Difference Amplifier Enables Low-Power, High-Performance Absolute Value Circuit
Traditional precision half- and full-wave rectifiers use carefully selected high-speed op amps, fast diodes, and precision resistors, but this expensive solution suffers from crossover distortion and temperature drift. A dual-channel difference amplifier—with no external components—can provide an absolute value output that achieves better precision, lower cost, and lower power consumption.
posted by Analog Dialogue @ 2:58 PM
5 comments
5 Comments:
Excellent article!! I understood it in a decent amount of time (30 to 45 min), from which I can then say it is well written and focused to "real people".
I found it very usefull, too. Kudos to both authors!
FTT
The article should have specifically pointed out that the resistors in the AD8277 absolute value schematic are actually a part of the AD8277 device itself. I didn't realize the resistors were on the IC until I pulled up the AD8277 datasheet and reverted back to the schematic. Pin numbers identified on the schematic and dashed lines enclosing each amp+resistors would have made this very apparent and added a bit more value to article. However, that said, great application of the AD8277 as an upgrade to an old problem.
Although some nice scope pics are shown there is no quantitative informatoion expect for part specs. I would like to see some performance information for low value signals in the mV range, (how close to zero can I go), and more on the high frequency effects.
In Fig 3 is there a reason that you use both resistors (in parallel) on the inputs of A1 (the unity gain amp) and the pos input of A2? Will leaving some of the SENSE or REF pins unconnected degrade results? For example, may I leave some SENSE and REF pins disconnected to provide an ABS function with 2X gain?
Hi Dave
There is a reason for the resistors to be connected the way they are. a) to get the absolute value with a gain of one and low vos, one has to match the impedance seen by all the amplifiers terminals(inverting and non inverting, or else the ibias becomes vos through the various resistors) b)not connecting resistors allow noise from external sources to couple to the ckt.
2) to address your other question as to the possibility to make a gain of two absolute value, the answer is yes, you can look at the data sheet of the AD8279 and try to wire it in a similar way to this application(not identical)
If you want to talk more call me. Moshe at 617 642 0013
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