Bristol Digest, Vol 539, Issue 5

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Today's Topics:

1. Re: Opamp - somewhat off topic (Dave SMITH)
2. Re: Opamp - somewhat off topic (Andrew)
3. Re: quad core blimey (Peter Hemmings)
4. Re: Opamp - somewhat off topic (david.smith@ds-electronics.co.uk)
5. Re: quad core blimey (bblug@gascoigne19.freeserve.co.uk)
6. Re: Opamp - somewhat off topic (Andrew)


----------------------------------------------------------------------

Message: 1
Date: Mon, 24 Feb 2014 17:48:11 +0100
From: Dave SMITH <Dave.Smith@st.com>
To: <bristol@mailman.lug.org.uk>
Subject: Re: [bristol] Opamp - somewhat off topic
Message-ID: <530B77CB.6020403@st.com>
Content-Type: text/plain; charset="ISO-8859-1"

On 24/02/2014 15:16, Peter Hemmings wrote:
> Generally if you are designating a circuit you should have an idea of
> what you want regards supply voltage and type of encapsulation etc. If
> you are makin a high gain amp you may have to remove any offset voltage
> or buy an amp that can compensate for it. Bandwidth also varies with
> feedback (as I recall) but should not be a problem with your use.

Andrew:

Yes, one thing you need to watch out for in particular is how close the
output can get to the power rails, and whether the OpAmp has single or
twin supply.

In some cases, you might have a minimum output voltage of a few hundred
millivolts above GND, and a maximum of a few hundred millivolts below
VDD. You probably want a "rail-to-rail" design which allows you to go
all the way to the supply voltage rails; at least all the way down to GND.

Also, some OpAmps require a dual supply (e.g. +5 V, GND, -5 V), so you
probably want one with a single supply.

I've not been able to check the spec of the one you mentioned; I'll do
it when I get the chance.



------------------------------

Message: 2
Date: Mon, 24 Feb 2014 16:54:05 +0000
From: Andrew <andrewsoltau@gmail.com>
To: Bristol and Bath Linux User Group <bristol@mailman.lug.org.uk>
Subject: Re: [bristol] Opamp - somewhat off topic
Message-ID: <530B792D.6000602@1dtv.com>
Content-Type: text/plain; charset="iso-8859-1"; Format="flowed"

On 24/02/14 16:48, Dave SMITH wrote:
> On 24/02/2014 15:16, Peter Hemmings wrote:
>> Generally if you are designating a circuit you should have an idea of
>> what you want regards supply voltage and type of encapsulation etc. If
>> you are makin a high gain amp you may have to remove any offset voltage
>> or buy an amp that can compensate for it. Bandwidth also varies with
>> feedback (as I recall) but should not be a problem with your use.
> Andrew:
>
> Yes, one thing you need to watch out for in particular is how close the
> output can get to the power rails, and whether the OpAmp has single or
> twin supply.
>
> In some cases, you might have a minimum output voltage of a few hundred
> millivolts above GND, and a maximum of a few hundred millivolts below
> VDD. You probably want a "rail-to-rail" design which allows you to go
> all the way to the supply voltage rails; at least all the way down to GND.
>
> Also, some OpAmps require a dual supply (e.g. +5 V, GND, -5 V), so you
> probably want one with a single supply.
>
> I've not been able to check the spec of the one you mentioned; I'll do
> it when I get the chance.
Thanks again. Great having someone knowledgable on the case.

Andrew

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Message: 3
Date: Mon, 24 Feb 2014 17:11:08 +0000
From: Peter Hemmings <peter@hemmings.eclipse.co.uk>
To: andrew@1dtv.com, Bristol and Bath Linux User Group
<bristol@mailman.lug.org.uk>
Subject: Re: [bristol] quad core blimey
Message-ID: <530B7D2C.8050106@hemmings.eclipse.co.uk>
Content-Type: text/plain; charset=ISO-8859-1; format=flowed



On 24/02/14 14:43, Andrew wrote:
> On 24/02/14 14:16, Peter Hemmings wrote:
>> "Core blimey" (London Accent) I never thought these still existed! I
>> remember them from many years ago (more the 40!).
> Things certainly change. It used to be Cor blimey. I presume you are on
> the new quad core blimey!

Very Good!

