> 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. > If I were to use a voltage divider for the sensor, I'd put it after the follower so it does not load the sensor. However, we're getting a lot of parts in here, and they all add inaccuracy. I'd probably just use an external ADC and be done with it. I think adding a bunch of analog circuitry for scaling is going to make the thing inaccurate. Run the ADC off the same 5V supply as the sensor. Level shift, if necessary, the SPI down to the PIC supply. The level shift can be done with a single chip. I'd choose the PIC voltage to minimize the number of lines that need level shifting. Good luck! Harold -- FCC Rules Updated Daily at http://www.hallikainen.com - Advertising opportunities available! -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist