AD6644: Vref load current

Document created by analog-archivist Employee on Feb 23, 2016
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He wants to know, how accurate the internal reference from the AD6644 is, and
how much current it can drive. He wants to use it for other parts, too.
Therefore he needs a current of 100uA out of the internal reference. Is this
possible?

 

The AD6644 internal +2.4 Vref is not capable of directly driving an external
100uA load. The Vref output would need to be buffered in that application.
ADI does not specify output accuracy on the Vref function since it is primarily
used as an internal AD6644 device reference only. Discussion with the Test
Engineer indicates the accuracy of the +2.4 Vref typically runs (+2.35 to 2.5 V
at +25°) and (+2.2 to 2.7 V from -25°C to +85°C).
The AD6644 Vref output would typically be used to provide a +2.4V DC
common-mode offset to the AD8138 diff amp for DC-coupled operation. This
configuration is shown in Fig 28 of the AD6644 datasheet. The input bias
requirement of the AD8138 VOCM input is only 0.5uA. Using the ADC's internal
+2.4V reference to establish the AD8138 level-shift insures excellent tempco
tracking, improves performance, reduces parts count, etc. If unused, the Vref
output should be tied through a 0.1uF cap to GND to bypass noise which could
affect the AD6644's performance.

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