On 8/2/06, John Pearson <xero@cmc.net> wrote:
> I need to amplify the signal from a sensor into an ADC. I have very little analog experience or knowledge.
>
> Circuit supply voltage is +3.3 volts.

use and OA with single suply and 3.3V in non inversor amplifier configuration
and rail to rail output
(input signal on +in, feedback reaction and ground resistor on -in,
input signal is considered to be positive in range +0...+35mV)

> Output impedance of transducer is 1K typical.

use an input impedance in the OA at least 10x greater (not inverted
configuration is ok)

> Full output voltage span of sensor is 35mV typical.

use a gain of 3300mV/35mV = 94 (~100) and Vdd-Vss as internal
references on the ADC. Any common amplifier will do that

> Nominal input impedance of ADC is 10K.

the output impedance from OA is near 0 ohm so ADC impedance is not an issue

>
> I want to amplify the DC voltage from the sensor to the ADC as much as possible. Linearity, temp drift and offset is not too much of an issue.

offset: yes. An ofset of +/-1mV at the OA input will create +/-100mV
at the output if the gain is 100

temp drift: an OA with 300uV/C will create an output thermal drift of about
+/-300uV * 100 (gain) = +/- 30mV/C which will be added with the
offset, choose an OA with maximum 30uV/C for your application
(nowadays will be difficult to found worse amplifiers than that...)

>
> Can this be done with perhaps a mosfet and a couple resistors?

not in easy way

Is there a small package (like a TO92 or 3 pin package) that will do the trick?

TO92 I don't know, but SM8 or SOT25-3 yes

>
> Thanks
>
> John

with pleasure,
Vasile
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