Q
We want to use the internal reference of the AD7856. We connect only one 100nF
on the Refin/Refout pin (4)... We have 3.84V on this pin, I suppose to have
4.096V... Can you confirm that the reference in is normaly at 4.096V if we
don't use a external reference on this pin....
A
1) The capacitors :
The recommendation that the 470nF capacitor be an NPO type is not really
required in practice. We have achieved specified performance without an NPO
capacitor in place. The reason for the dual decoupling capacitors is to
effectively decouple the high frequency bursts of charge which arise in the
DAC. So the absolute value of the capacitor is not so important. Any 470nF
ceramic capacitor with low ESR, ESL is suitable. The designation of a capacitor
as NPO means that it's value does not have a significant positive or negative
temperature coefficient. We originally thought that the high equivalent series
resistance of high value ceramic capacitors could cause a problem but this
turned out not to be the case. I'll ask if we can get the datasheet changed,
since this isn't the first enquiry we have had on this point.
2) Vref voltage :
Specified on page 7 of the datasheet -
4 REFIN/REFOUT Reference Input/Output. This pin is connected to the internal
reference through a series resistor and is the reference source for the
analog-to-digital converter.
The nominal reference voltage is 4.096 V and this appears at the pin.
This pin can be overdriven by an external reference or can be taken as high as
AVDD.
When this pin is tied to AVDD, or when an externally applied reference
approaches AVDD, the CREF1 pin should also be tied to AVDD.