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Registered Member #205
Joined: Sat Feb 18 2006, 11:59AM
Location: Skørping, Denmark
Posts: 741
My "new" DRSSTC controller relies on a rectified signal, to be able to predict the primary current zero crossing.
To insure proper linearity, a precision rectifier is needed.
Considering that it is non trivial to simulate, I am surprised that it is not just produced in a single chip. I mean, it is probably sitting inside every digital voltage meter anyway?.
Anyone got a pointer to a precision rectifier able at least up to 50kHz, that I can just go out and buy instead of having to fiddle with opamps and precision resistors.
Registered Member #205
Joined: Sat Feb 18 2006, 11:59AM
Location: Skørping, Denmark
Posts: 741
aaah well,
I bought some random opamps, ordinary ones and a couple fast ones.
In the accompanying application notes to one of them, an Intersil CA3130, there is a schematic of a full wave rectifier, utilizing only one opamp, and able to rectify up to 230 kHz.
So I guess things have evolved considerably since the textbook circuits were drawn.
I`l breadboard it this weekend and see if it fits the bill.
Registered Member #188
Joined: Thu Feb 16 2006, 05:18PM
Location:
Posts: 67
I highly recommend "Art of Electronics" by Horowitz/Hill, one of the best electronics books i have. Not as much math as in most uni books, easy to understand and practically oriented but still on a high level (no "recipe" style stuff without background). Theres some discussion on that topic in there too, very useful.
Registered Member #505
Joined: Sun Nov 19 2006, 06:42PM
Location: Yorkshire!
Posts: 329
Hi Finn, how are you?
I looked into fast precision rectifiers a few years ago when looking at synchronising SSTC drive signals to the base current of a resonator. I have built the circuit shown on page 19 of this datasheet for the AD8037 clamping op-amp
It worked well at 300kHz with very little apparent distortion although disadvantages are the need for a negative rail and the need for a good circuit layout.
I got samples from AD by asking nicely, might still have some around if you want them - I'm not going to use them anytime soon... let me know.
Registered Member #205
Joined: Sat Feb 18 2006, 11:59AM
Location: Skørping, Denmark
Posts: 741
Robert, I`l look into that book. James, Thanks, better than ever.
That AD8037 looks impressive, particularly the overdrive recovery time of 1.5nS might become usefull, see below.
Last night I breadboarded the intersil CA3130 opamp with the circuit shown on page 10 of it`s datasheet. At 52kHz, the performance was not good as shown on the first pic below . By bad performance I am referring to the lack of alignment at the zero crossing. One transition dips below zero, the other floats above zero
You can click the picture for full resolution
Although this opamp is able to function with a single supply, it needs a zero offset adjustment pot and a cap on the strobe pin, so why bother with it. However I am glad I bought this amp, because it made me aware of the ability to do full wave rectification with only one amp and one diode.
Next opamp in line is the LM7171, a fast video type. With the negative supply connected, at 48kHz, this, much better, performance was seen:
Zooming in on the zero transitions, this is the picture:
As you see, there is complete fidelity in the falling slope on both half waves, whereas one raising slope is a bit distorted. If this is due to the recovery time mentioned before, then the AD8037 will perform even better. I am only interested in the falling slope, and of course the peak value, so for now, I will conclude that the right topology and part has been found.
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Precision rectifiers.
