1. Home
  2. Electronics Design
  3. Technical question about good low power rail to rail opamp

Technical question about good low power rail to rail opamp

Technical question about good low power rail to rail opamp. I published 'scope_pic'

formatting link
and am getting questions as 'to where to connect the input etc..'.

The question is how to make a 1 MOhm, few pF, input with a bandwidth of about

1MHz, with as few components as possible, that also creates a +2V DC offset (to get the ADC midrange and allow negative signals),

Now My first idea was to make a JFET input, tried something in spice like this: ftp://panteltje.com/pub/fet.gif

There is a lot of spread in individual jfets, and the only negative voltage available is a tap of the -8 V from a MAX232.

So that leaves opamps, and the ones I found (5V rail to rail) have either low bandwidth, bad slew rates, incredible distortion, or all of those at the same time.

So I am looking for a decent quad 5V to 12 supply opamp with low current draw, only needs to drive a PIC input, say 10k in 25 pF, with low distortion, high gain, slew rate > 10V us, voltage swing to very near the positive supply, input impedance 00, input cap < 10 pF.

I looked at LMC6036, some others, but no good. Any ideas? So it is for the scope input amp.

Reply to
Jan Panteltje
Loading thread data ...

1MHz,

For 5V RR whats wrong with the OPA365. They also have an example for how to achieve 0V output for interfaceing to an ADC.

formatting link

The THS422X may be overkill but off the top of my head....

formatting link

Reply to
Hammy

On a sunny day (Mon, 12 Apr 2010 08:48:44 -0400) it happened Hammy wrote in :

1MHz,

This is limited to 5.5 V supply, so I think I cannot do +2 to -2 Vpp input swing for 0 to +4 V out. Unless I put a lithium cell in series with the output :-) The main purpose is level translation and impedance matching,

-2.5 to +2.5 in, versus 0 to 2.5 out,

1 MOhm about 10 pF in, versus 10k 15 pF out. Gain 1, (or higher of course), The opa range is interesting, will look up some more of that stuff.

Yes that is nice, but also consumes quite a bit of current... that I do not have.

Thank you.

Reply to
Jan Panteltje

TLV2372?

Kind of an LM358 including the top rail, with lower current... response about the same. Bandwidth isn't "low" but maybe it's not 10V/us.

The FET follower will work if you add a trimpot and another JFET to compensate it. If you don't want a trimpot and the extra 4 transistors, you might as well use an op-amp. Fewer pins for sure.

Tim

-- Deep Friar: a very philosophical monk. Website:

formatting link

Reply to
Tim Williams

1MHz,

Why are you stuck with only a +5v rail. Can't you use any voltage rails you like for the op amp and shift the output and alter the gain independently? (and if you need to add some diodes to keep the output clamped between 0 - 5 volts)

Reply to
David Eather

have.

Yea the THS422X IS a bit of a brute;-)

I went through my opamp data sheets and found this.4.3mA/ch .Availiable in a quad.Low power shutdown mode.

formatting link

The TLV2372 Is a decent low power opamp but its slow 2.4 V/us

Reply to
Hammy

n a sunny day (Mon, 12 Apr 2010 12:56:52 -0500) it happened "Tim Williams" wrote in :

That looks good. It is just fast enough and has a 4 in a package version And I can but it here and it is in stock. one for ch 1, one for ch 2, one for trigger, and one for DC shift perhaps.

Yes, need a lot more transistors then, more space, more expensive, I like this opamp, it does not have all those weird curves some of the others have. I came up with OP482 myself this afternoon, but that is more expensive. The TLV2374 quad goes for about 1 Euro here.

Reply to
Jan Panteltje

On a sunny day (Tue, 13 Apr 2010 04:21:36 +1000) it happened David Eather wrote in :

1MHz,

this:

I am using -8V from a MAX 232 output as negaive rail :-) and possibly the +8 from an other output as positive rail. That solution is hereby copyrighed. hehe

Reply to
Jan Panteltje

On a sunny day (Mon, 12 Apr 2010 14:26:02 -0400) it happened Hammy wrote in :

have.

OK, but those are +/-5V, and I want to run from +5 -8.

