Am I missing something in the understanding of the data sheet for AD7764?

Question asked by brennana on Apr 20, 2016
Latest reply on Apr 20, 2016 by brennana

Hi,

I noticed the numerical values seem to reflect differently against the meaning & specifications as shown on page 18 & 19 of the AD7764 datasheet.

1. Page 18

The page introduces the effective differential range of the ADC, then states only 80% of the range is available that can be applied to pins 5 & 6 of the modulators differential input. Then states this as +/- 3.2768 Vp-p. Then states that with the AD7764 being specified with an input -0.5dB down from full scale (-0.5dBFS). And the modulator must have a CM input voltage of 2.048V.

Q1. Being "specified" means performance as specified in data sheet provided you stay within these bounds right?

Then goes on to describe the figure showing the relationships between the voltages across the Modulator Input pins - corresponding codes and other pertinent information.

The Vin+ & Vin- values expressed at +ve FS and -ve FS are noted as 3.6855V & 0.4105V and 0.4105V & 3.6855V.

The issue is the modulator has differential inputs right...

Q2. So the difference in each limit should be -0.5dBFS right?

except 3.6855 V - 0.4105 V = 3.275 V and thus 0.4105 V - 3.6855 V = -3.275 V.

Curiously the sum of these limit voltages = 4.096 V which is the Vref??

Q3. Surely -0.5dBFS if FS is 3.2768 V is 3.0935 V [taken from manipulating: -0.5dB = 20log10(Max I/P/3.2768) ]

Or is 3.275 V an approximation for 3.2768 V and not actually --0.5dBFS?

2. Page 19. 2nd column 2nd para

Q4. Is the reference to 3.168 Vp-p (-0.5dBFS) approximate or approximate for FS?

Q5. 5 Vp-p into an approximate gain of (3.01/4.75) gives 3.168 Vp-p but how is 3.168V = -0.5dBFS?

cheers

Andrew