------=_Part_72817_7828013.1165603275816 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Content-Disposition: inline On 12/7/06, Gordon Williams wrote: > 1) The sensor output (up to 5V) is greater than the allowable ADC input > because the PIC is at 3.3V. A voltage divider would have to use large > resistances ~50 - 100 Kohm to limit the current from the sensor (max 0.1mA). > This exceeds the PIC ADC input impedance. Therefore what are the > alternatives? Could I use a capacitor in parallel with the bottom leg of > the voltage divider to supply the ADC input impedance requirements. Do I > need to use an op amp with a gain <1? I have a few LM158 on hand. Any > other ideas? I would use a simple opamp such as an LMV324 as a voltage follower after the divider you describe. This will make the output of the sensor and divider low impedance again and will also serve as a buffer for your ADC input. You should also clamp the output of the opamp with protection diodes. Attached is a small schematic drawn up by Olin which he posted on 9/9/04. I have used this circuit many times as a follower/buffer. You would have to change the diodes, but in general everything else would be the same. 2) What should I be using for the Vref for the ADC. Ideally I would want > to use the pressure sensor supply, but it will be at ~5 V. As the sensor > is > ratiometric I should have the ADC ratiometric using the same voltage. You can scale the reference down to 3V for Vref with a divider and opamp from the V+ voltage of the sensor, just like you can for the sensor output. 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