 It is a 2's compliment result so the 24 bit number has a sign bit therefore only 23 bits are available for a magnitude. 
 I worked this all out on a spreadsheet if you care to look at it. I did it in pieces then combined it over on the right where I tested the simplified formula. 
 Dave T 
 XLSX)

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AD1938 output coding