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From: Jan Panteltje on 12 Apr 2010 07:33
Technical question about good low power rail to rail opamp.
I published 'scope_pic'
http://panteltje.com/panteltje/pic/scope_pic/
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.
From: Hammy on 12 Apr 2010 08:48
On Mon, 12 Apr 2010 11:33:36 GMT, Jan Panteltje
<pNaonStpealmtje(a)yahoo.com> wrote:

>Technical question about good low power rail to rail opamp.
> I published 'scope_pic'
> http://panteltje.com/panteltje/pic/scope_pic/
>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.

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

http://focus.ti.com/lit/ds/symlink/opa365.pdf

>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.

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

http://focus.ti.com/lit/ds/symlink/ths4222.pdf

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


From: Jan Panteltje on 12 Apr 2010 09:41
On a sunny day (Mon, 12 Apr 2010 08:48:44 -0400) it happened Hammy
<spam(a)spam.com> wrote in <rb56s55cia8ci5lvp3qlis2klup2c50q3n(a)4ax.com>:

>On Mon, 12 Apr 2010 11:33:36 GMT, Jan Panteltje
><pNaonStpealmtje(a)yahoo.com> wrote:
>
>>Technical question about good low power rail to rail opamp.
>> I published 'scope_pic'
>> http://panteltje.com/panteltje/pic/scope_pic/
>>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.
>
>For 5V RR whats wrong with the OPA365. They also have an example for
>how to achieve 0V output for interfaceing to an ADC.
>
>http://focus.ti.com/lit/ds/symlink/opa365.pdf

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.


>>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.
>
>The THS422X may be overkill but off the top of my head....
>
>http://focus.ti.com/lit/ds/symlink/ths4222.pdf

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

Thank you.
From: Tim Williams on 12 Apr 2010 13:56
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: http://webpages.charter.net/dawill/tmoranwms

"Jan Panteltje" <pNaonStpealmtje(a)yahoo.com> wrote in message
news:hpv0ek$vfj$1(a)news.albasani.net...
> Technical question about good low power rail to rail opamp.
> I published 'scope_pic'
> http://panteltje.com/panteltje/pic/scope_pic/
> 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.


From: David Eather on 12 Apr 2010 14:21
On 12/04/2010 9:33 PM, Jan Panteltje wrote:
> Technical question about good low power rail to rail opamp.
> I published 'scope_pic'
> http://panteltje.com/panteltje/pic/scope_pic/
> 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.

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)
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