Spehro,

Do you have any recomendations for op amps.  I have an application circuit
going from an op amp to a DAC.  I got it off the web, but it did not
transfer very well.   From a 1993 Linear Edge, Issue No. 7.  I can see that
I need to attach VRef+ to an analog input, but the rest of the circuit is
not very clear.  Do you have a interconnect going from an Op Amp to a PIC
microC.
----- Original Message -----
From: "Spehro Pefhany" <speff@INTERLOG.COM>
To: <PICLIST@MITVMA.MIT.EDU>
Sent: Friday, March 02, 2001 1:29 PM
Subject: Re: [PIC]: A/D conversion (was: blank subject line)


> At 02:20 PM 3/2/01 -0800, you wrote:
> >I am attempting to do a A/D conversion to a signal ranging from -1 VDC to
> +5.5. VDC.  I am using a PIC16F877.  I understand to need to use the VREF,
> +VREF, and -VREF pins in order to do this.  The problem is that I have not
> found a good description of how to use/apply these pins.
>
> Have you had a look at the block diagram in section 11 of the data sheet?
> And table 15-12 ?
>
> You won't be able to directly cover that range with the A/D, you may need
> an op amp
> or just a couple of resistors to get it within the range of 0 < Vin < Vcc.
>
>
> (Once it's within that range, you can decide where you want the A/D
> converter to operate,
> within reason. Vref- should be between about Vss and Vref+ -2V. There has
> to be at
> least 2V between Vref+ and Vref- and Vref+ should not be higher than Vdd.)
>
> For example:
>
>            +5 = Vdd = Vref+
>                 |
>                 R1
>                 |
> Input -- 10K0 --x->>  Analog in 0..5V
> -1 ~ 5.5V       |
>                 R2
>                 |
>                0V = Vref -
>
> By inspection (at the limits) you can easily see that R1 must
> be 50 * 10K and R2 must be 10 * 10K0
>
> So, closest E-96 values, 10K0, 49K9, 100K
>
> (note that you should have a fairly low source impedance if you use this
> method, depending on the accuracy you need, less than 50 Ohms if you need
> the full 10 bits - if it's higher but *known* and *constant*, you can just
> subtract it from the 10K0).
>
> Best regards,
>
>
>
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
=
> Spehro Pefhany --"it's the network..."            "The Journey is the
reward"
> speff@interlog.com             Info for manufacturers:
http://www.trexon.com
> Embedded software/hardware/analog  Info for designers:
http://www.speff.com
> Contributions invited->The AVR-gcc FAQ is at:
http://www.bluecollarlinux.com
>
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>
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>

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