Reply to
Jan Panteltje

about 1MHz,

this:

draw,

Sorry - a few published PIC programmers have beaten you to that idea.

Reply to
David Eather

have.

Li'l zener diode in series?

--
Regards, Joerg

http://www.analogconsultants.com/

"gmail" domain blocked because of excessive spam.
Use another domain or send PM.
Reply to
Joerg

On a sunny day (Tue, 13 Apr 2010 05:07:58 +1000) it happened David Eather wrote in :

about 1MHz,

this:

draw,

Good, then we know it works :-)

Reply to
Jan Panteltje

1MHz,

I don't see low cost :-) ST has some nice rail-to-rail opamps. TS924 and TS974 come to mind.

--
Failure does not prove something is impossible, failure simply
indicates you are not using the right tools...
nico@nctdevpuntnl (punt=.)
--------------------------------------------------------------
Reply to
Nico Coesel

On a sunny day (Mon, 12 Apr 2010 21:26:28 GMT) it happened snipped-for-privacy@puntnl.niks (Nico Coesel) wrote in :

1MHz,

Nice, but a bit high current :-) Seems to be directed at the audiophile market?

Reply to
Jan Panteltje

1MHz,

!!MAXIM parts??? They actually _exist_??? Gag me with a spoon!

Reply to
Robert Baer

^^^^^^^^^^^^^^^^^^^^ Please, JP, this is primarily a political discussion group.

bout 1MHz,

Are you looking for gain of 1, or do you need other gains too (like a real 'scope)? The GBW of the amplifier will have to be much higher if you want to see a

1mV 1MHz signal fill the display.

I guess you could use the +/-8V supplies and avoid the need for a R-R amp?

Reply to
speff

bout 1MHz,

Op-amp idea:

The LT1352 gives a darn good performance for very little supply current.

It is not rail to rail but you could power it from the +/- 9V of an LT1081

A simple voltage follower and then an inverting amplifier with the noninverting pin at 1V seems like a good way to go.

PNP circuit: If you don't mind some drift and can afford to lose gain, you could use a PNP emitter follower with the emitter resistor being a voltage divider up to the +5V supply.

I very bad idea: You could use a flying capacitor switching circuit screaming along at

10 or more MHz.

this:

ge

The LT1081 is a little more robust than the MAX232.

draw,

igh

Reply to
MooseFET

On a sunny day (Mon, 12 Apr 2010 17:11:10 -0800) it happened Robert Baer wrote in :

How many MAX232 do you want? :-)

Reply to
Jan Panteltje

On a sunny day (Mon, 12 Apr 2010 19:16:32 -0700 (PDT)) it happened speff wrote in :

hehe :-)

Basically 1, but 10 would be OK too for an extra input range. If I set the internal reference of the PIC to 1.024 V (it is 4 x that now), then 64 pixels becomes 1 V on the LCD, or 16 mV per pixel. With a gain of 10 you get 1.6 mV per pixel.

GBW of the amplifier will have to be much higher if you want to see a

The math works like this for me: the fastest sample speed is 500 uS full screen (128 pixels horizontal). So per pixel 3.9 us. The opamp should be able to go from 0 to 1 V (or 0 to 4 V in the current setup) in 3.9 us. So there is plenty of room.

The +5 and the -8 will probably do.

Reply to
Jan Panteltje

On a sunny day (Mon, 12 Apr 2010 19:39:21 -0700 (PDT)) it happened MooseFET wrote in :

Ok, got it, but I already have a whole bunch of MAX232.

My other idea of an input was this:

+5 +5 | | e e PNP b-------- b PNP c | c +5 | |______|

-2 to | | | sets

+2 V c | R3 DC shift ---- b NPN e--------1 k ---> 0 to 4V | e ---------- b PNP R2 /// | c R1 | | -8 -8

This is two opposite connected emitter followers, the Vbe drops should cancel, The PNP current mirror adds a constant drop arcross R2 to shift the voltage to the positive side. The input impedanve is huge too. But an opamp is simpler. Have not tried this though. JFETS are nice as input.

Reply to
Jan Panteltje

ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.