SparkFun Forums

[axmanjr]

On a tutorial I read, it said that the accelerometer would need to have an output impedance under 10k (roughly), using a low input offset rail to rail op amp. How do I go about doing this? I seem to be having trouble finding this sort of op amp. I'm using a PIC18F4680.

[leon_heller]

You might not need the op amp, check the PIC data sheet and look at the ADC impedance.

Leon

[bungalow_steve]

On a tutorial I read, it said that the accelerometer would need to have an output impedance under 10k (roughly), using a low input offset rail to rail op amp. How do I go about doing this? I seem to be having trouble finding this sort of op amp. I'm using a PIC18F4680.

see
http://www.dimensionengineering.com/dat ... ACCM3D.pdf

[axmanjr]

I'll take a look at that bungalow_steve.

As for my current situation, the output impedance of the accelerometer is 100k-ohms. I believe the pic data sheet says something about 2.5k-ohms for the ADC to work properly.

[leon_heller]

Which accelerometer is it? I don't need an op amp with the Freescale one.

Leon

[axmanjr]

LIS3L02AS4. I get a voltage reading when I test the outputs with a voltmeter. But if I decide to plug in directly to the ADC pins on the PIC, I get nothing.

[bungalow_steve]

LIS3L02AS4. I get a voltage reading when I test the outputs with a voltmeter. But if I decide to plug in directly to the ADC pins on the PIC, I get nothing.

I have used analog devices AD8606 op amps with accelerometers. But you shouldn't get zero with no op amp, just the reading will have an offset error and the the frequency response will be terrible, you might have another problem. 100K is pretty big though, the accelerometer may not be able to drive the A/D capacitance at all. Speaking of which do you have a cap to ground at the output? The cap will charge up between conversions and will be able to supply the A/D with all the charge it needs as long as the sample rates are low enough.

[axmanjr]

putting a capacitor there messes up the output big time- a lot of garbage on the LCD screen. I ordered a couple of the TS924 opamps. I found the evaluation kit for the exact accelerometer that I am using. Unfortunately the company that sells them has a real crappy website, so I can't buy it. But the datasheet for the board shows them interfacing the accelerometer with this particular opamp. So I'll check back with you guys when I receive the order from digikey...

I may check out the AD8606 as well.

[blibot]

LIS3L02AS4. I get a voltage reading when I test the outputs with a voltmeter. But if I decide to plug in directly to the ADC pins on the PIC, I get nothing.

Same problem.what is the solution?

[axmanjr]

Same problem.what is the solution?

I read that you will need a low input rail to rail opamp buffer to drop the high impedance of the accelerometer to a "usable" level for the PIC chip. I ordered the TS924 from digikey, after finding this. Unfortunately, I can't find a link to buy this kit, but the manual shows them using this particular opamp.