Bob Blick wrote:

> On Fri, 11 Feb 2011 11:44 -0500, "V G"  wrote:
>> I'm trying to read the output from an amplifier that can potentially
>> swing from -3V to +3V with an ADC (on a PIC32). Should I use an
>> opamp (LTC to "shift" the signal into the positive voltage range? Or
>> can I just use two 10K resistors to scale the -3V to 3V output into
>> the 0V - 3V range?=20
>>=20
>> I'm leaning towards the resistor divider. Seems simple and
>> effective, but I wouldn't know any complicated implications it could
>> have. Also, what values of resistors should I use?
>=20
> Resistors are fine, since your source has low impedance (probably, I
> am assuming since you said "amplifier").=20
>=20
> What is the maximum recommended impedance for the PIC32 A/D? Choose
> your resistors to be four times that, or less.=20

How do you get to "four times that, or less"? Isn't that "two times
that, or less", for a simple divider like this:=20

opamp out (-3V...+3V) -- R1 -- output -- R2 -- +3V

> Remember that the accuracy will now also be dependent upon the +3
> source on the end of your resistor divider.=20

That also goes for the opamp summer/scaler if it uses the same +3V as
reference. OTOH, any difference is only a (small) DC offset on the ADC
input, and if this is still the ECG project, a (small) DC offset doesn't
do any harm here.

Gerhard
--=20
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