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Thread: [EE] 24-bit A/D. Are We in the Twilite Zone Here?
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face picon face BY : Sean Breheny email (remove spam text)



Hi Dan,

I think you went past the BW consideration too quickly. There is a BIG
difference between getting  uV resolution in a 3HZ BW and in a 1 MHz BW.

I have been working on Cornell's Autonomous Helicopter project and we are
pushing MuRata rate gyros to their limits. They give 640uV per deg/sec. I
measured their output noise using the best low-noise amp I could make and I
got 0.05 deg/sec noise floor in about a 50Hz BW (amp noise + gyro noise)
which corresponds to 32uV of noise.

I then designed a circuit on a double-sided circuit board with the top
almost dedicated to ground plane. I kept the digital and analog GND planes
separate except at the connection point of the main GND wire. I placed the
recommended bypass caps for the AD974 (the 16-bit 200ksamp/sec ADC we are
using) and I bypassed the supplies to the op-amps in the circuit). I got a
final noise of 0.066 deg/sec for the whole system, which corresponds to 42
uV. I obtained this on the first try, by just following the manufacturers
recommendations and using careful layout and limiting BW when I could (I
think this is key). I have an (AVR ;-) ) micro running on the same board,
inches away.

Also, I think I was able to improve the noise performance by amplifying my
signal so that my minimum resolution corresponded to a couple of bits on
the ADC, so I wasn't pushing the ADC to its limits.

Sean

At 02:49 PM 4/24/00 -0600, you wrote:
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| Sean Breheny
| Amateur Radio Callsign: KA3YXM
| Electrical Engineering Student
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Subject (change) [EE] 24-bit A/D. Are We in the Twilite Zone Here?

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