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AD7682 - reference drive

Question asked by larsen on Apr 14, 2014
Latest reply on Apr 15, 2014 by maithil

Here is the starting point of our circuit:

AD7682 reference driving.png

For AD7682, we need to use an external reference voltage as we have a non-standard voltage (~1V).  So we would like to use an op-amp to drive the AD7682’s reference. It appears – if I read correctly, that there are two ways of supplying an external reference:

Driving REFIN with an external reference like illustrated in figure 32 from the datasheet:

AD7682 Drive REFIN.png

Driving the REF pin with an external reference like illustrated in figure 36 from the datasheet.AD7682 Drive REF.png

Both methods seem to present similar problems to the op-amp driver: It has to be heavily decoupled with from 10uF to 100nF. This is a challenge for most op-amps as instability can occur. Incidentally the circuit in Figure 36 is likely to lack a snubber network to stabilize it – at least according to the datasheet for AD8605 (page 14 fig. 40). Anyway,

  1. What are the advantages and disadvantages of driving REF versus driving REFIN?
    I suppose the drift is better when not using the internal buffer (Driving REF), but how much, appears not to be clearly documented.
  2. Can you suggest a suitable opamp buffer which is low power (<50uA and <2.2V,  AD8605 is >2.7V)?
  3. Driving REFIN: Is it ok to do with just a few hundred pF of decoupling and avoid any snubber complexity (as opamp will be inherently stable with low capacity)  or is it better to go with 100nF and add sufficient snubber circuit to stabilize the circuit? Problem with the latter is that most op-amps are not spec’ed for this type of operation – not even AD8605! Adding a series resistor to the opamp output will deteriorate the output impedance.

 

Managing of reference power: About power consumption, the datasheet says on page 22

AD7682 refernce note.png

But unfortunately Table 10 on page 25 is unclear to me:

     4) What combination of Bit5..3 disable the internal buffer? In other terms what is the CFG[5..3] setup suggested to operate in modes: a) Driving REF (fig. 36) and b) Driving REFIN (fig. 32) respectively?

 

I hope you can guide us with at least some of the points in this rather lengthy question.

br henning

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