(sinks back into armchair and goes to sleep)

>
>
>
>
> _______________________________________________
> Bristol mailing list
> Bristol@mailman.lug.org.uk
> https://mailman.lug.org.uk/mailman/listinfo/bristol
>

I told you there were lots more experienced members!

--
Peter H



------------------------------

Message: 4
Date: Mon, 24 Feb 2014 18:17:32 +0000
From: david.smith@ds-electronics.co.uk
To: bristol@mailman.lug.org.uk
Subject: Re: [bristol] Opamp - somewhat off topic
Message-ID:
<d13b78b290fa71d38e213695e71d75a064d63a45@webmail.eclipse.net.uk>
Content-Type: text/plain; charset="utf-8"

?

----- Original Message -----
From: "Andrew"

On 24/02/14 14:02, Matt Dainty wrote:
I was looking at a circuit for use with a Pi that featured an OpAmp
and that used a CA3140 part. I note in the features it's a
replacement for the 741 that Dave suggested. HTH Matt Thank you
Matt, a help indeed. And they are cheap and plentiful on Ebay. Seems
like the problem is solved.
Actually, I think that the CA3140 probably isn't the best choice - it
has a minimum supply voltage of 4 V, and the Custard Pi seems to be
mostly 3.3 V based.? Also, the minimum output voltage is about 130
mV.

I think I would probably suggest something like the Microchip
MCP6231, as it can be supplied down to 3 V, and the output voltage
can swing to within 35 mV of both power rails.

As for price, both of them are available on eBay, although the eBay
price seems to be about 10X the RS price (although on RS you
typically have to order in quantities of 20, and you'll need to
collect it from the counter in Bristol if you want to avoid the
punitive postage charges).

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Message: 5
Date: Mon, 24 Feb 2014 18:18:35 -0000
From: <bblug@gascoigne19.freeserve.co.uk>
To: "'Bristol and Bath Linux User Group'" <bristol@mailman.lug.org.uk>
Subject: Re: [bristol] quad core blimey
Message-ID:
<01f501cf318c$d51a0b30$7f4e2190$@gascoigne19.freeserve.co.uk>
Content-Type: text/plain; charset="us-ascii"

>On 24/02/14 14:16, Peter Hemmings wrote:

>"Core blimey" (London Accent) I never thought these still existed! I
remember them from many years ago (more the 40!).

>Things certainly change. It used to be Cor blimey. I presume you are on the
new quad core blimey!



'New' quad-core? Surely we're up to hex-core now, possibly even oct-core?



John

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Message: 6
Date: Mon, 24 Feb 2014 18:23:28 +0000
From: Andrew <andrewsoltau@gmail.com>
To: Bristol and Bath Linux User Group <bristol@mailman.lug.org.uk>
Subject: Re: [bristol] Opamp - somewhat off topic
Message-ID: <530B8E20.6030000@1dtv.com>
Content-Type: text/plain; charset="iso-8859-1"; Format="flowed"

On 24/02/14 18:17, david.smith@ds-electronics.co.uk wrote:
>
> ----- Original Message -----
> From:
> "Andrew" <andrewsoltau@gmail.com>
>
>
> On 24/02/14 14:02, Matt Dainty wrote:
>
> I was looking at a circuit for use with a Pi that featured an
> OpAmp and that used a CA3140 part. I note in the features it's
> a replacement for the 741 that Dave suggested. HTH Matt
>
> Thank you Matt, a help indeed. And they are cheap and plentiful on
> Ebay. Seems like the problem is solved.
>
> Actually, I think that the CA3140 probably isn't the best choice - it
> has a minimum supply voltage of 4 V, and the Custard Pi seems to be
> mostly 3.3 V based. Also, the minimum output voltage is about 130 mV.
>
> I think I would probably suggest something like the Microchip MCP6231,
> as it can be supplied down to 3 V, and the output voltage can swing to
> within 35 mV of both power rails.
>
> As for price, both of them are available on eBay, although the eBay
> price seems to be about 10X the RS price (although on RS you typically
> have to order in quantities of 20, and you'll need to collect it from
> the counter in Bristol if you want to avoid the punitive postage charges).
>
Thanks for that.

Is there such a thing as 4 or 5 of these on a board, or even 10? We are
thinking of setting it up to do several in parallel.

Andrew
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End of Bristol Digest, Vol 539, Issue 5
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