Electronics > Projects, Designs, and Technical Stuff
Combining DACs for more current?
NiHaoMike:
One "hack" you can do with a single output DAC is to supply the DAC via a resistor, then wire in a PNP transistor so it adds to the output current when the DAC starts drawing much more than its quiescent current. You'll probably also want to add a capacitor across the resistor to prevent oscillation.
Another solution is the classic NPN transistor on the output to boost the current, just keep in mind the voltage drop varies significantly with temperature. Using a small linear regulator instead of the transistor will give you a positive offset instead of a negative one, plus it would be more stable over temperature. Then there's the LT3083 that has close to zero offset.
David Hess:
--- Quote from: Vovk_Z on August 12, 2020, 09:38:42 pm ---30 mA is not a big current for many of actual opamps, even some precision ones. For example AD823 (not very precision), AD8610 (precision), OPA189 (very precision one).
--- End quote ---
High output current is incompatible with precision, even on the chopper-stabilized OPA189, because of thermal effects from self heating. This is why precision operational amplifiers are characterized with high load resistance, and in applications where it matters, they will be externally buffered. They implement current limiting only for self protection and not with the idea that they should be operated with high output currents.
The above is also why lower power parts with lower input bias currents may outperform higher power parts with higher input bias currents; lower input bias current allows for higher impedance feedback which causes less self heating in both the operational amplifier and feedback network.
Vovk_Z:
You are right of cause in general. But in real life hot high loaded OPA189 will be still much more accurate than cold AD8610, for example. Hot AD8610 will be more accurate then AD820-823, etc.
We know very few about accurasy TS needs, but MCP4802 is a 8-bit DAC - it is hard to make it worse with hot precision opamp. I think any opamp with a maximum offset voltage (at 125 C) less then 1-2 mV should do. There are many of them. TS may need a low voltage rail-to-rail ones - I'm not familiar with them unfortunately and can't give a list without some investigation.
David Hess:
The AD8610 is not remotely competitive with the OPA189. The proper comparison is with parts like the bipolar OP77, OP177, AD708, and similar (1) which have offset voltage drifts of 100 nV/C compared to the typical chopper stabilized amplifier's offset drift of 50 nV/C.
What these parts all have in common is that their offset voltage drift is dominated by self heating and the immediate environment. Under ideal conditions, chopper stabilized parts have a lower drift but without thermal baffles and without high load resistance, they all perform equally poorly; thermocouple effects will dominate. With the AD8610, the difference in performance would be much less.
Where precision bipolar parts cannot compete is with lower flicker noise since chopper stabilization removes it.
(1) Since trimming the offset voltage of these types of precision bipolar operational amplifiers reduces offset voltage drift, even parts with higher drift are suitable if offset nulling is used.
Vovk_Z:
--- Quote from: David Hess on August 14, 2020, 01:10:30 am ---The AD8610 is not remotely competitive with the OPA189.
--- End quote ---
- no, it is competitive if we compare only its output current (and costs much less). :)
The TS question was only about current. So I proposed several options. :)
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