Are there any unity gain stable fast op-amps for single rail?

Lostgallifreyan wrote in news:Xns9F9DAC92FAB66zoodlewurdle@216.196.109.145:

As an example, I found LT1364 on Digikey, but while their filter implies operation on a single supply (5 V ~ 36 V, ±2.5 V ~ 18 V), the data sheet makes it clear that the output swing DOES NOT include the negative rail!

Please help me if you can, because it's clear I can't easily get any guidance I can trust, if I do what I'm 'supposed to do'.

If you can go with a smaller GBP (10 MHz typical, 7 MHz spec'd minimum) National's LM6132/6134 has an input CM range that includes the rails and can swing to within a few mV of both output rails (depending on how hard it's driving, of course). Unity gain stable.

Rich Webb wrote in news: snipped-for-privacy@4ax.com:

I almost certainly need a lot more GBP but thankyou, it's a nice suggestion to check out. It seems to have all the other virtues I need.

(As an aside, I'll mention that I sent a message to Digikey about their search filters returning dodgy results. I got a reply that tuned up just one amp, but it still wouldn't take output to negative rail. For whatever reason, Digikey do not seem to realise that this requirement is what 'signal rail' means, it's not just a different of perspective like stating +5V to +36V instead of ±2.5V to ±18V. The reply I got actually inferred that I'd asked for single stage, too... Ò^O)

Lostgallifreyan wrote in news:Xns9F9DD18B54BFEzoodlewurdle@216.196.109.145:

SINGLE... :)

Well, I don't know who you can trust? How ever, I looked at that spec sheet for the quoted part#. The problem isn't so much that is can't include the (+) rail, it does, it just does not get all the way down to the absolute common side. You'll see ~ .7 or more when the output is in the low swing. This also falls true for the high side with that amp.

Single rail circuits of this nature is tricky to get it right, have you thought about operating a small on board (-) voltage generator for the (-) rail of the op-amp or any op-amp not having rail abilities to the common? I mean, you only need like -1 volt or so.

Jamie

Lostgallifreyan schrieb:

Unfortunately, there is an antagonism between high speed and high supply voltage, and modern products tend to be low-voltage anyway. Nor do the tricks needed for full rail-to-rail inputs increase speed.

If you can dispense with the high supply voltage: TSH72 by STM. Input seems to include the negative rail, output incudes both rails, max. recommended supply 12V.

If you can dispense with the rail-to-rail property: One of the current-feedback amps dressed up as voltage-feedback types, like the LT1361 and LT1364 by LT, or the LM6171 by National. Or true voltage-feedback types like the LM6161 (unity-gain stable) or LM6164 (unstable) by National, or the THS4001 by TI, all with max. supply around 32V.

These are often labeled video amplifiers, so this should make a good search term.

Martin.

Jamie wrote in news:8ehwq.29352$v snipped-for-privacy@newsfe21.iad:

Not this time. :) I did that in another circuit where it had the extra advantage of some huge headroom (laser power meter, up to over 20 watts on a single scale, to be read by any voltmeter on a 20V range). In this case though, I want clipping at ground as part of protection for a laser diode. So the output (and input) must be able to reach the negative rail. To do anything else increases circuit complexity a lot, compared to what I have now.

Re trust, I'm gettting good answers here (yours too), a sense of what compromises make these particular amp specs hard to find. I knew they were rare, but not so much about why. (LT specifically mention LT1215 as unusual, I guess I was lucky to find it so easily).

snipped-for-privacy@freenet.de wrote in news: snipped-for-privacy@freenet.de:

Definitely possible. One use I had in mind could use up to 36V supply, but that scheme didn't need speed, was just a neat way to use the existing circuit. The real need that made me build it was for a laser diode, not an LED array, and I can use a supply as low as 6V, but I will try for more because some people like to drive multiple diodes in series, so the more headroom the better..

TSH72 looks nice, but has very small differential input max, 4V effectively, and it seems limited to symmetry about ground. I don't need very much difference, but I do need at least 5V. (Also, once I settle on a really good one, I know I might reach for it more often if it has some generous allowance all round. Ideal fantasy: CA3140 on steroids, GBP around 70 MHz :)

I found an LM6142 (after the suggestion given to me, of LM6132), which looks like a proper contender, so I'll certainly look at these others you named. Thankyou. (I don't need to-rail on the postive supply, just the negative..)

Definitely. I'll try that too. Ideally I'll end up with one that is as futureproof as the CA3140 has turned out to be, but I guess no-one knows the form book for new devices, so maybe all bets are off.

