[Quoting cobaltmute]What about bypass on the buffers?
I considered it, but decided not to for a few reasons. I've got this buffer running on a PIMETA v1 board, and it's stable, at least with the op-amps I tried. That board has no op-amp bypassing, unless you count C4. And if C4 is enough bypass in PIMETA v1, then it's certainly enough here, given that the op-amp's bypass caps are pretty close, connected through a plane on both ends so inductance is all but zero.If there weren't a real estate cost, the decision might have gone differently. As it is, I'm talking myself into using what little real estate there is for CL resistors.
[Quoting dsavitsk]My sense would be that with spots for them, people can jumper.
Leave open, actually. The resistor funnels tiny currents into the chip from V+ to clamp down on the output current. With no currents flowing in, no clamp.
[Quote]If you don't put them in, then you might want to put the buffers on adapters so that they are easily replaceable.
Others reports about this buffer say it's not really stable on BrownDog adapters in PIMETAv1. My tests referenced above are with the chip soldered to the board, so that's how it will be in PIMETAv2.As for replacement, it's actually not that difficult to remove an SO-8 chip from a PCB. You don't need anything but a soldering iron to do it. I show how in Tangent Tutorial 4.
[Quote]Didn't someone else (Linear, National?) start producing buf634's with a different name?
You're thinking of the LME49600, which I rejected because it only comes in a big SMT power pad type package. (Think of a TO-220 soldered flat to the board...kind of like that). It's bigger even than a DIP-8. You'd have to sacrifice some of the cool bits in PIMETAv2 to make room for three of them.
[Quote]What about using your Pint setup for the rail splitter?
I was forced into that, by the desire to use the same type of chip for all channels, and not having a single-channel version of the AD8397, the main reason for creating the PINT. (And, when it didn't work out, the main reason for discontinuing the PINT.)When you're not so forced, you might choose not to pay the costs:
- Another op-amp channel is generally more expensive than a TLE2426
- The extra op-amp draws a lot more current than a TLE2426
- The two external resistors you have to add take more board space than a TO-92, unless you tombstone them
[Quote]thus doing away with the TLE once and for all.
Aside from availability difficulties, is there anything you see wrong with the TLE2426, as compared to an op-amp and a pair of resistors?
[Quoting PRR]This may well be flawed models.
That makes me feel better, thanks. 
I had my heart set on this scheme for lots of reasons, and was willing to pay some performance cost to get it. Maybe I can have at least some of the cake...
[Quote]And the AC impedance may be quite different from DC readings. And real hard to measure meaningfully.
I tried to simulate it, but stacking a 1 VAC source on top of the DC source and then running an AC analysis seemed to ignore the DC source, giving a 1V rail to rail AC source driving the circuit!I'm pretty sure SPICE can be made to do the right thing -- 18 VDC + 1 VAC ripple -- but I gave up on it. I attached the circuit file above, so if someone gives it a go and figures it out, please post it.
[Quote]The Diode-bias has two points: the base-bias current is quite small, starving the 1N914s
Yeah, on purpose. If I allow enough current to make the diodes happier, I lose the advantage of not using something here that has to throw off photons.That brings me to option 3, which I just thought of today: put pads on the board to allow an LED in either position. You then have several options in populating the board. You can configure it like either bipolar CCS above, by suitable use of jumpers and strings of diodes in a loop in the LED pads. Or, you can use both, giving more interior illumination for a clear enclosure, or just to get better performance.
[Quote]Emitter resistors under the 2N3904s might bump the plate resistance; also unless you are getting very well-matched '3904s then another 20mV-100mV of dead resistance might improve mirror consistency.
Is it going to matter? Is the bias difference going to be any worse than trying to match your own JFETs for PIMETA v1, or taking the Tangent lottery on matched sets?
posted 02-09-2009 07:32 PM CST (US) 
