Need Help - Gyro with OpAmp Question
Posted: Sat Dec 04, 2004 9:37 am
Hello All,
We've been playing with the ADXRS401 gyro for a bit now and have experienced some interesting phenomena that I'm hoping some of you can explain...
The ADXRS series has a nice 2.5V reference pin. It's uber-regulated and should be dead on. Hooking up a cheapo multimeter, I get just that - 2.5V, dead-on, ok cool.
Attaching this 2.5V to a PIC, I run an ADC on the voltage. To my mistification, I get all sorts of variances. Usually lower, like 498/1024 = 2.43V. Why is this? I believe it's a current issue.
The maximum recommended input impedance for the PIC 16F88 is 2.5kOhm. The ADXRS401 has a datasheet spec for the 2.5V reference called "Load Drive to Ground" which is 200uA at 2.5V. I assume this translates to a 2.5V/0.0002A = 12.5kOhm output impedance. Therefore, we've violated the PIC's ADC input impedance. Simply put, the ADXRS is not pumping out enough juice to correctly and repeatably charge up the internal capacitor that is the PIC's Analog to Digital Converter. I am even close to being on track here?
Okay, so after four blissful years of EE training, if you didn't have the word OpAmp burned into your brain, you were probably sleeping the entire time. Well it turns out, I was sleeping...
Question 1) How would I hookup an opamp to be used only as a current source / signal buffer? I don't want to amplify the signal, I just what the 2.5V reference pin (and other ADXRS pins) to have enough current to be correctly read by the PIC's ADC.
Question 2) How do I hook up a simple opamp circuit for small amplification of the ADXRS signal? All the opamp examples I remember are for 100-1000x amplification. This would saturate the ADC immediately. I am looking for a 2-3x amplification.
I really appreciate any ideas or links you can point me to,
-Nathan
We've been playing with the ADXRS401 gyro for a bit now and have experienced some interesting phenomena that I'm hoping some of you can explain...
The ADXRS series has a nice 2.5V reference pin. It's uber-regulated and should be dead on. Hooking up a cheapo multimeter, I get just that - 2.5V, dead-on, ok cool.
Attaching this 2.5V to a PIC, I run an ADC on the voltage. To my mistification, I get all sorts of variances. Usually lower, like 498/1024 = 2.43V. Why is this? I believe it's a current issue.
The maximum recommended input impedance for the PIC 16F88 is 2.5kOhm. The ADXRS401 has a datasheet spec for the 2.5V reference called "Load Drive to Ground" which is 200uA at 2.5V. I assume this translates to a 2.5V/0.0002A = 12.5kOhm output impedance. Therefore, we've violated the PIC's ADC input impedance. Simply put, the ADXRS is not pumping out enough juice to correctly and repeatably charge up the internal capacitor that is the PIC's Analog to Digital Converter. I am even close to being on track here?
Okay, so after four blissful years of EE training, if you didn't have the word OpAmp burned into your brain, you were probably sleeping the entire time. Well it turns out, I was sleeping...
Question 1) How would I hookup an opamp to be used only as a current source / signal buffer? I don't want to amplify the signal, I just what the 2.5V reference pin (and other ADXRS pins) to have enough current to be correctly read by the PIC's ADC.
Question 2) How do I hook up a simple opamp circuit for small amplification of the ADXRS signal? All the opamp examples I remember are for 100-1000x amplification. This would saturate the ADC immediately. I am looking for a 2-3x amplification.
I really appreciate any ideas or links you can point me to,
-Nathan