Lostgallifreyan schrieb:

Unfortunately ST datasheets are hardly complete. The GHz input transistors very likely need protection since transients of the order of the supply voltage always have to be reckoned with. This is probably supplied on-chip using diodes in combination with series resistors. The max. admissible +-2V (not 4V as far as I can see) between inputs presumably refers to a steady-state stress. If you want to exceed this value, external series resistors should keep the junctions happy. To determine the resistor values, measure the current actually drawn at 2V. As not to compromise speed, tiny capacitancies may be added in parallel.

You may also want to look at Linear's LT1632: rail-to rail input and output, 45MHz, 45V/us, max. admissible supply 36V. The center figure on page 9 of my datasheet (last change appears to have been 2007) is incompatible with a GBP of 45MHz; I suppose the gain scale is off by

Martin.

snipped-for-privacy@freenet.de wrote in news: snipped-for-privacy@freenet.de:

I'll pass on that one. I like the idea of the current limiting resistors, but in my current circuit I have already made boards, and need to keep their size down while using through-hole parts. (I'll aim for a second edition using SMT some time but first things first. :)

Btw, isn't 4V centred about zero just another way to say ±2V? I'm ok with saying ±nV if it removes ambiguity though.

Now that DOES look nice. :) Thankyou. Wasn't part of my last LTspice update, but I swapped it in for LT1215 in my circuit model and it works well enough that I'll try to get some to try in the real one soon.

The circuit as it is now is here:

Currently, when I crank the input frequency (using square waves) to above about 350 MHz I get some nasty spiking that would threaten the laser diode that the circuit is driving. That ONLY happens if the dual pot that mixes between DC 5V internal reference, and 5V peak input, were to fail in such a way as to make the total difference of input voltage greater than 5V. With a good pot this is unlikely, but if I can reduce the risk further, I will. Spice didn't reveal this problem, so I'm not yet sure what the cause is, other than it seems related to cliping at zero volts. THis cliiping is a clear advantage at up around 300 Hz but apparently a major risk above this. Ò^O

The first thing to test seems to be a different amp, one that is more stable with unity gain (the first stage in that circuit has LESS gain, as it has to divide by four, whatever total results from the mixed input). The CA3240 never showed this behaviour, but then it wouldn't, it's not a fast amp. :) So I'm still left wondering if changing the amp will fix this, or whether I need to look elsewhere. Figuring out what other IC's work well here seemed like a good first test...

Lostgallifreyan wrote in news:Xns9FA19959150AAzoodlewurdle@216.196.109.145:

Clipping. Just so people know.... >:)

Lostgallifreyan wrote in news:Xns9FA19959150AAzoodlewurdle@216.196.109.145:

KHz. This typo actually mattered...

Lostgallifreyan schrieb:

My TSH7x datasheet says on page 2:

ABSOLUTE MAXIMUM RATINGS

Symbol | Parameter | Value | Unit ---------------------------------------------------------------- Vid | Differential Input Voltage (2) | +- 2 | V

with the footnote:

(2) Differential voltages are non-inverting input terminal with respect to the inverting terminal

It is listed by ST as being replaceable by their TSH72! They fail to mention the much higher max. supply of the LT1632 though.

Martin.

snipped-for-privacy@freenet.de wrote in news: snipped-for-privacy@freenet.de:

Oh. So literally ONLY 2V, from each other. I didn't notive that restriction, I assumed they meant ground, therefore total peak difference of 4 when both were were at opposite maximum. (Normally when I see ± anything, that's what is meant).

Anyway, if you looked at the thing I linked to, you'll see I have weird ideas about versatility and use of widely available parts with a good degree of futureproofing. So a wide-range differential input is a must.

I can see that recommending parts can be more than awkward. We're expected to specify minutiae to limit choices, but that's a raw call when makers are failing to do what we must do, when we can't if they don't. Again, this is ahy I try to choose parts carefully. I like to make choices I can stand by for years. I have no clue what LT's record is of pulling IC's out from under people's feet, but surely as times get harder, they might want to limit the number of different devices to get reliable sales. I don't know how to figure out likely stocks that last, it seems that a non-technical approach can work just as well, like seeing what gets sold widely enough to end up on eBay and lots of small hobbyist supplies. Apart from "if it works and people want it enough" I can't see how to make choices.

Btw, the CA3240 works a damn sight better than I said it does in that laser driver page I linked to. I just coped it again today, and was more than impressed. It's actually better at 500 KHz that some are at 100 or 200 KHz when using video bandwidth amps, if it doesn't have to do any gain. It seems to have enough slew rate and stability at unity gain to make me wonder why I'm searching so hard to better it at all!