On Fri, 24 Aug 2001, Lawrence Lile wrote:

> I've got a situation where a high impedance sensor is feeding analog
> information into a PIC A/D input.  The datasheets sez not to use PIC A/D's
> with input impedances > 10Kohms.  The sensor is basically a phototransistor,
> with a 2 meg collector resistor for high gain.
>
> So I either need to use a buffer, or do what the data sheet says not to do.

Lawrence,

You may wish to consider an N-Channel JFET configured as a unity gain
buffer. Connect the gate to the collector/resistor junction of the
phototransistor, the source to +5, and the drain to ground through an Nk
ohm (N depends on the JFET). Take the output at the JFET's drain. JFET's
like negative bias between GD junction, so ideally you'd want to tie the
drain resistor to say -5V, which you probably don't have. However, at cost
of reducing some of the signal swing in the photo transistor, you can
add an emitter resistor and thus introduce a little extra DC bias to the
gate.

The benefit of a JFET amplifier is that its input impedance is extremely
high. It's hard (but not impossible) to achieve this with a BJT
configuration.

Scott

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